conceptual physics fundamentalsdurandet/ppt_lectures/...equilibrium of moving things equilibrium...

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

Conceptual Physics Fundamentals

Chapter 3: EQUILIBRIUM AND LINEAR

MOTION


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


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

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

– any object not in its proper place will strive to get there

examples: •  stones fall •  puffs of smoke rise


Aristotle on Motion

•  natural motion (continued) – straight up or straight down for all things on – beyond Earth, motion is circular example: Sun and Moon continually circle Earth

•  violent motion – produced by external pushes or pulls on

objects example: wind imposes motion on ships


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)


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 Inertia CHECK YOUR NEIGHBOR


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 Inertia CHECK YOUR ANSWER

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Mass—A Measure of Inertia

Mass •  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)


The concept of inertia mostly involves

A. mass. B.  weight. C.  volume. D.  density.

Mass—A Measure of Inertia CHECK YOUR NEIGHBOR


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 Inertia CHECK YOUR ANSWER

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

A. halved. B.  twice. C.  depends on location. D.  none of the above.



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

A. halved. B.  twice. C.  depends on location. D.  none of the above.



Mass—A Measure of Inertia Mass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight. Mass versus weight •  on Moon and Earth

– weight of an object on Moon is less than on Earth – mass of an object is the same in 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


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.



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


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:




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


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



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 Force CHECK YOUR NEIGHBOR


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 Force CHECK YOUR ANSWER


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 vectors

–  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: ΣF = 0


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 equal in magnitude and opposite in direction when added, cancel to zero bag of flour remains at rest


The equilibrium rule, ΣF = 0, applies to

A. vector quantities. B.  scalar quantities. C.  both of the above. D.  neither of the above.

The Equilibrium Rule CHECK YOUR NEIGHBOR

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The equilibrium rule, ΣF = 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 Rule CHECK YOUR ANSWER


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 table a compresses atoms in the

table, and the compressed atoms produce the support force


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 Force CHECK 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 ΣF = 0. The sum of the two upward support forces is equal to your weight.

The Support Force CHECK YOUR ANSWER


Equilibrium of Moving Things

Equilibrium •  a state of no change with no net force acting

– static equilibrium example: hockey puck at rest on slippery ice

– dynamic equilibrium example: hockey puck sliding at constant speed on

slippery ice


Equilibrium of Moving Things

Equilibrium test •  whether something undergoes changes in

motion example: 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 Things CHECK YOUR NEIGHBOR


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 Things CHECK YOUR ANSWER


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 wooden floor is less than that on a rough floor


The force of friction can occur

A. with sliding objects. B.  in water. C.  in air. D.  all of the above

The Force of Friction CHECK YOUR NEIGHBOR


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 Friction CHECK 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.



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




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.



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 sec. Her speed is 6 m/s.


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

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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


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 Velocity CHECK YOUR NEIGHBOR


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 Velocity CHECK 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.


Acceleration

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


Acceleration

Acceleration •  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.


An automobile cannot maintain a constant speed when

A. accelerating. B.  rounding a curve. C.  both of the above D.  neither of the above

Acceleration CHECK YOUR NEIGHBOR


An automobile cannot maintain a constant speed when

A. accelerating. B.  rounding a curve. C.  both of the above D.  neither of the above

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

Acceleration CHECK 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.



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.



Acceleration

Free-fall •  falling under the influence of gravity

only—with no air resistance –  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. C.  both of the above D.  neither of the above



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. C.  both of the above D.  neither of the above

Explanation: It is common to express 10 m/s per second as 10 m/s/s, or 10 m/s2.



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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