Chapter 4: Forces and Motion - Don Voorhees

In the span of a few extremelyexhilarating seconds, this bungyjumper has experienced a crash

course in forces and motion—as wellas speed, velocity, acceleration, grav-ity, mass, weight, and inertia. Aftershe passes the course, she wouldprobably love to explain these con-cepts to you—from somewhere onsolid ground. In this chapter, you’llstudy them from the safety of a chair.You’ll also learn how Newton’s laws ofmotion describe what happened tothe bungy jumper after she fell.

What do you think?Science Journal Look at this photowith a friend. Discuss what this mightbe or what is happening. Here’s ahint: In a few more seconds, a drasticreduction in motion will be welcomed.Write your answer in your ScienceJournal.

100

Forces and MotionScience TEKS 6.5 A; 6.6 A, B

44

101

Why can you slide down pavement when it is covered with ice,but not when it is dry?

In both cases, the force of friction brings you to a stop,but ice has much less friction than concrete allowingyou to glide farther. The force of friction usually gets

larger as a surface becomes rougher. Explore how the roughness of differentsurfaces affects the motion of a moving object.

Measure the effect of friction1. Lay your textbook flat on the floor.

Place the clip end of a clipboardon the edge of your textbook.

2. Lay a path of fine-grained sand-paper on the floor leading awayfrom the clipboard.

3. Roll a toy car down the boardonto the path.

4. Measure the distance the car traveled on the sandpaper path.

5. Repeat the same procedure using medium-grained and then coarse-grained sandpaper.

ObserveRecord your measurements in your Science Journal. Write a paragraphexplaining the relationship between roughness of each type of sandpaper andthe distance that the car traveled on the sandpaper.

EXPLOREACTIVITY

101101

Making a Cause and Effect Study Fold Make thefollowing Foldable to help you understand thecause and effect relationship of forces and motion.

1. Place a sheet of paper in front of you so the long side is at the top. Fold thepaper in half from the left side to the right side. Fold top to bottom andcrease.Then unfold.

2. Through the top thickness of paper, cut along the middle fold line to formtwo tabs as shown.

3. Label the tabs Motion and Forces as shown.

4. As you read the chapter, list information about what causes forces and howforces cause motion under the tabs.

FOLDABLESReading & StudySkills

FOLDABLESReading &Study Skills Motion

Forces

102 CHAPTER 4 Forces and Motion

Describing MotionS E C T I O N

MotionEvery day you are surrounded by motion. You recognize

motion when you see cars and people move around you. Yousense that air moves as it blows against your face—or that piecesof Earth’s crust move if you have ever felt the shaking of anearthquake.

Even as you sit in a quiet country field, Earth is movingaround the Sun, and the Moon is moving around Earth. As yousit on a bus that is too crowded for the passengers to move, yousee buildings and other objects seemingly slide past you. Are youmoving or are they moving? Can you tell? So how do you knowwhen something is moving?

Motion Is RelativeThis might have happened to you. You are sitting in a parked

car and are startled to notice that the car is rolling backwards.After you look around, you realize that it is the car next to youthat is moving forward. Why does your car seem to be movingbackwards? At first you could see that your car was getting far-ther away from the other car. As shown in Figure 1, if youthought that the other car was not moving, then it was your carthat had to be moving. Yet after you looked around, you couldtell that your car was in the same place in the parking lot. Youknew then that your car hadn’t moved, and it was the other carthat had moved forward.

■ Identify when motion occurs.■ Explain relative motion.■ Compare speed, velocity, and

acceleration.

Vocabularydisplacement velocityspeed acceleration

You must interpret movement every day.

Figure 1Two cars are sitting side by side.One of them moves. How canyou tell which one has moved?

Grade 5 TEKS Review

For a review of the Grade 5 TEKSChange Occurs in Cycles, see page 499.

SECTION 1 Describing Motion 103

Figure 2Sometimes it is obvious whichobject has moved. The tree doesn’t move, so the skatermust have moved.

Reference Points You used the parking lot as a referencepoint to determine which car moved. You realized that whereyou were in the parking lot had not changed. In other words,your position relative to the parking lot didn’t change. You knewthen that relative to the parking lot, you were not moving, so itwas the other car that moved.

Motion always is described relative to some reference point.Figure 2 shows an in-line skater in a park. Suppose the tree inthe figure is the reference point. The skater is in motion relativeto the tree because the position of the skater has changed rela-tive to the tree.

Reference points are needed to describe the motion of largerobjects as well—such as Earth. Look at Figure 3. Earth is mov-ing around the Sun. If the Sun is the reference point, Earthmoves in a nearly circular path around the Sun. However, theSun is moving relative to the Milky Way Galaxy and the MilkyWay is in motion relative to other galaxies in the universe. Themotion of Earth through space depends on the reference pointthat is chosen.

Choosing a Reference Point Suppose you were todescribe the motion of a baseball as it sped past a batter. Youwouldn’t worry about the motion of the ball relative to the solarsystem or the galaxy. You would be concerned only with itsmotion relative to the player waiting to hit the ball. In otherwords, you would choose the batter as the reference point. Youselect the reference point to describe the motion that is impor-tant to you and the situation you are in. If you are walking on abus that is moving, you could describe your motion relative tothe bus or relative to the ground. Depending on the referencepoint you choose, how would your speed be different?


Figure 3

VISUALIZING EARTH’S MOTION

The Milky Way Galaxy is moving relative to the center of acluster of galaxies called the Local Group. So you can think ofEarth’s motion this way: Earth orbits the Sun, which movesaround the Milky Way Galaxy, which in turn is moving aroundthe center of the Local Group.

C

The Sun belongs to a collection of several billion stars thatmake up the Milky Way Galaxy. Viewed from the “top” of thegalaxy, the Sun moves clockwise in a nearly circular orbit around the galaxy’s center. Earth’s orbit around the Sun is not in thesame plane as the galaxy. As a result, Earth’s motion traces out

a corkscrew path* as it moves with the Sun relative to the center of thegalaxy.

B

In the vastness of space, Earth’s motion can be described onlyin relation to other objects, such as stars and galaxies.The illustration here shows how Earth moves relative to the

Sun and to the Milky Way Galaxy, which is part of a cluster of galaxies called the Local Group.

Imagine you are looking downon the Sun’s North Pole. From thisperspective, Earth traces out a nearlycircular path, moving counterclock-wise in its orbit around the Sun.

A

*Earth’s corkscrew path not shown to scale

LocalGroup

Sun

Earth

Milky WayGalaxy

Changing PositionWhen you ride a bike, you are in motion rela-

tive to the ground. That motion can be describedin many ways. How far did you travel? Where didyou go? How long did it take? How fast were youriding? Did you speed up or slow down?

When something is moving, its location rela-tive to a reference point is changing. The loca-tion of an object is how far the object is from areference point. This also is called the position ofthe object. All motion involves a change of posi-tion. The position is measured relative to a refer-ence point, as shown in Figure 4.

Distance One way to describe your change in position is totell how far you went, or the distance you traveled. Suppose youtravel in a straight line, and don’t change direction. Then youcan calculate the distance you traveled by subtracting your finalposition from your starting position.

For example, suppose you are 50 m from your house andyou want to ride to a friend’s house. You start pedaling your bikeand ride in a straight line. You stop when you are 150 m fromyour house, as shown in Figure 5. Your final position is 150 m,and your starting position is 50 m. So the distance you traveledis: 150 m � 50 m � 100 m.

What does distance measure?


Figure 4Suppose the house is the refer-ence point. The position of thebike rider is 50 m away from thehouse.

50 m

150 m

50 m

Initialposition

Finalposition

Figure 5The position of the bike rider haschanged. The distance traveled isthe difference between the finalposition and the initial position.In this example the distance traveled is 100 m.

Displacement Sometimes motion is notin a single direction. For example, supposeyou are walking to school, which is a 1-kmtrip. You get halfway to school when yourealize you forgot your lunch. You turnaround, walk home, get your lunch, andthen walk to school. How far did you travel?

Look at Figure 6. Your trip to schoolinvolves three parts. In the first part, youget halfway to school. The distance youtravel is 0.5 km. In the second part, youwalk back home. The distance you travel is0.5 km. In the third part, you walk all theway to school and travel a distance of 1 km.To find the total distance you travel, add the

distances traveled for all three parts: 0.5 km � 0.5 km � 1.0 km.The total distance you walked is 2.0 km.

Even though you walked a distance of 2.0 km, you are only1.0 km from you house when you get to school. In other words,the distance between your starting position and your final posi-tion is 1.0 km. The direction and the distance between the finalposition and the starting position is the displacement. Your dis-placement is 1 km east. Displacement and distance traveleddescribe motion in different ways. Distance traveled depends onthe entire path you walked. Displacement depends on only yourstarting position and your final position.

Distance and displacement are usefulin different ways for describingmotion. A biologist studying behav-

ior in bees might record the entire path a bee took when itsearched for food, located it, collected it, returned to the hive,and interacted with other bees. On the other hand, a biologiststudying the migration of monarch butterflies might recordonly where a butterfly stops each night. This biologist is inter-ested only in the butterflies’ displacement after each day.

What is speed?When you ride your bike to your friend’s house, how fast do

you ride? Suppose Monday it takes 30 min to get there, but onFriday it takes only 20 min. On which day were you movingfaster? You were moving faster on Friday, because it took lesstime. When you move faster, it takes less time to travel the samedistance. A way to describe how fast you are moving is by mea-suring your speed. Speed is the distance traveled divided by thetime needed to travel that distance.


Figure 6Your travel distance and displace-ment can be different. How is thedisplacement different from the dis-tance traveled in this example?

1 km

0.5 km

Math Skills Activity

Example ProblemOn a short bike ride, you ride 20 km. It takes you an

hour and a half to complete your ride. What was your average speed?

Solution

This is what you know: distance � 20 kmTime � 1.5 h

This is what you need to know: average speed

This is the equation you need to use: average speed � total distance/total time

Substitute the known values: average speed � 20 km/1.5 h � 13.3 km/h

Check your answer by multiplying by the time. Do you calculate the same distance that was given?

Calculating Average Speed

For more help, refer to the Math Skill Handbook.

Practice Problem

On a long cross-country trip, you cover 3,600 km in about 50 h.What was your average speed for the trip?

Average Speed As you biked to your friend’s house, youprobably slowed down as you had to climb a hill and speeded upas you came down the other side. Perhaps you had to stop at anintersection. Your speed probably changed several times for var-ious reasons.

If someone were to ask you what your speed was for yourride, what would you say? If your speed changed, what speedwould describe how fast you traveled for the entire trip? Oneway to answer this is to determine your average speed. Averagespeed is found by dividing the total distance traveled by the totaltime.

average speed �

If you bike 5 km in half an hour, you can calculate your averagespeed like this:

average speed � � 10 km/h5 km�0.5 h

total distance��

total time


Math TEKS 6.2 C; 6.4 A;6.5; 6.11 A, D; 6.12 A

Graphing Motion The way in which some-thing has moved can be shown with a dis-tance-time graph. On a distance-time graph,time is plotted along the horizontal axis. Thedistance traveled is plotted on the verticalaxis. Each point plotted on the graph showshow far something has moved, and howmuch time was used to travel that distance.

Figure 7 shows the distance-time graphfor a sprinter running a 100-m dash. Thedistance the sprinter moved was plotted onthe graph for each second of the race.According to the graph, after 2 s she hastraveled 10 m. How far did the sprinter runbetween 8 s and 9 s? What is her averagespeed during this time interval?

VelocityWhen school is over for the day, you and a classmate walk

home. Both of you leave at the same time, but walk in differentdirections. If you walk at the same speed, after 10 min you havetraveled the same distance. However, you are not at the sameplace after walking for 10 min because you are walking in differ-ent directions. Even though your speed and the distance traveledare the same, your displacement is different. Sometimes thedirection of motion is important. The direction and speed of anobject are described by the object’s velocity. Velocity is the speedin a particular direction.


0 21 3 4 5 6 7 8 9 10 11 120

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50

60

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100

Time (s)

Dis

tanc

e (m

)Distance vs. Time for 100-m Dash

Speedingup

Changingdirection

Slow

ing

dow

n

Figure 7This distance-time graph showshow the distance traveled by asprinter changed during a 100-mrace. What was the sprinter’s aver-age speed over the entire race?

Figure 8The arrow shows the bike rider’svelocity. The direction of thearrow shows the direction ofmotion. The arrow’s length showsthe bike rider’s speed. The veloc-ity of an object changes when itsspeed or direction changes.

Changing Velocity The velocity of an object can change asit moves along. For example, look at Figure 8. As you rode toyour friend’s house, your beginning velocity might have been 15 km/h north. When you slowed and turned the corner, yourvelocity changed to 8 km/h east. When you turned, your speedand direction changed. Then you pedaled harder to speed up,and your velocity was 15 km/h east. This time your directionstayed the same but your speed changed. Is your velocity nowthe same as when you began? No, because you are now goingeast instead of north. Your speed is the same, but your directionhas changed.

What two things does a velocity measurementdescribe?

AccelerationDistance, displacement, speed, and velocity tell where, how

far, how fast, and in what direction. Sometimes it is importantto know how motion is changing. Is the approaching train goingfaster and faster or slowing to a stop? Is the in-line skater chang-ing direction? The acceleration of an object describes how itsvelocity is changing. Acceleration is a change in velocity dividedby the amount of time over which the change occurs. Thechange in velocity can be due to a change in speed, a change indirection, or both. An object is accelerating if it is speeding up,slowing down, or turning. A figure skater moving in a circle atconstant speed is still accelerating because the direction ofmotion is changing.


Section Assessment

1. Compare and contrast velocity and acceleration.

2. You walk 200 m north, then 275 m south.What total distance did you walk?

3. In question 2, how far are you from yourstarting position? In what direction?

4. Why is a reference point needed to describe motion?

5. Think Critically What does it meanabout your trip if your displacement isequal to the distance you traveled?

6. Recording Data Mark off a short distance on the floor and use a stopwatch to time your-self walking slowly, walking at a moderatepace, and walking quickly. Calculate yourspeed in each case. For more help, refer tothe Science Skill Handbook.

7. Solving One-Step Equations A car is moving forward at a rate of 30 m/s. How farwill the car have traveled after 4 s? For morehelp, refer to the Math Skill Handbook.

Modeling AccelerationProcedure1. Use masking tape to lay a

course on the floor. Mark astart, and place marks alonga straight path 10 cm,40 cm, 90 cm, 160 cm,and 250 cm from the start.

2. Clap a steady beat. On thefirst beat, the person walk-ing the course is at Start. Onthe second beat, the walkeris on the first mark and soon. The walker is moving ata constant acceleration.

Analysis1. Describe what happens to

your speed as you movealong the course. Infer whatwould happen if the coursewere extended farther.

2. Repeat step 2, starting atthe other end of the course.Are you still accelerating?Explain.


ForcesS E C T I O N

What is a Force?When you pedal your bike to speed up, you push against

pedals. When you want to slow down, you pull on the brakelevers. In both cases you exerted a force on the bike. A force is apush or a pull.

A force has size and direction. For example, pushing thisbook from the side will cause it to slide across the desk. How-ever, pushing downward on the book will not cause it to move.The direction of a force is important.

Force and Change in MotionWhen you push or pull on something you change its posi-

tion and its motion. Look at Figure 9. When a book is sitting onthe desk, pushing on it starts it moving. By applying a force, youchanged the position of the book. You push or pull on a door tomake it open or close. You bring a moving shopping cart to astop by pushing or pulling on it. In all these cases, the positionand motion of something changed when a force was applied.

In all of these cases, the motion of an object changes when aforce acts on it. A force makes an object speed up, slow down, orchange direction. Recall that an object accelerates when it speedsup, slows down, or changes direction. So, when a force acts onsomething, the force causes it to accelerate.

■ Identify force, balanced forces,and net force.

■ Explain friction.

Vocabularyforce frictioninertia gravitybalanced forces

Forces cause every change in motionyou can see around you.

Figure 9A force causes the motion of eachof these objects to change. Howis each object accelerating?

Force exerted by man pulling on cart

Motion of cart

Force exerted by girl on door

Motion of door

Inertia and Mass How doesan object respond to a force? Ifyou give this book a push, youcan slide it off your desk. How-ever, if you give a car the samepush, you know the car won’tmove. Some moving objects areeasy to stop. It’s easy to stop amoving basketball but hard tostop a rolling car. Compared tothe book or the basketball, a carhas more inertia. Inertia mea-sures an object’s tendency to resist changing its motion. Themore inertia an object has, the harder it is to start the objectmoving or to slow it down.

Inertia depends on the amount of matter in an object, or itsmass. The more inertia an object has, the less effect a given forcehas on its motion. Imagine trying to push a toy truck comparedwith pushing a full-sized automobile, as shown in Figure 10. Asthe object becomes more massive, it becomes harder to move.

Balanced Forces Sometimes more than one force acts onan object. Suppose you and a friend are pushing on a chest ofdrawers, as shown in Figure 11. If you push on opposite sides,but with the same force, the chest doesn’t move. It’s as if noforce were acting on the chest. The two equal forces acting inopposite directions have canceled each other out. Forces thatcancel each other are called balanced forces. If the forces actingon something are balanced, its motion doesn’t change.

SECTION 2 Forces 111

If the forces are oppositeand equal, the chest won’tmove. If they are opposite but

unequal, the chest will move in thedirection of the stronger force.

If the forces are inthe same direction, theforces add together.

Figure 11The motion of an object dependson the direction of the forcesapplied, as well as how strongthe forces are.

Figure 10The more mass a vehicle has, themore inertia it has. It will takemore force to start it moving.Which vehicle has more inertia?

Total force

Total force

Unbalanced Forces If both of you push on the same side ofthe chest, it moves faster than if only one of you pushed. Theforces exerted by each of you were in the same direction. Theseforces added together to form the net force. If you push on oneside of the chest and your friend pushes on the other side, howwill the chest move? If you push harder, the chest will move in the

direction you are pushing. However, the chestwill move more slowly than if you were pushingalone. When the two forces are in oppositedirections, the net force is the difference betweenthe forces. Unbalanced forces do not cancel eachother. If more than one force acts on an object,its motion will change only if the forces actingon it are unbalanced.

What is the differencebetween balanced andunbalanced forces?

FrictionRub your hand on the top of your desk. You can feel a force

on your hand, which slows it. This force is friction, which resistsmotion between two touching surfaces. The rougher the sur-faces are, the greater the friction is. Friction can be reduced bymaking the surfaces smoother. This is sometimes done by apply-ing oil or grease to the surfaces.

Friction is present in almost all motion.Figure 12 shows some examples of friction. When a hockey

puck slides across the ice, friction between thepuck and the ice slows the puck down andmakes it stop. When you ride on a skateboard,friction between your feet and the board keepsyou from sliding off when you start and stop.When you ride a bike, friction keeps the wheelsfrom slipping on the ground.

Air Resistance Place your hand in the airrushing past a moving car and you can feel aforce pushing on it. This force is air resistance,which is a type of friction. Air resistance slowsobjects moving through the air. For example,air resistance on an open parachute slows afalling skydiver.


Figure 12Friction occurs between anytwo surfaces that slide pasteach other.

Friction slows the puck and brings it to a stop.

Friction between your shoes and the skateboardkeeps you from sliding off as you slow down.

Friction between the tire and the ground helpswheels turn without slipping.

Motionof puck

Friction betweenpuck and ice

Motion

Friction

Motionof bike

Friction betweentire and ground

GravityIf you hold a tennis ball and then let it go, the tennis ball falls

to the ground. The motion of the tennis ball changed after youlet it go, so a force was acting on it. The force pulling it down tothe ground is gravity. Gravity is the pull that all matter exerts onother matter. When you dropped the tennis ball, Earth pulledthe tennis ball downward.

Earth and the tennis ball exerted a gravitational pull on eachother, even though they weren’t touching. However, the force ofgravity between two objects becomes weaker as the objects getfarther apart. Also, the gravitational force is weaker if the mass ofthe objects is less. For example, gravitational pull exists betweenyou and the chair across the room. However, the mass of the chairis much less than Earth’s mass so the gravitational pull betweenyou and the chair is so small that you don’t even notice it.

Mass and Weight Do you know how much you weigh? Isyour weight the same thing as your mass? The answer is no.Weight and mass are different. Weight is the gravitational forcebetween an object and the planet or moon where the object is.On Earth, your weight is the strength of the gravitational pullon you due to Earth’s gravity. When you stand on a scale, youare measuring the pull of Earth’s gravity. Because weight is aforce, it is measured in newtons. Recall that mass is the amountof matter in an object. Mass is measured in kilograms. Howwould your weight change if you were far from Earth?

SECTION 2 Forces 113

Section Assessment

1. What is a force?

2. As you sit, the force of gravity is pullingyou toward the ground. Are the forces acting on you balanced or unbalanced? Explain.

3. When you stand on a slope, why don’t you slide?

4. Explain the differences between inertia,mass, and weight.

5. Think Critically A skater pushes off with one foot and then glides in a circle.Is a net force acting on the skater? How can you tell?

6. Forming Operational Definitions You standon a rickety box. If it is not strong enough tohold you, it collapses. Use this example to formoperational definitions of the terms balancedand unbalanced forces. For more help, refer to the Science Skill Handbook.

7. Communicating In your Science Journal,describe all the forces that act on a ball whenyou toss it into the air and then catch it.When are forces balanced? When are theyunbalanced? For more help, refer to the Science Skill Handbook.

Research Visit the Glencoe Science Web site attx.science.glencoe.comfor more information abouthow gravity acts betweenastronomical objects such asthe sun and planets. Com-municate to your class whatyou learn.

http://tx.science.glencoe.com

Based on your results, label each toy accord-ing to its behavior; for example, fastest toy,or curviest path. For more help, refer to theScience Skill Handbook.

Toys in Motion


2. Make a coupleof test runswith the toy.Choose appro-priate starting and ending points on the floor.Mark and measure the distance betweenthem. Let the toy go, and time how long ittakes to move from start to finish. Record thedistance and time in your data table.

3. Repeat step 2 for four more toys.

4. With a calculator, calculate the speeds of thedifferent toys and record them in your datatable.

5. Create a bar graph showing the speeds of thetoys. Put the names of the toys on the x-axisand the speeds on the y-axis.

Conclude and Apply1. Does a relationship exist between the size of

the toy and its speed? Explain. Compare youranswers to your classmates’answers. Didthey find the same relationship? Why or why not?

2. What forces made the different toys slowdown or stop moving?

3. Is your speed calculation the same whetheryou measure distance traveled or displace-ment? Which should you measure?

A ll around you are examples of movingobjects. To determine an object’s speed, two

things must be measured.One is the distance theobject moved.The other is the time it took tomove that distance.

What You’ll InvestigateHow can you calculate the speed of moving toys?

Materialsmeterstickstopwatch*clock with a second handtoys that move—spring-wound,

battery-operated, or push toysgraph papercalculator*Alternate materials

Goals■ Demonstrate that changes in motion can

be measured and represented graphically.■ Measure distance and time using SI units.■ Calculate and graph speeds.

Safety Precautions

Procedure1. On your paper, or with a computer, prepare a

data table like the one shown below.

Toy Total Time (s) Speed Distance (m) (m/s)

Toy Speeds

Newton’s Laws of MotionWhat kinds of things have you seen in motion today? Maybe

you saw cars starting, stopping, and turning on busy streets. Per-haps you saw airplanes flying beneath clouds that seemed todrift along. Maybe you watched the wind scramble a pile ofleaves or saw the Moon rise. Cars, clouds, leaves, wind, and theMoon—all these things seem so different. However, theirmotion can be explained using only three rules.

These three rules are known as Newton’s laws of motion. Thelaws were presented together for the first time by Isaac Newton in1687. They explain how forces can change the motion of anobject. These rules apply to all objects on Earth and in space. Thesame laws that describe the motion of a skateboard or the curvedpath of a batted ball also can predict the motion of the planets.

The First Law of MotionWhen your friend holds the football on the tee and you give it

a kick, the ball is put into motion as your foot hits it. In a basketballgame, a teammate throws the ball to you, but you can’t reach it,and the ball sails out of bounds. In both cases the motion of theballs can be explained by Newton’s first law of motion. Accordingto the first law of motion, an object will remain at rest, or keepmoving in a straight line with constantspeed, unless an unbalanced force acts on it.

Suppose you take a shot on the basket-ball court. After the ball leaves your hand, itdoesn’t travel in a straight line. Instead, itspath curves downward and goes throughthe hoop, as shown in Figure 13. Why did-n’t the ball obey the first law of motion andtravel in a straight line with constant speed?The force of gravity pulled the ball down-ward. Objects that are rolling or sliding areslowed by the force of friction. Here onEarth, gravity and friction keep objectsfrom moving in a straight line with con-stant speed.

■ Analyze motion using Newton’slaws.

■ Calculate acceleration using force.

Vocabularyfirst law of motionsecond law of motionthird law of motion

Newton’s three laws will help youunderstand motion.

The Laws of MotionS E C T I O N

Figure 13As a result of gravity, the balldoesn’t move in a straight linewith constant speed. Withoutgravity, the ball would follow thepath shown by the dashed line.

Force ofgravity

Path of ballwithout gravity

Path of ball

115

Understanding Motion It took along time to understand motion. Onereason was the people did not under-stand the behavior of friction, or thatfriction is a force. Because frictioncauses moving objects to stop, peoplethought the natural state of an objectwas to be at rest. For an object to be inmotion, something had to be continu-ously pushing or pulling it. As soon asthe force stopped, nature would bringthe object to rest.

The sixteenth-century Italian sci-entist Galileo was one of the first tounderstand that an object in constant

motion is as natural as an object at rest. It was usually the forceof friction that made moving objects come to a stop. To keep anobject moving, a force had to be applied to overcome the effectsof friction. If friction could be removed, once an object was in motion, it would continue to move in a straight line withconstant speed.

Newton’s first law means that an object can speed up, slowdown or change direction only if a force acts on it as shown inFigure 14. Only forces can cause changes in motion.

The Second Law of MotionHow do forces cause motion to change? Suppose you pick

up your backpack when it’s full of books. As you lift it, youchange its motion. The backpack was at rest, and you caused itto move when you pulled it upward. Recall that anytime themotion of something changes, it is accelerating. The backpackwent from being at rest to moving. When you pulled the back-pack upward, you caused it to accelerate. In other words, theacceleration of the backpack was caused by the force you exertedon it. Newton’s second law of motion says that an unbalancedforce acting on an object causes the object to accelerate in thedirection of the force.

Force, Mass, and Acceleration According to the secondlaw of motion, acceleration can be calculated by dividing theunbalanced force exerted on an object by the mass of the object.This relationship between force, mass, and acceleration can bewritten as this equation:

a � F/m


Figure 14The ball was moving downwarduntil it struck the floor andchanged direction. According toNewton’s first law of motion, thefloor exerted a force on the ball.

The May 18, 1980 eruptionof Mount Saint Helens inWashington included a lat-eral blast of volcanic mate-rial. Research the eventsthat led to this famousexplosion of ash andsteam. Identify the forcebehind the eruption andthe direction of the forcethat propelled ash and gas.

Calculating Acceleration In this equation, a stands for theacceleration, F is the unbalanced force, and m is the mass of theobject. Recall that force, mass, and acceleration have units. Forceis measured in newtons (N), acceleration is measured in units ofmeters per second squared (m/s2), and mass has units of kilo-grams (kg). The newton is a unit equal to the combined units kg • m/s2.

You can calculate the acceleration of an object when youknow the force exerted on the object and the mass of the object.For example, suppose you exert a force of 10 N on a basketball.The basketball has a mass of 1.0 kg. The acceleration can be cal-culated as shown below:

a � F/m

� 10 N/1.0 kg

� 10 m/s2

Math Skills Activity

Example ProblemYou have to push your 60-kg basketball hoop off

of the driveway. The unbalanced force on the hoop due to your push and friction is 6 N. Find the acceleration.

This is what you know:force: F � 6 Nmass: m � 60 kg

This is what you need to know:acceleration: a

This is the equation you need to use:a � F/m

Substitute the known values:a � (6 N) / (60 kg) � 1 m/s2

Check your answer by multiplying by the mass. Do you calculate the same forcethat was given?

Acceleration, Force and Mass

For more help, refer to the Math Skill Handbook.

Practice Problems

You are pushing a 3-kg lawn chair across the patio. You exert a force of 12 N. Findthe acceleration.

SECTION 3 The Laws of Motion 117

MATH TEKS 6.2 C; 6.8 B

How does acceleration affect howyou feel on a roller coaster? Tofind out more about accelerationand amusement park rides, seethe Amusement Park RidesField Guide at the back of thebook.

Acceleration and Direction According to the second lawof motion, when a force acts on an object, its acceleration is inthe same direction as the force. If you pull on a wagon that is atrest, the wagon starts moving in the same direction as your pull.When a tennis player serves the ball, the ball changes directionand moves in the direction of the force exerted by the racket, asshown in Figure 15.

What if a soccer ball comes rolling toward you and you putout your foot to stop it? The force of your foot was opposite tothe motion of the ball, so it slowed to a stop. The ball has accel-eration, but it is in a direction opposite to its motion.

Acceleration and Force Acceleration is related to change inmotion. The greater the acceleration is, the faster the motion of anobject changes. Look at the equation for acceleration. What hap-pens to the acceleration of a 1-kg basketball if the force is doubledfrom 10 N to 20 N? Then the acceleration is doubled to 20 m/s2. Ifthe force exerted on the object becomes larger, the accelerationbecomes larger. How does the speed of a baseball change if you hitit hard or if you just nudge it with the bat? The ball moves fasterwhen you hit it hard. According to the second law of motion, alarger force on an object causes a larger acceleration.

How are acceleration and force related?


Figure 15A tennis player’sserve changesthe motion ofthe tennis ball.

The motion of the ball changes directionwhen the racket exerts a force on it. What otherforce is acting on the ball while it is in the air?

Collect Data Visit the Glencoe Science Web site attx.science.glencoe.comfor data about accelerationsfor different vehicles.Communicate to your classwhat you learn.

The ball moves in a straightline when tossed upward.


Acceleration and Mass How does acceleration depend onthe mass of an object? In the previous example, what happens tothe acceleration if the ball is a softball instead of a baseball? Asoftball has a greater mass than a baseball. If the force that is usedto hit the ball remains the same, the acceleration is less for themore massive softball. Likewise, if your backpack is full of books,you have to pull hard to lift it off your desk. If it’s empty, the sameforce causes it to move quickly. When the backpack is full ofbooks, its mass is larger. If the same amount of force is exerted,the object with the smaller mass has a larger acceleration.

The Third Law of MotionHow high can you jump? Think about the forces acting on

you when you jump. Gravity is pulling you downward, so anupward force must be exerted on you that is greater than theforce of gravity. Where does this force come from? Maybe youthink it comes from your legs and feet, pushing you upward.You’re partly right.

Look at Figure 16. Your legs and feet push downward on theground. In response, the ground pushes upward on you. This isthe force that enables you to leave Earth for an instant. Newton’sthird law of motion describes how objects exert forces on eachother. According to the third law of motion, when a force isapplied on an object, an equal force is applied by the object inthe opposite direction. When you pushed down on the ground,the ground pushed back up on you.


The forces that move thecontinental plates are large.The motion produced, how-ever, is small because themasses are so large.Research how fast Earth’splates are moving andwrite a paragraph in yourScience Journal describingwhat you find.

Figure 16When you jump, you are usingthe third law of motion. Youexert a force on the ground, andthe ground exerts an equal forceon you.

Force exertedby ground Force exerted on ground

Force Pairs The forces exerted on an object and by the objectare called a force pair. The force pairs act in opposite directionsand are always equal in size. Therefore, to jump higher, youmust push harder on the ground. Then the ground pushes backharder on you.

You might think that if force pairs are equal in size and inopposite directions, they must cancel out. But remember thatforce pairs act on different objects. When you push on theground, you exert a force on Earth, but because Earth has somuch mass, your force has no noticeable effect on its motion.Your mass is much smaller than that of Earth, so the same sizeforce causes you to spring into the air. Only if equal and oppo-site forces act on the same object do they cancel out.

Action and Reaction According to the third law of motionforces always act in pairs. When you push on a wall, the wallpushes back on you. One force of the force pair is called theaction force, and the other is called the reaction force. Your pushon the wall is the action force, and the wall pushing back on youis the reaction force. For every action force, there is a reactionforce that is equal in size but opposite in direction. Every timeyou push on an object, the object pushes back on you. Everytime you pull on an object, the object pulls back on you.

Examples of Action and Reaction If you swing a ham-mer and hit a nail, you know what happens. The hammer wasmoving before it hit the nail but stopped moving after it hit thenail. The change in motion of the hammer indicates that a forceacted on it. The hammer exerted a force on the nail. At the sametime, the nail exerted a force on the hammer, which caused it tostop moving. The action force was the force the hammer exerted

on the nail. The reactionforce was the force the nailexerted on the hammer.

Sometimes the action andreaction forces are hard toidentify. In Figure 17, whenthe swimmer pushes hishands through the water, heexerts a force on the water.Even though the water isn’t asolid, such as a wall, thewater still exerts a reactionforce on the swimmer. This isthe force that causes theswimmer to move forward.

Figure 17The swimmer exerts a force onthe water, and the water exerts areaction force on the swimmer.

Demonstrating theThird Law of MotionProcedure1. Blow up a balloon to maxi-

mum size. Hold its end toprevent air from escaping.

2. Tape the center of the balloon to a small, toy carso the opening of the balloon points backward.

3. Set the car on the floor and release the air in theballoon.

Analyze1. Describe how the toy car

moved when you let go ofthe balloon.

2. How does Newton’s thirdlaw of motion explain whatyou observe?


Force exerted by swimmeron water

Force exerted by wateron swimmer

Applications of Newton’sLaws of Motion

Newton’s ideas about motion have beentested many times and have been found toapply to objects everywhere. This is whythey are called laws. They apply in outerspace and to large objects like planets as wellas they do to small objects here on Earth.You experience examples of Newton’s lawsevery day. When you walk, your feet push onthe ground. According to the third law ofmotion, the ground then pushes on you, andyou move forward, as shown in Figure 18.When you bang your shin on a piece of fur-niture, it hurts. You pushed on the furniturewith your shin, and the furniture pushedback on your shin. Can you see other exam-ples of Newton’s laws of motion in youreveryday life?

Under what circumstances do the laws ofmotion apply?


Section Assessment

1. The forces acting on an object are balanced.What do Newton’s first and second laws sayabout the motion of the object?

2. Suppose you are sitting still in a chair.According to Newton’s second law ofmotion,are the forces acting on you bal-anced or unbalanced? Explain.

3. Compare the acceleration of a 5-kg objectacted on by forces of 1 N and 2 N.

4. When you jump upward, you slow downand eventually fall back to Earth. Whichlaw of motion explains this?

5. Think Critically Use Newton’s laws ofmotion to explain why a large predatormight have trouble pursuing a smaller prey animal that changes direction quicklyduring the chase.

6. Comparing and Contrasting Compare and contrast Newton’s second law of motionwith his third law of motion. Use at least two examples of objects in motion from everyday experience to demonstrate the similarities and differences in how the two laws describe motion. Build or display a model to show one of your examples.For more help, refer to the Science Skill Handbook.

7. Solving One-Step Equations A studentstanding on an icy parking lot throws a ball with a force of 20 N. If friction is ignored,and the student has a mass of 60 kg, what is the student’s acceleration? For more help,refer to the Math Skill Handbook.

Figure 18When you walk, the force exertedon you by the ground causes youto move forward.

Force exertedby ground on person

Force exerted by person on ground


Test Your Hypothesis

Analyze Your Data

Draw Conclusions

3. Find out the space shuttle’s averagespeed for each of the missions.To do this, divide the distance (in kilometers) by the time in hours.This will give you the average speedfor the mission in km/h.

Do1. Make sure your teacher approves

your plan before you start.

2. Go to the Glencoe Science Web siteat tx.science.glencoe.com topost your data.

Plan1. In order to compare average

speed data for five space shuttlemissions, find out the total distance traveled and the time duration for each mission.

2. Review the time duration data.Make sure that the data representsthe number of hours of the mission for each mission. You will need to convert valuesexpressed in days, minutes, andseconds to hours in order to calcu-late the average speed.

3. Which of the missions had the slowest average speed?

4. What is the difference between thefastest average speed and the slowest average speed?

1. For each shuttle mission you investi-gated, determine the average speedfor that mission.

2. Which of the missions had thefastest average speed?

4. Calculate the range in the averagespeeds by subtracting the smallestaverage speed from the largestaverage speed. What factors couldmake the average speeds different?

1. Find this Use the Internet activity on the Glencoe Science Web site attx.science.glencoe.com. Postyour data in the table provided.Study other students’ data and compare them to your data.Review data that other students entered.

2. Does the total mission time affectthe average speed? Why or why not?

3. What factors affect the averagespeed for each mission?

ACTIVITY 123

Once you’ve compared data with others onthe web site, communicate to your class as awhole—especially any major differences.Discuss those differences.



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Science & Language ArtsandandandScience Language Arts

Respond to the ReadingRespond to the Reading

1. If the horse and Rayona are similar toa “wind-up toy thathas been turned onetwist too many,”what kind of motioncan the reader expectthem to have?

2. To what animal doesRayona compare herself?

3. Where does the storytake place?

The sounds of the rodeo around me fade in my con-

centration. There’s a drone in my ears that blocks out

everything else, pasts and futures and long-range

worries. The horse and I are held in a vise1, a wind-up toy

that has been turned one twist too many, a spring coiled

beyond its limit.

“Now!” I cry, aloud or to myself I don’t know. Every-

thing has boiled down to this instant. There’s nothing in the

world except in the hand of the gate judge, lowering in slow

motion to the catch that contains us. . . .

Wheeling and spinning, tilting and beating, my breath

the song, the horse the dance. Time is gone. All the ordinary

ways of things, the gettings from here to there, the one and

twos, forgot. The crowd is color, the whirl of a spun top. The

noises blend into a waving band that flies

around us like a ribbon on a string. Beneath

me four feet dance, pounding and leaping and

turning and stomping. My legs flap like wings.

I sail above, first to one side, then the other,

remembering more than feeling the slaps of

our bodies together. Things happen faster

than understanding, faster than ideas. I’m a

bird coasting, shot free into the music, spiral-

ing into a place without bones or weight. I’m

on the ground. Unmoving. The heels of my

hands sunk in the dust of the arena....1 a device used to hold something tightly in place

“Rayona’s Ride”from the novel A Yellow Raft in Blue Water

by Michael Dorris


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Write a Story Write the climaxof your own coming-of-age story.Write about a time in your lifethat represents the momentwhen you went from being ayoung person to a young adult.

andandandLinking Science Linking Science

Nadia Roberts is a lecturer and engineering instructor at theNational Test Pilot School in Mojave, California. There, she teachescourses for test pilots and flight test engineers. A test pilot or flightengineer must make sure that airplanes can safely do what they weredesigned to do. They also study the interaction between humans andaircraft. This means that the test pilot spends a lot of time flying newor updated airplanes. If something does not work the way it should,the test pilot must determine the problem.

What might pilots use as reference points?

To learn more about careers in aeronautics, visitthe Glencoe Science Web site at tx.science.glencoe.com.

Test Pilot and Flight Test Engineer�

CareerCareer ConnectionConnection

Understanding LiteratureComing-of-Age Story “Rayona’s Ride” is a chapter inthe book A Yellow Raft in Blue Water, which is a com-ing-of-age story. A coming-of-age story describes ayoung person’s growing up and maturing. The passageyou just read is a turning point or climax for Rayona,the 15-year-old narrator. A climax is the point in thestory with the greatest interest or suspense. The climaxsignals the moment of Rayona’s growing up. What signsin the passage show that this is an important moment?

Science Connection Reference points are needed todescribe the motion of objects. In Rayona’s Ride, Ray-ona’s motion on her horse is described in relation todifferent reference points throughout the ride. As theride begins, Rayona uses herself as the reference pointand sees the crowd as moving like a ribbon on a stringaround her. At the end of her ride, Rayona uses Earth as a reference point to describe her motion when she spirals through the air and lands motionlesson the ground. The reference points during Rayona’sride change depending on her movement on or off the horse.

Coming-of-Age Story

SCIENCE AND LANGUAGE ARTS 125


126 CHAPTER STUDY GUIDE

3. Balanced forces cancel each other. Whenforces are unbalanced, a change in motionwill occur.

4. Friction is the force that resists motionbetween two surfaces that are in contact.Gravity is a force that acts between allobjects that have mass.

Section 3 The Laws of Motion1. Newton’s first law of motion states that an

object will remain at rest or move at con-stant speeduntil anunbalancedforce acts onit. How doesNewton’s firstlaw explainthe motion ofthis baseball?

2. Newton’s second law of motion states thatan object acted on by an unbalanced forcewill accelerate in the direction of the forceaccording to the following formula.

acceleration �

3. Newton’s third law of motion states thatobjects exert forces on each other that areequal, but in opposite directions.

force�mass

Section 1 Describing Motion1. Motion is

described rela-tive to a refer-ence point.What referencepoints couldyou use todescribe themotion of the lead goose?

2. Motion can be measured in two ways. Dis-tance is the total path traveled. Displace-ment includes the distance and directionfrom the starting point.

3. Speed measures how fast you cover a givendistance.

average speed �

Velocity includes speed and direction.

4. An object that accelerates speeds up, slowsdown, or turns.

Section 2 Forces

1. A force is a push or a pull. It has a size and adirection.An object acted on by an unbalancedforce will change its motion and accelerate.

2. The tendency of an object to resist changingits motion iscalled inertia.Inertia dependson the amountof mass anobject has.Which dumbbellin the photo hasthe most inertia?

total distance��

total time

On the front of the tabs ofyour Cause and EffectStudy Fold describe how

the size and direction of a force affect motion.

After You ReadFOLDABLESReading & StudySkills

FOLDABLESReading &Study Skills

Study GuideChapter XXXX Study GuideChapter 44

CHAPTER STUDY GUIDE 127

Vocabulary Wordsa. acceleration h. inertiab. balanced forces i. second law ofc. displacement motiond. first law of motion j. speede. force k. third law off. friction motiong. gravity l. velocity

Using VocabularyGive the vocabulary word that answers each

question.

1. What can be stated as “for every action,there is an equal and opposite reaction”?

2. What is a measure of speed and direction?

3. What acts on a motionless object?

4. What describes how an object speeds up,slows down, or turns?

5. What force resists motion between two surfaces?

6. What property explains why a heavy objectis harder to move than a light one?

7. What causes motion to change?

Make flashcards for new vocabulary words. Putthe word on one side and the definition on theother. Use them to quiz yourself.

Study Tip

Complete the following concept map about motion.

Study GuideChapter XXXX Study GuideChapter 44

effects on objects

effect on motion

effect on motion

forces betweenobjects

change

describedby

describedby

describedby

1st law

Newton’s laws of motion

velocity acceleration

Forces

Choose the word or phrase that best answersthe question.

1. A push is an example of what?A) acceleration C) displacementB) velocity D) force

2. What force slows a sliding box?A) acceleration C) velocityB) friction D) inertia

3. Which of these is not a unit for speed?A) m/s C) cm/s2

B) km/h D) m/day

4. Which of the following would NOT includea direction?A) force C) velocityB) acceleration D) distance

5. What causes a dropped coin to fall?A) inertia C) gravityB) velocity D) friction

6. What is an object’s resistance to changingits motion called?A) forceB) gravityC) accelerationD) inertia

7. An unbalanced force to the left acceleratesan object in what direction?A) rightB) leftC) forwardD) depends on initial velocity of object

8. A 10-N force is exerted on an object with amass of 2 kg. What is the acceleration of theobject?A) 20 m/s2

B) 10 m/s2

C) 5 m/s2

D) depends on the direction of the force

9. You are riding a bike. Which of the follow-ing is an example of balanced forces?A) You pedal to speed up.B) You turn at constant speed.C) You coast to slow down.D) You pedal at constant velocity.

10. You push against a stationary wall with aforce of 20 N forward. What is the force thewall exerts on you?A) 20 N backward C) 10 N backwardB) 20 N forward D) 10 N forward

11. A baseball is pitched east at 40 km/h. A bat-ter hits it so it moves west at 40 km/h. Didthe ball accelerate? Explain.

12. Maureen walked 3 km east in 0.75 h. It took0.3 h to jog back to her starting point. Whatwas her velocity in each direction?

13. A 200-N net force is applied to a 40-kgobject and a 10-kg object. Which one accel-erates more? By how much? Explain.

14. A 20-N force is applied to a 5-kg object. Itdoes not change its motion. What couldexplain this?

15. A box with a weight of 500 N is placed on atable that can support 450 N. Are the forcesbalanced? What happens?

128 CHAPTER ASSESSMENT

Assessment & ReviewChapter 1515 Assessment & ReviewChapter 44

200 N

200 N10 K

40 K

CHAPTER ASSESSMENT 129

16. Comparing and Contrasting Compare andcontrast displacement and distance.

17. Making and Using Graphs Make a distance-time graph for a person walking at 1.5 m/sfor 10 s.

18. Identifying and Manipulating Variables andControls You use a spring scale to apply thesame force to pull 1-kg, 2-kg, and 4-kg objects.What remains constant? What is the variable?

19. Making Models Use spring scales todemonstrate Newton’s third law of motion.

20. Making and Using Graphs Analyze thegraph below. What is the speed between 20 sand 40 s? What is the acceleration between20 s and 40 s?

21. Oral Presentation Research the science ofmotion used by manufacturers of sportsequipment. What variables affect themotion? Present your findings in a speechto your class.

0 2010 30 40 50 60

Dis

tanc

e (m

)

0

100

6080

2040

Time (s)

Go to the Glencoe Science Web site at tx.science.glencoe.com or use theGlencoe Science CD-ROM for additionalchapter assessment.

TECHNOLOGY

AssessmentChapter 1515

A map is a tool that can be used to findyour way or to trace a route that you havetaken. The map below shows the pathway a sixth-grade student took as she walkedfrom her home to a friend’s house afterschool one afternoon. Study the map tofind out about the total distance the stu-dent walked and to find out about her displacement. TEKS 6.6 B

Study the illustration and answer thefollowing questions.

1. According to the map, what is the totaldistance traveled by the student?A) 0.5 km C) 1.5 km B) 1.0 km D) 2.5 km

2. If the whole walk took the student 1 h,what was her average speed?F) 1.0 km/h H) 2.5 km/hG) 1.5 km/h J) 3.5 km/h

3. If the student walks back home later,what is her total displacement?A) 0.0 km C) 1.5 kmB) 1.0 km D) 2.5 km

AssessmentChapter 44

1.0 km

0.5 km

0.5 km

TAKS Practice


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Chapter 4: Forces and Motion - Don Voorhees