Conceptual Physics Fundamentals

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

Conceptual Physics Fundamentals

Chapter 3:

EQUILIBRIUM AND LINEAR MOTION

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This lecture will help you understand:

Aristotle on Motion Galileo’s Concept of Inertia Mass—A Measure of Inertia Net Force The Equilibrium Rule Equilibrium of Moving Things The Force of Friction Speed and Velocity Acceleration

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Equilibrium and Linear Motion

“When you’re over the hill, that’s when you pick up speed.”

—Quincy Jones

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Aristotle on Motion

Aristotle’s classification of motion natural motion

o Every object in the universe has a proper place determined by a combination of four elements: earth, water, air, and fire

o Any object not in its proper place will strive to get there.

Example: stones fall; puffs of smoke rise

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Aristotle on Motion

natural motion (continued)o Straight up or straight down for all things on

Eartho Beyond Earth, motion is circular.

Example: Sun and moon continually circle the Earth.

violent motiono produced by external pushes or pulls on

objects

Example: Wind imposes motion on ships.5


Galileo’s Concept of Inertia

Italian scientist Galileo demolished Aristotle’s assertions in early 1500s.

Galileo’s discovery: Objects of different weight fall to the ground at

the same time in the absence of air resistance. A moving object needs no force to keep it

moving in the absence of friction.

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Galileo’s Concept of Inertia

Force is a push or a pull

Inertia is a property of matter to resist changes in

motion depends on the amount of matter in an object

(its mass)

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The use of inclined planes for Galileo’s experiments helped him to _______.

A. eliminate the acceleration of free fall

B. discover the concept of energy

C. discover the property called inertia

D. discover the concept of momentum

Galileo’s Concept of InertiaCHECK YOUR NEIGHBOR

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The use of inclined planes for Galileo’s experiments helped him to _______.

A. eliminate the acceleration of free fall

B. discover the concept of energy

C. discover the property called inertia

D. discover the concept of momentum

Comment:

Note that inertia is a property of matter, not a reason for the behavior of matter.

Galileo’s Concept of InertiaCHECK YOUR ANSWER

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Mass—A Measure of InertiaMass a measure of the inertia of a material object independent of gravity greater inertia greater mass unit of measurement is the kilogram (kg)

Weight the force on an object due to gravity scientific unit of force is the Newton (N)

unit is also the pound (lb)

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The concept of inertia mostly involves _______.

A. mass

B. weight

C. volume

D. density

Mass—A Measure of InertiaCHECK YOUR NEIGHBOR

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The concept of inertia mostly involves _______.

A. mass

B. weight

C. volume

D. density

Comment:

Anybody get this wrong? Check the title of this slide! :-)

Mass—A Measure of InertiaCHECK YOUR ANSWER

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If the mass of an object is halved, the weight of the object is _______.

A. halved

B. doubled

C. depends on location

D. none of the above


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If the mass of an object is halved, the weight of the object is _______.

A. halved

B. doubled

C. depends on location

D. none of the above


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Mass—A Measure of InertiaMass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight.

Mass versus weight On Moon and Earth

o Weight of an object on the Moon isless than on the Earth.

o Mass of an object is the samein both locations.

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Mass—A Measure of Inertia

One Kilogram Weighs 9.8 Newtons.

Relationship between kilograms and pounds 1 kg = 2.2 lb = 9.8 N at Earth’s surface 1 lb = 4.45 N

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When the string is pulled down slowly, the top string breaks, which best illustrates the _______.

A. weight of the ball

B. mass of the ball

C. volume of the ball

D. density of the ball


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When the string is pulled down slowly, the top string breaks, which best illustrates the _______.


B. mass of the ball



Explanation:

Tension in the top string is the pulling tension plus the weight of the ball; both of which break the top string.


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When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______.


B. mass of the ball




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When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______.


B. mass of the ball



Explanation:

It is the “laziness” of the ball that keeps it at rest, resulting in the breaking of the bottom string.


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Net Force

• Net force is the combination of all forces that change an object’s state of motion.Example: If you pull on a box with 10 N and a friend

pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling.

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A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______.

A. 5 N to the left

B. 5 N to the right

C. 25 N to the left

D. 25 N to the right

Net ForceCHECK YOUR NEIGHBOR

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A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______.

A. 5 N to the left

B. 5 N to the right

C. 25 N to the left

D. 25 N to the right

Net ForceCHECK YOUR ANSWER

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Net Force

Vector quantity a quantity whose description requires both

magnitude (how much) and direction (which way) can be represented by arrows drawn to scale,

called vectorso Length of arrow represents magnitude, and arrowhead

shows direction

Examples: force, velocity, acceleration

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The Equilibrium Rule

The equilibrium rule the vector sum of forces acting on a non-

accelerating object equals zero in equation form: F = 0

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The Equilibrium Rule

Example: A string holding up a bag of flour

Two forces act on the bag of flour: tension force acts upward weight acts downward

The forces are equal in magnitude and opposite in direction; when the forces are added they cancel to zero and the bag of flour remains at rest.

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The equilibrium rule, F = 0, applies to _______.

A. vector quantities

B. scalar quantities

C. both of the above

D. neither of the above

The Equilibrium RuleCHECK YOUR NEIGHBOR

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The equilibrium rule, F = 0, applies to _______.

A. vector quantities

B. scalar quantities

C. both of the above

D. neither of the above

Explanation:

Vector addition takes into account + and - quantities that can cancel to zero. Two forces (vectors) can add to zero, but there is no way that two masses (scalars) can add to zero.

The Equilibrium RuleCHECK YOUR ANSWER

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Support Force

Support force (normal force) is an upward force on an object that is opposite to the force of gravity.Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force.

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When you stand on two bathroom scales with one foot on each scale, and with your weight evenly distributed, each scale will read _______.

A. your weight

B. half your weight

C. zero

D. more than your weight

The Support ForceCHECK YOUR NEIGHBOR

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When you stand on two bathroom scales, with one foot on each scale and with your weight evenly distributed, each scale will read _______.

A. your weight

B. half your weight

C. zero

D. more than your weight

Explanation:

You are at rest on the scales, so F = 0. The sum of the two upward support forces is equal to your weight.

The Support ForceCHECK YOUR ANSWER

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Equilibrium of Moving Things

Equilibrium a state of no change with no net force acting

o static equilibrium

Example: Hockey puck at rest on slippery ice

o dynamic equilibrium

Example: Hockey puck sliding at constant speed on slippery ice

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Equilibrium of Moving Things

Equilibrium test whether something undergoes changes in

motionExample: A refrigerator at rest is in static equilibrium. If it is

moved at a steady speed across a floor, it is in dynamic equilibrium.

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A bowling ball is in equilibrium when it _______.

A. is at rest

B. moves steadily in a straight-line path

C. Both of the above

D. None of the above

Equilibrium of Moving ThingsCHECK YOUR NEIGHBOR

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A bowling ball is in equilibrium when it _______.

A. is at rest

B. moves steadily in a straight-line path


D. None of the above

Equilibrium of Moving ThingsCHECK YOUR ANSWER

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The Force of Friction

Friction occurs when objects rub against one another applies to solids, liquids, and gases acts in a direction to oppose motion

Example: When an object falls down through air, the force of friction (air resistance) acts upward.

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The Force of Friction depends on the kinds of material and how much they are

pressed together is due to tiny surface bumps and to “stickiness” of the

atoms on a material’s surface

Example: Friction between a crate on a smooth woodenfloor is less than that on a rough floor.

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The force of friction can occur _______.

A. with sliding objects

B. in water

C. in air

D. All of the above

The Force of FrictionCHECK YOUR NEIGHBOR

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The force of friction can occur _______.

A. with sliding objects

B. in water

C. in air

D. All of the above

Comment:

Friction can also occur for objects at rest. If you push horizontally on your book and it doesn’t move, then friction between the book and the table is equal and opposite to your push.

The Force of FrictionCHECK YOUR ANSWER

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When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______.

A. less than Josh’s push

B. equal to Josh’s push

C. equal and opposite to Josh’s push

D. more than Josh’s push


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When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______.






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When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______.






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When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______.





Explanation:

The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium.


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Speed and Velocity

Speed defined as the distance covered per amount of

travel time units are meters per second in equation form

Example: A girl runs 6 meters in 1 second. Her speed is 6 m/s.

speed = distance coveredtime

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Speed and Velocity

Average speed the entire distance covered divided by the total

travel time doesn’t indicate various instantaneous speeds

along the way in equation form:

Example: Drive a distance of 80 km in 1 hour and your

average speed is 80 km/h.

total distance coveredaverage speed = travel time

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Speed and Velocity

Instantaneous speed is the speed at any

instant.

Velocity a description of how fast and in what direction a vector quantity

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The average speed of driving 30 km in 1 hour is the same average speed as driving _______.

A. 30 km in one-half hour

B. 30 km in two hours

C. 60 km in one-half hour

D. 60 km in two hours

Speed and VelocityCHECK YOUR NEIGHBOR

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The average speed of driving 30 km in 1 hour is the same average speed as driving _______.

A. 30 km in one-half hour

B. 30 km in two hours

C. 60 km in one-half hour

D. 60 km in two hours

Speed and VelocityCHECK YOUR ANSWER

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Speed and Velocity

Constant speed is steady speed, neither speeding up nor slowing down.

Constant velocity is constant speed and constant direction (straight-line path with no acceleration).

Motion is relative to Earth, unless otherwise stated.

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Acceleration

Galileo first formulated the concept of acceleration in his experiments with inclined planes.

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Acceleration

Acceleration is the rate at which velocity changes over time involves a change in speed, direction, or both

speed and direction

Example: Car making a turn

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Acceleration

in equation form:

Example: If in 1 second you steadily increase your velocity from 30 km/h to 35 km/h, and in the next 1 second you steadily increase your velocity from 35 km/h to 40 km/h, you change your velocity by 5 km/h each second. Your acceleration is 5 km/h/s.

acceleration = change of velocitytime interval

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An automobile cannot maintain a constant speed when _______.

A. accelerating

B. rounding a curve


D. Neither of the above

AccelerationCHECK YOUR NEIGHBOR

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An automobile cannot maintain a constant speed when _______.

A. accelerating

B. rounding a curve



Comment:

When rounding a curve, the automobile is accelerating because it is changing direction.

AccelerationCHECK YOUR ANSWER

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Acceleration and velocity are actually _______.

A. the same

B. rates, but for different quantities

C. the same when direction is not a factor

D. the same in free-fall situations


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Acceleration and velocity are actually _______.

A. the same

B. rates, but for different quantities

C. the same, when direction is not a factor

D. the same in free-fall situations

Explanation:

Velocity is the rate at which distance changes over time; acceleration is the rate at which velocity changes over time.


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Acceleration

Free-fall• falling under the influence of gravity

only—with no air resistanceo Freely falling objects on Earth gain

speed at the rate of 10 m/s each second (more precisely, 9.8 m/s2).

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If a falling object gains 10 m/s each second it falls, its acceleration is _______.

A. 10 m/s

B. 10 m/s per second




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If a falling object gains 10 m/s each second it falls, its acceleration is _______.

A. 10 m/s

B. 10 m/s per second



Explanation:

It is common to express 10 m/s per second as 10 m/s/s, or

10 m/s2.


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A free-falling object has a speed of 30 m/s at one instant. Exactly one second later, its speed will be _______.

A. the same

B. 35 m/s

C. more than 35 m/s

D. 60 m/s


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A free-falling object has a speed of 30 m/s at one instant. Exactly one second later, its speed will be _______.

A. the same

B. 35 m/s

C. more than 35 m/s

D. 60 m/s

Explanation:

One second later, its speed will be 40 m/s, which is more than

35 m/s.


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Conceptual Physics Fundamentals