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Force and Motion How is it moving? So…what is motion?

So…what is motion? So…what is motion? In math terms…

• A “change in position over time” is the same as saying:

How can we describe motion?• Motion can be described by:

– DISTANCE (how far did it travel?)

– TIME (how long did it travel?)

– SPEED (how fast did it travel?)

– DIRECTION (which way did it go?)

– ACCELERATION (does the motion change?)

What does “speed” mean?

• Some examples of speed:

60 miles/hour 100 meters/minute

• Let’s break it down…

If you travel 60 miles per hour, how far do you travel in 1 hour?

• 60 miles/hour is the same as 60 miles

1 hour

Let’s practice calculating speed

• If you travel 100 km in 2 hours, what is your speed?

• Speed = Distance

Time

• Distance = 100 km Time = 2 hours

• Speed = 100 km

2 h

• Speed = 50 km/h

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Try it out!• Time yourself moving across the 5 meter

tracks on the floor.

• What is the DISTANCE?

• What is your TIME?

• Speed = Distance

Time

What is your SPEED?

Speed, Distance & Time

Speed = Distance

Time

Time = Distance

Speed

Distance = Speed x Time

S

D

T

Speed Practice Problems

A family takes a car trip heading northeast from Durham, NC to Washington, DC. They travel for 4 hours and cover 360 km.

What was their average speed?

Speed = Distance

Time= 360 km

4 h

= 90 km/h

Speed vs. Velocity

SPEED – tells you have fast or slow something is moving (changing position).

Example = 25 km/h

VELOCITY – tells you speed AND DIRECTION! (changing position in a certain direction)

Example = 25 km/h EAST

Speed Practice Problems

A family takes a car trip heading northeast from Durham, NC to Washington, DC. Their speed was 90 km/h. What was their VELOCITY?

Velocity is SPEED and DIRECTION!

Velocity = 90 km/h NORTHEAST

Speed Practice Problems

After school, your teacher went for a jog along Cornwallis Rd. She ran for 30 minutes at a speed of 150 m/min. How far did she run?

Distance = Speed x Time

Distance = 150 m/min x 30 min

Distance = 4500 meters (4.5 km)

S

D

T

Speed Practice Problems

Mr. Sawyer goes on a long bike ride in the country. He rides his bike 35 km at a speed of 20 km/h. For how long was he riding his bike?

Time = Distance

Speed

Time = 35 km

20 km/h

S

D

T

= 1.75 h

Acceleration Practice Problems

Ms. Litwak buys a new car that can accelerate from rest (0 m/s) to 24 m/s in 8 seconds. What is the car’s rate of acceleration?

Acceleration = Final speed – Initial SpeedTime

Acceleration = 24 m/s – 0 m/s8 s

= 3 m/s2

What was the average speed of the runner in the first 5

seconds of the race?

Speed = 25 m5 s

Speed = 5 m/s

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A snail slowly slithers down the sidewalk. It travels at a speed of 3 cm/min. How far would it travel in 20 minutes?

Distance = Speed x Time

Distance = 3 cm/min x 20 min

Distance = 60 cm

The fastest man on earth, Usain Bolt, runs at a speed of 10 m/s. How long would it take him to run 160 meters?

Time = Distance

Speed

Time = 160 m

10 m/s

Time = 16 seconds!

Acceleration

Acceleration

• Acceleration is a CHANGE in motion

• An object is accelerating if it is:

– Speeding up

– Slowing down

– Changing direction

Using an Accelerometer

• An accelerometer measures acceleration.

• CAREFULLY push your accelerometer so it slides across the table and comes to rest.

• How does the paper clip show acceleration?

• In your notebook, complete the chart for each situation.

Taking a ride on the school bus…

• YOU act like an accelerometer when you ride the school bus.

• What happens to you,

when the bus:– Speeds up suddenly

– Turns a corner

– Stops quickly at a red light

– Rides along at

a steady speed

Taking a ride on the school bus…

• When your MOTION CHANGES, that’s a sign that the bus is ACCELERATING.

Is it accelerating?

• Your car speeds up when the light turns green.

• A racecar goes at a constant speed around a curved track.

• A toy car moves in a straight line across the room at a steady speed of 0.5 m/s.

• A roller coaster car slows down as it climbs a hill.

Graphing Acceleration

• On a position-time graph, changes in speed are shown by curved lines.

Straight line – Not

accelerating!

Curving up –

Speeding up

Curving down –

Slowing down

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• Look carefully at each car

below.

• Decide which car matches

graphs A, B, and C.

Graph B – Constant Speed

Graph C – Accelerating Slowly

Graph A – Accelerating Quickly

Calculating Acceleration

• Acceleration is the change in speed per unit of time.

• Acceleration is measured in units of meters per second per second, or m/s2.

Acceleration = Final speed – Initial Speed

Time

Calculating Acceleration

• A car leaving traffic changed its speed from 10 m/s to 25 m/s in 7.5 seconds. What was its acceleration?

Acceleration = Final speed – Initial Speed

Time

Acceleration = 25 m/s – 10m/s

7.5 s= 2.0 m/s2

Introduction to

FORCES

FORCES

When you ride a bike, your foot PUSHES against the

pedal. The push makes the wheels of the bike move.

When you drop something, it is PULLED to the ground

by gravity.

A FORCE is a PUSH or PULL in a

particular DIRECTION.

Forces can affect motion in the following ways:

They can make objects:

i) START MOVING

ii) MOVE FASTER

iii) MOVE SLOWER

iv) STOP MOVING

v) CHANGE DIRECTION

vi) CHANGE SHAPE

FORCESFORCES AFFECT HOW OBJECTS MOVE.

BIG

SCIENCE

IDEA

FORCESIdentify each picture as a PUSH or a PULL. Is the

force causing a change in speed or direction or both?

FORCES

Since forces cause changes in SPEED

or DIRECTION of an object, we can say

that forces change VELOCITY, so….

Forces cause

ACCELERATION.

FORCES

More than one force can act on an object

at one time. What happens to the object

when forces act depends on 2 things:

1) Strength of the Forces

2) Direction of the Forces

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FORCES

When 2 or more forces act on an object,

the forces combine to form a net force.

or OPPOSE each other.

Forces may WORK TOGETHER

FORCESIf the forces cancel each other out, and do

not cause the object to move, the forces

are said to be BALANCED.

If the forces don’t cancel each other out – 1

force is stronger than the others – the forces

are UNBALANCED and will cause a

CHANGE IN MOTION.

MEASURING

FORCE

The strength of a force is

measured in NEWTONS.

The symbol is (N).

We use a SPRING SCALE

to measure force.

MEASURING

FORCE

- Always “zero” your balance

before use.

- Pull gently and with

constant force.

-Practice using your spring

scale to drag items across

your desk.

COMBINING

FORCES

Two forces in the same direction can add

together to produce a larger net force.

5 N

right

5 N

right

+ =

10 N

right

COMBINING

FORCESTwo forces in opposite directions can

subtract to produce a smaller net force in

the direction of the larger force.

5 N

right10 N

left

- =

5 N

left

COMBINING

FORCESTwo forces may cancel each other out (if

equal and opposite) to produce NO NET

FORCE.

5 N

right

- =

5 N

left

0 N

(No Net

Force)

Circle the best answer:

1) The forces shown above are PUSHING / PULLING forces.

2) The forces shown above are WORKING TOGETHER / OPPOSITE FORCES.

3) The forces shown above are EQUAL / NOT EQUAL.

4) The forces DO / DO NOT balance each other.

5) The net force is 1000 N TO THE RIGHT / 1000 N TO THE LEFT / ZERO.

6) There IS / IS NO motion.

Circle the best answer:

7) The forces shown are PULLING / PUSHING forces.

8) The forces shown are WORKING TOGETHER / OPPOSITE FORCES.

9) The forces shown are EQUAL / NOT EQUAL.

10) The forces DO / DO NOT balance each other.

11) The stronger force is pulling RIGHT / LEFT.

12) Motion is the to the RIGHT / LEFT.

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13) Two movers are trying to move a heavy box. One mover pushes to

the right with a force of 150 N. The other mover pushes to the left

with a force of 200 N.

a) Draw & label the forces on the diagram.

b) What is the net force? 50 N LEFT

c) Will the box move? YES

d) If yes, in what direction? LEFT

150 N 200 N

50 N NET FORCE 14) Two movers are trying to move a heavy chair. One mover PULLS to the left with a force of 200 N. The other mover PUSHES to the left with a force of 200 N.

a) Draw & label the forces on the diagram.

b) What is the net force?

400 N LEFT

c) Will the chair move?

YES

d) If yes, in what direction?

LEFT200 N 200 N

400 N NET FORCE

15) Four children are fighting over the

same toy. Mike is pulling North with

a 50 N force, Justin is pulling East

with a 40 N force, Chantal is pulling

South with a 50 N force, and Tykera

is pulling West a 30 N force.

a) Draw & label the forces on the

diagram.

b) Is there a net force on the toy?

YES = 10 N EAST

c) In which direction will the toy

move?

EAST

d) Who gets the toy?

JUSTIN

50

N

50

N

30 N 40 N

10 N

Net Force

MIKE

JUSTIN

CHANTAL

TYKERA

FRICTION

FRICTION

• What will happen when the ball is released?

• When the ball reaches the bottom of the slope, will it keep moving forever?

Since the ball stops, there must be a force acting to slow the ball down.

The force that slows the ball to a stop is FRICTION.

What is Friction?

• Friction is a force that two surfaces exert on each other when they rub against each other.

• The direction of the friction force is always OPPOSITE to the direction of the motion.

Friction Force

Direction of Motion

Types of Friction

• Static Friction opposes the motion of an object that is at rest

• To make the object move, you have to exert a force larger than the force of static friction.

Direction of Intended Movement

Static Friction Force

Types of Friction

• Sliding friction occurs when two solids slide over each other.

• Sliding friction makes car brakes work and stops athletes from slipping.

Direction of Slide Sliding Friction Force

Types of Friction

• Rolling friction occurs when an object rolls across a surface.

• Rolling friction is easier to overcome than sliding friction for the same materials.

Friction Force

Direction of Motion

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Types of Friction

• Fluid friction occurs when a solid object moves through a liquid or gas.

• Air resistance is a type of fluid friction.

Friction Force

Direction of Motion

Which type of friction is slowing down the object in each situation?

• You are slipping down a waterslide at Emerald Pointe.

• You use a lot of force to slide a desk across the floor.

• You’re riding a skateboard down the street and it slowly rolls to a stop.

• You try to push the couch, but can’t seem to move it. Friction Force

SLIDING FRICTION

ROLLING FRICTION

FLUID FRICTION

STATIC FRICTION

Friction Thought Questions• Why would you add oil to a rusty bike

chain?

• Why would you add sand to an icy driveway or road?

• Why is it easier to move heavy furniture using a handcart rather than pushing it?

• Why would a shoe company be interested in studying friction?

• What would happen if we repeated the tug-of-war and one team had only socks on?

Review of Friction Forces

Review - What is Friction?

• Friction is a force that two surfaces exert on each other when they rub against each other.

• The direction of the friction force is always OPPOSITE to the direction of the motion.

• It SLOWS down moving objects!

Friction Force

Direction of Motion

What factors affect Friction?

• In your homework, you found out 2 factors affect friction:

– Types of surfaces involved

– How hard the surfaces push together

• Today, we’re going to investigate DIFFERENT SURFACES to see which ones create the most friction.

CARPET SANDPAPER DESK

What factors affect Friction?

• In your homework, you found out 2 factors affect friction:

– Types of surfaces involved

– How hard the surfaces push together

• Today, we’re going to investigate DIFFERENT SURFACES to see which ones create the most friction.

CARPET SANDPAPER DESK

Conclusions Questions1. Which surface material created the

MOST frictional force?

2. Which surface material created the LEAST frictional force?

3. Give an example of a situation where we use a certain surface material to:

a) REDUCE friction

b) INCREASE friction

GRAVITY& AIR RESISTANCEThe physics of falling

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The Force of Gravity

• Gravity is the force that pulls all objects down to the earth.

• Rain falls from the sky down to earth…

• If you drop a book, it falls to the ground…

• If you trip, you’ll fall down…

• Actually in science, gravity is a force ofattraction that acts between ALL objects(the earth, you, the desk, a book)

• The force of gravity is much STRONGERfor LARGER objects (more mass).

Universal Gravitation Universal Gravitation• Because the Earth is by far, the largest

and closest object around, it has the greatest force of attraction...

• So, no matter where you are on earth, all things fall to the ground due to gravity…

What is “free fall”?

• When gravity is the ONLY forceacting on an object, it is in free fall.

• In that case, gravity is an UNBALANCED FORCE which causes the object to accelerate.

Acceleration due to Gravity

• Calculate the acceleration of an object in free fall.

A = Final speed – initial speed

time

A = 50 m/s – 0 m/s

5 s

Acceleration = 10 m/s2

Objects in Free Fall

• Do all objects fall at the same rate?

• If we dropped a bowling ball and a tennis ball from the same height, which would land first?

• Let’s try it!

Mass and Gravity Oct. 24, 2008

Question: How does mass affect the speed of a falling object?

Hypothesis: (What do you think will happen AND WHY?)

Observations & Data Collection:Repeat each trial twice and record your observations.

Ping pong ball vs. Wooden ball:

Wooden ball vs. Metal ball:

Ping pong ball vs. Metal ball:

Conclusion: (One sentence)

Objects in Free Fall

• Do all objects fall at the same rate?

• ALL objects in free fall travel at the same rate, regardless of mass!

• In free fall, heavy objects and light objects fall at the same rate!

So, which will land first?• WHY does the penny

land first?

• Remember the force that opposes motion (slows things down)? FRICTION!

• Falling objects experience friction with the air called AIR RESISTANCE that slows them down.

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Air Resistance

• The larger the object (more surface area), the more air resistance.

• That’s why parachutes work! The upward force of the air acting on the LARGE parachute slows you down as you fall.

Air Resistance

• Draw a diagram showing the forces…

• Downward force of gravity is same on both.

• Upward force of air resistance is greater on the feather.

• The net force (down) is greater on the

penny.

Air

Resistance

Gravity

Net Force on Feather

Net Force on Penny

Without air resistance, all objects would fall at the same rate…

Galileo Drops the Ball

Hammer and Feather Drop on the Moon

Gravity on the moon?• The force of gravity is much weaker on

the moon because…– It is much farther away from earth.

– The moon is much smaller than earth.

That’s why astronauts weigh less on the moon!

Gravity Review

• Gravity is the force that pulls all objects down to the earth.

• When gravity is the ONLY force acting, ALL objects accelerate at a rate of 10 m/s2.

• Mass doesn’t matter – in free fall, heavy objects and light objects fall at the same rate!

Air Resistance Review

• Some objects take longer to fall – they are slowed down by FRICTION with the air called AIR RESISTANCE.

• The larger the surface area, the greater the force of air resistance pushing up.

• Without air resistance, all objects would fall at the same rate…

• Draw a free body diagram of the sky diver and label ALL the forces.Gravity = 1000 N

Air Resistance = 800 N

• What is the net force?

Net Force

= 200 N

Air

Resistance

= 800 N

Gravity =

1000 N

Newton’s 1st Law

Newton’s 1st Law of Motion

• After the ball is kicked, what forces are acting on it while it rolls?

• What if we could remove those forces? What would happen then if we kicked the ball?

Friction

GRAVITY

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Newton’s 1st Law of Motion

• Newton’s 1st law of motion states:

An object at rest will remain at rest,

-and-

an object moving at a constant velocity will continue moving at a constant velocity,

-UNLESS-

it is acted upon by an unbalanced force.

Newton’s 1st Law of Motion

That’s because larger objects have more inertia (more resistance to a change in

their motion)!

Objects resist any change to their motion!

This resistance is called INERTIA.

Which one would be easier to push?

or…

BASICALLY…

Newton’s 1st Law of Motion

• Unfortunately, your bed really doesn’t make itself…

• And dirty clothes won’t pick themselves up off the floor!

Make your bed!Do the laundry!

• That’s because things at rest will stay at rest until an unbalanced force (like your arms lifting the sheets) acts on them.

Newton’s 1st Law of Motion

• Inside a moving vehicle, everything is moving at the same velocity – your body, the objects in the car, and the car itself.

• What would happen to the things inside the car if the car hit a wall?

• That’s because things moving at a constant velocity will stay at a constant velocity unless acted upon by an unbalanced force.

• The car was acted upon by an outside force – the force of the wall hitting the car.

• The person inside the car wasn’t hit by the force of the wall, so he kept moving at the same speed and in the same direction.

Newton’s 1st Law of Motion

• Explain this animation:

• The truck is stopped by the force of the impact with the car, but the ladder continues to move at its original speed and in its original direction because of inertia.

Newton’s 1st Law of Motion

• So, how did the tablecloth demo work?

• The cloth experienced a pulling force that caused it to start moving. The dishes did not have a direct force applied to them, so they remained in their places because of inertia.

Newton’s 1st Law of Motion

• Following the instructions on your sheet, do each of the activities at your desk.

• In your notebook, write the title of each activity and explain in at least two complete sentences how it works, using Newton’s 1st Law. Newton’s 2nd Law

Let’s review Newton’s 1st Law

• This law deals with situations where forces are balanced…

• When forces are balanced, objects resist any changes in their motion… that’s called INERTIA.

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Newton’s 1st Law of Motion

• If no one kicks the ball, what will happen?

• Objects at rest will remain at rest…

Newton’s 1st Law of Motion

• If there was NO FRICTION (no unbalanced force) to slow down the ball, what would happen?

• Objects in motion will remain in motion …

Newton’s 1st Law of Motion

• The ladder’s inertia keeps it moving forward after the unbalanced force (car) stops the truck…

Newton’s 2nd Law of Motion• Newton’s 2nd law of motion describes how

UNBALANCED FORCES and MASS affect the ACCELERATION of an object.

• Let’s try it!

Test 1 – Increasing Force• Small Force - Blow through the straw lightly

toward the marble. Observe the marble’s motion.

• Large Force – Blow the straw with more force toward the marble. Observe the marble’s motion.

How does increasing the force affect the marble’s acceleration?

Test 2 – Increasing Mass• Small Mass (marble) - Blow through the straw

as hard as you can. Observe the marble’s motion.

• Large Mass (golf ball) – Blow the straw as hard as you can. Observe the golf ball’s motion.

How does increasing the mass affect the the object’s acceleration?

So, how do unbalanced forces affect an object’s motion?

• How does a batter’s swing affect the acceleration of a baseball?

Sacrifice Bunt Homerun Hit

So, how do unbalanced forces affect an object’s motion?

• The harder you hit, the faster it goes!

• The greater the force, the greater the acceleration…

Sacrifice Bunt Homerun Hit

So, how does mass affect an object’s motion?

• Which shopping cart would move faster with a single push?

Empty Cart Full Cart

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So, how does mass affect an object’s motion?

• The fuller the cart, the slower it goes!

• The more mass, the less acceleration…

Empty Cart Full Cart

Newton’s 2nd Law of MotionIt states:

ACCELERATION depends on the object’s MASS, and the net FORCE acting on the object.

• We can also write it mathematically:

Force = Mass x Acceleration

Newton’s 2nd Law

Force = Mass x Acceleration

M F A

If you raise the mass but keep force the same,

acceleration will decrease.

If you lower the mass but keep force the same,

acceleration will increase.

If you want more acceleration with the same force, you must decrease

the mass.

If you want less acceleration with the same force, you must increase

the mass.

Newton’s 2nd Law

Force = Mass x Acceleration

If you want more acceleration with the same mass, you must increase

the force.

If you want less acceleration with the same mass, you must decrease

the force.

Newton’s 2nd Law

Force = Mass x Acceleration

If you raise the mass but want the same

acceleration, you must increase the force.

If you lower the mass but want the same

acceleration, you must decrease the force.

Newton’s 2nd Law of Motion

Force = Mass x AccelerationMr. Sawyer’s car ran out of gas. How much force does Mr. Sawyer need to push his 750kg car at an acceleration of 1 m/s2?

F = m x a

F = 750 kg x 1 m/s2

F = 750 N right

1 m/s2750 kg

Try one on your own…

Ms. Litwak’s van runs out of gas. How much force does she need to push the 2000kg van at an acceleration of 0.5 m/s2?

F = m x a

F = 2000 kg x 0.5 m/s2

F = 1000 N right

0.5 m/s2

Speed, Distance & Time

We can also write the formula like this:

Acceleration = Force

Mass

Mass = Force

Acceleration

Force = Mass x Acceleration

m

F

a

Newton’s 2nd Law of Motion Newton’s 2nd Law of Motion

Find the golf ball’s acceleration.

a) The putter hits the 0.05 kg golfball with a force of 1 N.

b) The driver hits the 0.05 kg golfball with a force of 8 N.

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Newton’s 2nd Law of Motion

• Use Newton’s 2nd law of motion to explain in words the difference in the motion of the golf balls.

Newton’s 3rd Law

Which forces are acting to get this guy up in the air?

His feet push DOWN on the ground.

• But wait… His downward push can’t be causing his upward motion.

Which forces are acting to get this guy up in the air?

His feet push DOWN on the ground.

• There must be a force pushing UP!• The force of the ground pushes him UP!

The ground pushes UP on the man.

Newton’s 3rd Law of Motion

• Newton’s 3rd law says that: For every action force, there is an equal and opposite reaction force.

Action Force: Man’s feet push DOWN on the ground.

Reaction Force: Ground pushes UP on the man.

ALL forces act in PAIRS!

Let’s Demonstrate…

• Stand up and face a partner with your palms touching.

• Push on your partners hands. Don’t move your feet. The first partner to step back loses…

• How can Newton’s third law explain what happens?

Newton’s 3rd Law of Motion

• Explain this animation using Newton’s third law.

Newton’s 3rd Law of Motion

• The man’s foot exerts a backward push on the boat (action force), while the boat exerts a forward push on the man (reaction force).

A PAIR OF FORCES:EQUAL FORCES, BUT

IN OPPOSITE DIRECTIONS

Let’s Demonstrate…

• Two people (the same size) in rolling chairs face each other with their feet touching.

• Only 1 student pushes. What will happen?

• How can Newton’s third law explain what happens?

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Identify the force pairs in each situation

•A person fires a rifle.

•Action Force – gun pushes the bullet out at high speed.

•Reaction Force – the bullet pushes back on the gun (recoil).

Identify the force pairs in each situation

•A space shuttle lifts off.

•Action Force – engine pushes gases down & out.

•Reaction Force – the gases push the rocket up.

*This upward force must be stronger than gravity pulling down on the rocket!

Identify the force pairs in each situation

•A person stands still.

•Action Force – gravity pulls the person down to the floor.

•Reaction Force – the floor pushes up on the person.

*You don’t need MOTION for force pairs. They are everywhere!

Let’s Demonstrate…

• The toy clackers work 2 different ways… (watch your teacher)….

• How can Newton’s third law explain what happens?

If forces are equal and in opposite directions, why don’t they cancel out (and balance)?

•Forces only cancel if they act on the same object.(Think about a tug of war –all forces act on the rope).

•These forces are acting on different objects!

These forces are acting on different objects!

•Action Force – Rocket engine pushing on gases.

•Reaction Force – Gases push on the rocket.

Think about it . . .

• Why does it hurt so much when you stub your toe?

• When your toe exerts a force on a table, the table exerts an equal force back on your toe.

• The harder you hit your toe against it, the more force the the table exerts back on your toe (and the more your toe hurts).

What is Momentum?• Momentum is a measure of how much motion

object has.

• It is affected by mass and velocity. The heavier an object is, the more momentum it has.

• It’s easier to stop soccer ball coming towards you at 20 m/s than a car coming at 20 m/s.

• It’s easier to stop car travelling at 1 km/h than a car travelling 60 km/h!

Conservation of Momentum• When objects collide, their total momentum is

conserved (stays the same), unless outside forces act.

• The total amount of motion coming into a collision will also come out of the collision.

1. Place 4 coins in a row, touching each other.

2. Place the 5th coin about 2 inches away from the end of the row, keeping it in line.

3. Lightly flick your finger forward, propelling the single coin against the others.

4.What do you observe?

5.Try it again, flicking 2 coins into a row of 3 coins. What do you observe?

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MomentumMomentum can be calculated using this formula:

Momentum = mass x velocity

A golf ball with a mass of 0.05 kg travels at 16 m/s.

A baseball with a mass of 0.15 kg travels at 7 m/s.

Which ball has the greater momentum?

Golf ball’s momentum = 0.05 kg x 16 m/s

Baseball’s momentum = 0.15 kg x 7 m/s

= 0.8 kg m/s

= 1.05 kg m/s

Work, Power & Simple Machines(Making work easier…phew!)

What is WORK?

If you put a lot of effort into doing something and are worn out at the end, you think you’ve done a lot of WORK, right?

Not necessarily….

If you haven’t exerted a force AND moved an object some distance, you haven’t done any WORK at all!

What is WORK?

In scientific terms, you do WORK when you exert a FORCE that causes an object to move some DISTANCE in the SAME DIRECTION of the force.

Examples:

– Pushing a lawn mower

– Lifting books out of your bag

– Pulling a suitcase on wheels

What 2 things must happenfor WORK to be done?

MOTION – The object must move.

If the object doesn’t move,

there is no work done.

FORCE & MOTION IN THE SAMEDIRECTION

Movement must be in the same direction as force.

If the motion is in a different direction than the force, there is no work.

Is WORK being done?

Pushing a car that’s stuck in snow.

NO! (No work because the car doesn’t move).

Lifting a baby out of his stroller.

YES! (Baby moves in same direction as you lift)

Carrying your bookbag to class.

NO! (Force is pulling up, but motion is sideways)

Pushing a lawn mower.

YES! (Mower goes in same direction as you push)

Calculating WORK

WORK = FORCE x DISTANCE

The greater the distance, the more you work.

Eg. Pushing a car 100 m vs.

Pushing a car 200 m (more work!)

The greater the force, the more you work.

Eg. Lifting 1 book onto a table vs.

Lifting 10 books onto a table (more work!)

Calculating WORK

You carry a baby that weighs 30 N upstairs to his room (3 meters above you). How much work is done?

WORK = FORCE X DISTANCE

WORK = 30 N x 3 meters

WORK = 90 N·m (90 J)

Work is measured in Joules (J)

1 Joule = 1 N·m

What is POWER?

Power is the rate at which work is being done (or how much work is being done in a unit of time).

POWER = WORK ÷ TIME More power means less time to do the

same work OR more work done in the same amount of time.

Power is measured in Watts (W).

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POWER

Think about 2 cars – one with a 200 horsepower engine, and one with a 500 horsepower engine.

Which one has the more powerful

engine?

Which one will go further in 10 minutes?

Which one will go 10 miles in the shortest amount of time?

Calculating POWER

A motor exerts a force of 2,000 N to lift an elevator 8.0 m in 4.0 seconds. What is the power of the motor?

Power = Work = Force x Distance

Time Time

Power = 2,000N x 8 m

4 s

Power = 4,000 J/s (4,000 Watts)

= 16,000 J

4 s

What are MACHINES?

Most people think of complex, automated, technical, or electronic gadgets with motors…, but

machines can be much simpler.

A machine is any device that lets you do WORK in an easier or more effective way.

How do Machines do work?

Machines make work easier by changing3 things about the FORCE you exert to do work:

AMOUNT of force you exert

DISTANCE over which you exert force

DIRECTION in which you exert force

How do Machines work?

In other words, a machine changes the strength, distance, directionof your push or pull.

What is the mechanical advantage of a machine?

A machine’s mechanical advantage is the number of times a machine

increases a force exerted on it.

Mechanical = Output ForceAdvantage Input Force

What is the mechanical advantage of a machine?

You exert 10 N of force on a can opener. The can opener exerts 30 N

of force on the can. What is the mechanical advantage?

Mechanical = Output Force = 30 NAdvantage Input Force 10 N

Mechanical Advantage = 3

What are SIMPLE MACHINES?

There are only 6 basic kinds of simple machines that make work

easier.

These 6 simple machines make up all the other compound machines we use everyday.

SIX SIMPLE MACHINES

The six simple machines are:

Inclined Plane

Wedge

Screw

Lever

Wheel & Axle

Pulley

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INCLINED PLANE

An inclined plane is a flat, sloped surface connecting a lower level with a higher level.

INCLINED PLANE

It lets you use less force over a longer distance to raise a load to a higher level.

Input Force

Output Force

INCLINED PLANE:Examples

Ramps (Boat ramps, wheelchair ramps)

Propeller

Ladders/Stairs

WEDGE

A wedge has slanting slides that taper to a thin edge – it splits material apart. (A moving inclined plane!)

It converts motion in one direction, into a splitting motion that acts at right angles to the blade.

WEDGE

Input Force

Output ForceOutput Force

WEDGE:Examples & Uses

Ax, Knife, etc.

Zippers

Used in all cutting machines (to split materials apart)

Lifting machines may use wedges to slide under loads

SCREW

A screw has a “thread” or “groove” wrapped around a central cylinder. (Another inclined plane - wrapped around a cylinder!)

SCREW

While turning, it converts a twisting motion into a forward or backward motion.

Input Force

Output Force

SCREW:Examples & Uses

Screws can holds things together or lift materials.

Screws

Screw top lids for jars/bottles

Light bulb

Swivel stools/chairs

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LEVER

A lever is rigid bar that pivots/rotates on a fixed point. The fixed point is called the “fulcrum”.

LEVER

Levers may increase the size or distance of force or change direction of the force.

There are 3 types of levers.

LEVERS:Examples & Uses

First Class Levers:– Scissors, See-saws, Pliers

Second Class Levers:– Staplers, Nutcrackers,

Wheelbarrows

Third Class Levers– Shovels, baseball bats, tweezers

WHEEL & AXLE

A wheel and axle are 2 circles or cylinders attached together around a common axis.

The larger circle is the “wheel”, the smaller cylinder/rod is called the “axle”.

WHEEL & AXLE

The wheel is locked to the central axle –when one turns, so does the other one.

A short powerful force at the axle, will move the wheel’s edge a long distance.

A long motion at edge of wheel, moves the axle with great force.

Input

Force

Output

Force

Output

Force

Input

Force

WHEEL & AXLE:Examples & Uses

Screwdriver

Windmill

Cars/Bicycles

Rolling Pin

Door Knob

Fan

PULLEY

A pulley is a grooved wheel with a rope, used to raise/lower/move a load.

PULLEY

A simple fixed pulley only changes the direction of force.

Pulley systems decreases the input force, allowing you to move heavier loads.

Output

Force

Input

Force

Output Force

Input

Force

PULLEY:Examples & Uses

Cranes

Raising a flag on a pole

Window Blinds

Raising a sail on a boat

Clothesline

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COMPOUND MACHINES

Most machines are combinations of 2 or more simple machines.

For example, a simple can opener is a combination of 3 simple machines:

– Lever

– Wheel & axle

– Wedge

Simple Machines(Making work easier…phew!)

Machines make work easier by changing 3 things about the FORCE:

The amount of force

The distance of the force

The direction of the force

Machines make work easier by changing 3 things about the FORCE:

The amount of force(eg. A ramp lets you lift a heavy object

with LESS force)

Machines make work easier by changing 3 things about the FORCE:

The distance of the force(eg. A baseball bat lets you move your

arms a short distance, but move the end of the bat a large distance).

Machines make work easier by changing 3 things about the FORCE:

The direction of the force(eg. The pulley on a set of window blinds

lets you move the blinds UP with a DOWNWARD pull.

What is the mechanical advantage of a machine?

A machine’s mechanical advantageis the number of times a machine increases a force exerted on it.

Mechanical = Output ForceAdvantage Input Force

What is the mechanical advantage of a machine?

You exert 10 N of force on a can opener. The can opener exerts 30 N of force on the can. What is the mechanical advantage?

Mechanical = Output Force = 30 N

Advantage Input Force 10 N

Mechanical Advantage = 3

What is the efficiency of a machine?

The EFFICIENCY compares:

– the work you put IN to

– the work the machine puts OUT.

An IDEAL machine is 100% efficient.INPUT WORK = OUTPUT WORK

In the real world, some input work is always lost due to FRICTION between the moving parts of the machine.

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What is the efficiency of a machine?

EFFICIENCY = Output Work x 100%Input Work

You mow the lawn with a rusty lawn mower. You do 50,000 J of work on the lawn mower but only 25,000 J go to cutting the lawn. What is the efficiency of the lawn mower?

What is the efficiency of a machine?

You mow the lawn with a rusty lawn mower. You do 50,000 J of work on the lawn mower but only 25,000 J go to cutting the lawn. What is the efficiency of the lawn mower?

EFFICIENCY = Output Work x 100%Input Work

Efficiency = 25,000 J x 100%

50,000 J

Efficiency = 50%

Try the rest of the practice problems on your own…

Mechanical = Output ForceAdvantage Input Force

EFFICIENCY = Output Work x 100% Input Work

The Physics of

Rollercoasters

The Early Days of Rollercoasters

The first American

rollercoaster, the

Mauch Chunk

Switchback Railway,

c. 1870

Length: 40 miles

Speed: over 100 mph

Wooden Coasters

Racer at King’s Island

1972

Length: 3415 feet

Speed: 61 mph

Steel Coasters

Revolution at Magic Mountain

1977

First Loop of

the Modern Era

Classic RollercoastersViper at Magic Mountain

1990

3 loops, 4 corkscrews

Speed: 73 mph

Suspension Coasters

Batman The Ride

Magic Mountain

1994

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Stand-Up Coasters

Riddler’s Revenge

Magic Mountain

1998

Tallest, Fastest

Stand-Up Coaster

SupermanSuperman

Magic Mountain

1997

100 mph

415 feet tall

Six Flags Great

Adventure

New Jersey

2005

Speed: 128 mph

Height: 456 feet

Kingda Ka

Rollercoaster Forces

• There are two main

forces which need to be

considered when

designing rollercoasters:

• Gravity

• Friction

How Rollercoasters Work

• Rollercoasters are

driven by gravity

• There is no engine

attached to the cars

• The cars are pulled to

the top of a hill and

released

Potential and Kinetic Energy

• Every rollercoaster relies on conservation of

energy

• At the top of the hill, the rollercoaster has

potential energy

• At the bottom of the hill, the potential

energy has been converted into kinetic

energy

Potential Energy

• Potential Energy = mass x gravity x height

• Heavier objects have more potential energy

• The higher an object, the greater its potential

energy

Kinetic Energy

• Kinetic Energy = 1/2 x mass x velocity2

• Heavier objects have more kinetic energy

• The faster an object is moving, the greater

its kinetic energy

Hills

• K.E. = 1/2 mv2, P.E. = mgh

• When cars go up a hill, their height

increases and their velocity decreases

• When cars go down a hill, their height

decreases and their velocity increases

• A rollercoaster can never go higher than the

top of the first hill

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Loops

• Loops are treated like hills, with one

difference

• Cars must have enough energy to reach the

top of the loop

• Cars must have a certain speed at the top of

a loop so that they don’t fall