conceptual physics fundamentals
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Conceptual Physics Fundamentals
Chapter 3:
EQUILIBRIUM AND LINEAR MOTION
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
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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
Mass—A Measure of InertiaCHECK YOUR NEIGHBOR
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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
Mass—A Measure of InertiaCHECK YOUR ANSWER
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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
Mass—A Measure of InertiaCHECK YOUR NEIGHBOR
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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
Explanation:
Tension in the top string is the pulling tension plus the weight of the ball; both of which break the top string.
Mass—A Measure of InertiaCHECK YOUR ANSWER
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When the string is pulled down quickly, the bottom 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
Mass—A Measure of InertiaCHECK YOUR NEIGHBOR
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When the string is pulled down quickly, the bottom 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
Explanation:
It is the “laziness” of the ball that keeps it at rest, resulting in the breaking of the bottom string.
Mass—A Measure of InertiaCHECK YOUR ANSWER
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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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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
C. Both of the above
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
The Force of FrictionCHECK YOUR NEIGHBOR
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
The Force of FrictionCHECK YOUR ANSWER
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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 _______.
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
The Force of FrictionCHECK YOUR NEIGHBOR
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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 _______.
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
Explanation:
The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium.
The Force of FrictionCHECK YOUR ANSWER
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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
C. Both of the above
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
C. Both of the above
D. Neither of the above
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
AccelerationCHECK YOUR NEIGHBOR
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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.
AccelerationCHECK YOUR ANSWER
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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
C. Both of the above
D. Neither of the above
AccelerationCHECK YOUR NEIGHBOR
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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
Explanation:
It is common to express 10 m/s per second as 10 m/s/s, or
10 m/s2.
AccelerationCHECK YOUR ANSWER
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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
AccelerationCHECK YOUR NEIGHBOR
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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.
AccelerationCHECK YOUR ANSWER
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