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Chapter 10 Lecture
physics
FOR SCIENTISTS AND ENGINEERS
a strategic approach
THIRD EDITION
randall d. knight
© 2013 Pearson Education, Inc.
Chapter 10 Preview
© 2013 Pearson Education, Inc.
Slide 10-6
Reading Question 10.3
Mechanical energy is
A. The energy due to internal moving parts.
B. The energy of motion.
C. The energy of position.
D. The sum of kinetic energy plus potential
energy.
E. The sum of kinetic, potential, thermal, and elastic
energy.
© 2013 Pearson Education, Inc.
Slide 10-13
Reading Question 10.3
Mechanical energy is
A. The energy due to internal moving parts.
B. The energy of motion.
C. The energy of position.
D. The sum of kinetic energy plus potential
energy.
E. The sum of kinetic, potential, thermal, and elastic
energy.
© 2013 Pearson Education, Inc.
Slide 10-14
The Basic Energy Model
 Within a system, energy can
be transformed from one
type to another.
 The total energy of the
system is not changed by
these transformations.
 This is the law of
conservation of energy.
 Energy can also be transferred
from one system to another.
 The mechanical transfer of energy to a system via
forces is called work.
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Slide 10-23
Kinetic Energy and Gravitational Potential
Energy
Define kinetic energy as an energy of motion:
Define gravitational potential energy as an
energy of position:
The sum K + Ug is not changed when an object is in
free fall. Its initial and final values are equal:
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Slide 10-25
Example 10.1 Launching a Pebble
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Slide 10-31
Example 10.1 Launching a Pebble
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Slide 10-32
Example 10.1 Launching a Pebble
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Slide 10-33
Energy Bar Charts
 A pebble is tossed up into the air.
 The simple bar charts below show how the sum of K + Ug
remains constant as the pebble rises and then falls.
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Slide 10-34
QuickCheck 10.4
Rank in order, from largest to
smallest, the gravitational
potential energies of the balls.
A. 1 > 2 = 4 > 3
B. 1 > 2 > 3 > 4
C. 3 > 2 > 4 > 1
D. 3 > 2 = 4 > 1
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Slide 10-38
QuickCheck 10.4
Rank in order, from largest to
smallest, the gravitational
potential energies of the balls.
A. 1 > 2 = 4 > 3
B. 1 > 2 > 3 > 4
C. 3 > 2 > 4 > 1
D. 3 > 2 = 4 > 1
© 2013 Pearson Education, Inc.
Slide 10-39
QuickCheck 10.6
A small child slides down the four frictionless slides A–
D. Rank in order, from largest to smallest, her speeds at
the bottom.
A. vD > vA > vB > vC
B. vD > vA = vB > vC
C. vC > vA > vB > vD
D. vA = vB = vC = vD
© 2013 Pearson Education, Inc.
Slide 10-51
QuickCheck 10.6
A small child slides down the four frictionless slides A–
D. Rank in order, from largest to smallest, her speeds at
the bottom.
A. vD > vA > vB > vC
B. vD > vA = vB > vC
C. vC > vA > vB > vD
D. vA = vB = vC = vD
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Slide 10-52
Example 10.3 The Speed of a Sled
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Slide 10-53
Example 10.3 The Speed of a Sled
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Slide 10-54
Example 10.3 The Speed of a Sled
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Slide 10-55
Problem-Solving Strategy: Conservation
of Mechanical Energy
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Slide 10-56
QuickCheck 10.7
Three balls are thrown from
a cliff with the same speed
but at different angles.
Which ball has the greatest
speed just before it hits the
ground?
A.
Ball A.
B.
Ball B.
C.
Ball C.
D.
All balls have the same speed.
© 2013 Pearson Education, Inc.
Slide 10-57
QuickCheck 10.7
Three balls are thrown from
a cliff with the same speed
but at different angles.
Which ball has the greatest
speed just before it hits the
ground?
A.
Ball A.
B.
Ball B.
C.
Ball C.
D.
All balls have the same speed.
© 2013 Pearson Education, Inc.
Slide 10-58
QuickCheck 10.8
A hockey puck sliding on smooth ice at 4 m/s comes to a
1-m-high hill. Will it make it to the top of the hill?
A.
Yes.
B.
No.
C.
Can’t answer without knowing the mass of the puck.
D.
Can’t say without knowing the angle of the hill.
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Slide 10-59
QuickCheck 10.8
A hockey puck sliding on smooth ice at 4 m/s comes to a
1-m-high hill. Will it make it to the top of the hill?
A.
Yes.
B.
No.
C.
Can’t answer without knowing the mass of the puck.
D.
Can’t say without knowing the angle of the hill.
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Slide 10-60
QuickCheck 10.9
The restoring force of three
springs is measured as they are
stretched. Which spring has the
largest spring constant?
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Slide 10-64
QuickCheck 10.9
The restoring force of three
springs is measured as they are
stretched. Which spring has the
largest spring constant?
Steepest slope.
Takes lots of force for
a small displacement.
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Slide 10-65
Elastic Potential Energy
 The figure shows a beforeand-after situation in which
a spring launches a ball.
 Integrating the net force
from the spring, as given by
Hooke’s Law, shows that:
 Here K = ½ mv2 is the kinetic
energy.
 We define a new quantity:
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Slide 10-74
QuickCheck 10.10
A spring-loaded gun shoots a plastic ball with a launch
speed of 2.0 m/s. If the spring is compressed twice as
far, the ball’s launch speed will be
A.
B.
C.
D.
E.
1.0 m/s.
2.0 m/s.
2.8 m/s
4.0 m/s.
16.0 m/s.
© 2013 Pearson Education, Inc.
Slide 10-76
QuickCheck 10.10
A spring-loaded gun shoots a plastic ball with a launch
speed of 2.0 m/s. If the spring is compressed twice as
far, the ball’s launch speed will be
A.
B.
C.
D.
E.
1.0 m/s.
2.0 m/s.
2.8 m/s
4.0 m/s.
16.0 m/s.
© 2013 Pearson Education, Inc.
Conservation of energy:
Double x double v
Slide 10-77
QuickCheck 10.11
A spring-loaded gun shoots a plastic ball with a launch
speed of 2.0 m/s. If the spring is replaced with a new
spring having twice the spring constant (but still
compressed the same distance), the ball’s launch
speed will be
A.
B.
C.
D.
E.
1.0 m/s.
2.0 m/s.
2.8 m/s.
4.0 m/s.
16.0 m/s.
© 2013 Pearson Education, Inc.
Slide 10-78
QuickCheck 10.11
A spring-loaded gun shoots a plastic ball with a launch
speed of 2.0 m/s. If the spring is replaced with a new
spring having twice the spring constant (but still
compressed the same distance), the ball’s launch
speed will be
A.
B.
C.
D.
E.
1.0 m/s.
2.0 m/s.
2.8 m/s.
4.0 m/s.
16.0 m/s.
© 2013 Pearson Education, Inc.
Conservation of energy:
Double k  increase
v by square root of 2
Slide 10-79
Example 10.6 A Spring-Launched Plastic Ball
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Slide 10-80
Example 10.6 A Spring-Launched Plastic Ball
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Slide 10-83
QuickCheck 10.12
A particle with the potential
energy shown is moving
to the right. It has 1.0 J of
kinetic energy at x = 1.0 m.
In the region 1.0 m < x < 2.0 m,
the particle is
A.
Speeding up.
B.
Slowing down.
C.
Moving at constant speed.
D.
I have no idea.
© 2013 Pearson Education, Inc.
Slide 10-99
QuickCheck 10.12
A particle with the potential
energy shown is moving
to the right. It has 1.0 J of
kinetic energy at x = 1.0 m.
In the region 1.0 m < x < 2.0 m,
the particle is
A.
Speeding up.
B.
Slowing down.
C.
Moving at constant speed.
D.
I have no idea.
© 2013 Pearson Education, Inc.
Slide 10-100
Chapter 11 Preview
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Slide 11-3
Chapter 11 Preview
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Slide 11-8
QuickCheck 11.3
A crane lowers a girder into place at constant speed.
Consider the work Wg done by gravity and the work
WT done by the tension in the cable. Which is true?
A.
Wg > 0 and WT > 0
B.
Wg > 0 and WT < 0
C.
Wg < 0 and WT > 0
D.
Wg < 0 and WT < 0
E.
Wg = 0 and WT = 0
© 2013 Pearson Education, Inc.
Slide 11-34
QuickCheck 11.3
A crane lowers a girder into place at constant speed.
Consider the work Wg done by gravity and the work
WT done by the tension in the cable. Which is true?
A.
Wg > 0 and WT > 0
B.
Wg > 0 and WT < 0
C.
Wg < 0 and WT > 0
D.
Wg < 0 and WT < 0
E.
Wg = 0 and WT = 0
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The downward force of gravity is in the
direction of motion  positive work.
The upward tension is in the direction
opposite the motion  negative work.
Slide 11-35
QuickCheck 11.4
Robert pushes the box to the
left at constant speed. In doing
so, Robert does ______ work
on the box.
A.
positive
B.
negative
C.
zero
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Slide 11-39
QuickCheck 11.4
Robert pushes the box to the
left at constant speed. In doing
so, Robert does ______ work
on the box.
A.
positive
B.
negative
C.
zero
Force is in the direction of displacement  positive work
© 2013 Pearson Education, Inc.
Slide 11-40
QuickCheck 11.6
Which force below does the most work? All three
displacements are the same.
A.
B.
C.
D.
The 10 N force.
The 8 N force
The 6 N force.
They all do the same work.
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sin60 = 0.87
cos60 = 0.50
Slide 11-47
QuickCheck 11.7
A light plastic cart and a heavy
steel cart are both pushed with
the same force for a distance
of 1.0 m, starting from rest.
After the force is removed, the
kinetic energy of the light
plastic cart is ________ that of
the heavy steel cart.
A.
B.
C.
D.
greater than
equal to
less than
Can’t say. It depends on how big the force is.
© 2013 Pearson Education, Inc.
Slide 11-49
Power
 The rate at which energy is transferred or transformed
is called the power P.
 The SI unit of power is the
watt, which is defined as:
Highly trained athletes have a tremendous
power output.
1 watt = 1 W = 1 J/s
 The English unit of power
is the horsepower, hp.
1 hp = 746 W
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Slide 11-98
Example 11.13 Choosing a Motor
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Slide 11-99
Example 11.13 Choosing a Motor
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Slide 11-100