Download Conceptual Physics Fundamentals Equilibrium and Linear Motion

Conceptual Physics
Fundamentals
Chapter 3:
EQUILIBRIUM AND LINEAR
MOTION
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
This lecture will help you
understand:
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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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1
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
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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
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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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Galileo’s Concept of Inertia
CHECK YOUR NEIGHBOR
The use of inclined planes for Galileo’s experiments helped
him to
A.
B. 
C. 
D. 
eliminate the acceleration of free fall.
discover the concept of energy.
discover the property called inertia.
discover the concept of momentum.
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Galileo’s Concept of Inertia
CHECK YOUR ANSWER
The use of inclined planes for Galileo’s experiments helped
him to
A.
B. 
C. 
D. 
eliminate the acceleration of free fall.
discover the concept of energy.
discover the property called inertia.
discover the concept of momentum.
Comment:
Note that inertia is a property of matter, not a reason for the behavior of matter.
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3
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)
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Mass—A Measure of Inertia
CHECK YOUR NEIGHBOR
The concept of inertia mostly involves
A.
B. 
C. 
D. 
mass.
weight.
volume.
density.
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Mass—A Measure of Inertia
CHECK YOUR ANSWER
The concept of inertia mostly involves
A.
B. 
C. 
D. 
mass.
weight.
volume.
density.
Comment:
Anybody get this wrong? Check the title of this slide! :-)
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4
Mass—A Measure of Inertia
CHECK YOUR NEIGHBOR
If the mass of an object is halved, the weight of the object is
A.
B. 
C. 
D. 
halved.
twice.
depends on location.
none of the above.
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Mass—A Measure of Inertia
CHECK YOUR ANSWER
If the mass of an object is halved, the weight of the object is
A.
B. 
C. 
D. 
halved.
twice.
depends on location.
none of the above.
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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
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Mass—A Measure of Inertia
CHECK YOUR NEIGHBOR
When the string is pulled down slowly, the top string
breaks, which best illustrates the:
A.
B. 
C. 
D. 
weight of the ball.
mass of the ball.
volume of the ball.
density of the ball.
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Mass—A Measure of Inertia
CHECK YOUR ANSWER
When the string is pulled down slowly, the top string
breaks, which best illustrates the:
A.
B. 
C. 
D. 
weight of the ball.
mass of the ball.
volume of the ball.
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.
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Mass—A Measure of Inertia
CHECK YOUR NEIGHBOR
When the string is pulled down quickly, the bottom string
breaks, which best illustrates the:
A.
B. 
C. 
D. 
weight of the ball.
mass of the ball.
volume of the ball.
density of the ball.
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Mass—A Measure of Inertia
CHECK YOUR ANSWER
When the string is pulled down quickly, the bottom string
breaks, which best illustrates the:
A.
B. 
C. 
D. 
weight of the ball.
mass of the ball.
volume of the ball.
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.
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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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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.
B. 
C. 
D. 
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
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Net Force
CHECK YOUR ANSWER
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.
B. 
C. 
D. 
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
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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 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 nonaccelerating object equals zero
•  in equation form: ΣF = 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
equal in magnitude and opposite in direction
when added, cancel to zero
bag of flour remains at rest
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The Equilibrium Rule
CHECK YOUR NEIGHBOR
The equilibrium rule, ΣF = 0, applies to
A.
B. 
C. 
D. 
vector quantities.
scalar quantities.
both of the above.
neither of the above.
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The Equilibrium Rule
CHECK YOUR ANSWER
The equilibrium rule, ΣF = 0, applies to
A.
B. 
C. 
D. 
vector quantities.
scalar quantities.
both of the above.
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.
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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 table a compresses atoms in the
table, and the compressed atoms produce the
support force
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The Support Force
CHECK YOUR NEIGHBOR
When you stand on two bathroom scales with one foot on
each scale and with your weight evenly distributed, each
scale will read
A.
B. 
C. 
D. 
your weight.
half your weight.
zero.
more than your weight.
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The Support Force
CHECK YOUR ANSWER
When you stand on two bathroom scales, with one foot on
each scale and with your weight evenly distributed, each
scale will read
A.
B. 
C. 
D. 
your weight.
half your weight.
zero.
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.
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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
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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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11
Equilibrium of Moving Things
CHECK YOUR NEIGHBOR
A bowling ball is in equilibrium when it A.
B. 
C. 
D. 
is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
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Equilibrium of Moving Things
CHECK YOUR ANSWER
A bowling ball is in equilibrium when it
A.
B. 
C. 
D. 
is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
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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 wooden
floor is less than that on a rough floor
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The Force of Friction
CHECK YOUR NEIGHBOR
The force of friction can occur
A.
B. 
C. 
D. 
with sliding objects.
in water.
in air.
all of the above
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The Force of Friction
CHECK YOUR ANSWER
The force of friction can occur
A.
B. 
C. 
D. 
with sliding objects.
in water.
in air.
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.
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13
The Force of Friction
CHECK YOUR NEIGHBOR
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.
B. 
C. 
D. 
less than Josh’s push.
equal to Josh’s push.
equal and opposite to Josh’s push.
more than Josh’s push.
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The Force of Friction
CHECK YOUR ANSWER
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.
B. 
C. 
D. 
less than Josh’s push.
equal to Josh’s push.
equal and opposite to Josh’s push.
more than Josh’s push.
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The Force of Friction
CHECK YOUR NEIGHBOR
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.
B. 
C. 
D. 
less than Josh’s push.
equal to Josh’s push.
equal and opposite to Josh’s push.
more than Josh’s push.
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The Force of Friction
CHECK YOUR ANSWER
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.
B. 
C. 
D. 
less than Josh’s push.
equal to Josh’s push.
equal and opposite to Josh’s push.
more than Josh’s push.
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 sec. Her speed is 6 m/
s.
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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
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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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Speed and Velocity
CHECK YOUR NEIGHBOR
The average speed of driving 30 km in 1 hour is the same
average speed as driving
A.
B. 
C. 
D. 
30 km in one-half hour.
30 km in two hours.
60 km in one-half hour.
60 km in two hours.
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Speed and Velocity
CHECK YOUR ANSWER
The average speed of driving 30 km in 1 hour is the same
average speed as driving
A.
B. 
C. 
D. 
30 km in one-half hour.
30 km in two hours.
60 km in one-half hour.
60 km in two hours.
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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
•  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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17
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.
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Acceleration
CHECK YOUR NEIGHBOR
An automobile cannot maintain a constant speed when
A.
B. 
C. 
D. 
accelerating.
rounding a curve.
both of the above
neither of the above
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Acceleration
CHECK YOUR ANSWER
An automobile cannot maintain a constant speed when
A.
B. 
C. 
D. 
accelerating.
rounding a curve.
both of the above
neither of the above
Comment:
When rounding a curve, the automobile is accelerating because it
is changing direction.
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18
Acceleration
CHECK YOUR NEIGHBOR
Acceleration and velocity are actually
A.
B. 
C. 
D. 
the same.
rates, but for different quantities.
the same, when direction is not a factor.
the same in free-fall situations.
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Acceleration
CHECK YOUR ANSWER
Acceleration and velocity are actually
A.
B. 
C. 
D. 
the same.
rates, but for different quantities.
the same, when direction is not a factor.
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 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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19
Acceleration
CHECK YOUR NEIGHBOR
If a falling object gains 10 m/s each second it falls, its
acceleration is
A.
B. 
C. 
D. 
10 m/s.
10 m/s per second.
both of the above
neither of the above
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Acceleration
CHECK YOUR ANSWER
If a falling object gains 10 m/s each second it falls, its
acceleration is A.
B. 
C. 
D. 
10 m/s.
10 m/s per second.
both of the above
neither of the above
Explanation:
It is common to express 10 m/s per second as 10 m/s/s, or
10 m/s2.
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Acceleration
CHECK YOUR NEIGHBOR
A free-falling object has a speed of 30 m/s at one instant.
Exactly one second later its speed will be
A.
B. 
C. 
D. 
the same.
35 m/s.
more than 35 m/s.
60 m/s.
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20
Acceleration
CHECK YOUR ANSWER
A free-falling object has a speed of 30 m/s at one instant.
Exactly one second later its speed will be
A.
B. 
C. 
D. 
the same.
35 m/s.
more than 35 m/s.
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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