Lecture Outline
Chapter 7:
Energy
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This lecture will help you understand:
• Energy
• Work
• Power
• Mechanical Energy: Potential and Kinetic
• Work-Energy Theorem
• Conservation of Energy
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Work
• Work
– involves force and distance.
– is force x distance.
– in equation form: W = Fd.
• Two things occur whenever work is done:
– application of force
– movement of something by that force
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Work
CHECK YOUR NEIGHBOR
If you push against a stationary brick wall for
several minutes, you do no work
A. on the wall.
B. at all.
C. Both of the above.
D. None of the above.
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Work
CHECK YOUR ANSWER
If you push against a stationary brick wall for
several minutes, you do no work
A. on the wall.
B. at all.
C. Both of the above.
D. None of the above.
Explanation:
You may do work on your muscles, but not on the wall.
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Work
• Examples:
– Twice as much work is done in
lifting 2 loads 1 story high versus
lifting 1 load the same vertical
distance.
• Reason: force needed to lift twice the
load is twice as much.
– Twice as much work is done in
lifting a load 2 stories instead of 1
story.
• Reason: distance is twice as great.
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Work
• Example:
– a weightlifter raising a
barbell from the floor
does work on the
barbell.
• Unit of work:
– newton-meter (Nm) or
joule (J)
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Work
CHECK YOUR NEIGHBOR
Work is done in lifting a barbell. How much work is
done in lifting a barbell that is twice as heavy the
same distance?
A. Twice as much
B. Half as much
C. The same
D. Depends on the speed of the lift
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Work
CHECK YOUR ANSWER
Work is done in lifting a barbell. How much work is
done in lifting a barbell that is twice as heavy the
same distance?
A. Twice as much
B. Half as much
C. The same
D. Depends on the speed of the lift
Explanation:
This is in accord with work = force x distance. Twice the force for the
same distance means twice the work done on the barbell.
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Work
CHECK YOUR NEIGHBOR
You do work when pushing a cart with a constant
force. If you push the cart twice as far, then the
work you do is
A. less than twice as much.
B. twice as much.
C. more than twice as much.
D. zero.
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Work
CHECK YOUR ANSWER
You do work when pushing a cart with a constant
force. If you push the cart twice as far, then the
work you do is
A. less than twice as much.
B. twice as much.
C. more than twice as much.
D. zero.
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Power
• Power:
– Measure of how fast work is
done
– In equation form:
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Power
• Example:
– A worker uses more power running up the
stairs than climbing the same stairs slowly.
– Twice the power of an engine can do twice
the work of one engine in the same amount of
time, or twice the work of one engine in half
the time or at a rate at which energy is
changed from one form to another.
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Power
• Unit of power
– joule per second, called the watt after James
Watt, developer of the steam engine
• 1 joule/second = 1 watt
• 1 kilowatt = 1000 watts
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Power
CHECK YOUR NEIGHBOR
A job can be done slowly or quickly. Both may
require the same amount of work, but different
amounts of
A. energy.
B. momentum.
C. power.
D. impulse.
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Power
CHECK YOUR ANSWER
A job can be done slowly or quickly. Both may
require the same amount of work, but different
amounts of
A. energy.
B. momentum.
C. power.
D. impulse.
Comment:
Power is the rate at which work is done.
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Mechanical Energy
• Mechanical energy is due to position or to
motion, or both.
• There are two forms of mechanical energy:
– Potential energy
– Kinetic energy
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Potential Energy
• Stored energy held in readiness with a potential
for doing work
• Example:
– A stretched bow has stored energy that can
do work on an arrow.
– A stretched rubber band of a slingshot has
stored energy and is capable of doing work.
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Potential Energy—Gravitational
• Potential energy due to elevated position
• Example:
– water in an elevated reservoir
– raised ram of a pile driver
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Potential Energy—Gravitational
• Equal to the work done (force required to move it
upward x the vertical distance moved against
gravity) in lifting it
• In equation form:
– Potential energy
= mass x acceleration due to gravity x
height
= mgh
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Potential Energy
CHECK YOUR NEIGHBOR
Does a car hoisted for repairs in a service station
have increased potential energy relative to the
floor?
A. Yes
B. No
C. Sometimes
D. Not enough information
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Potential Energy
CHECK YOUR ANSWER
Does a car hoisted for repairs in a service station
have increased potential energy relative to the
floor?
A. Yes
B. No
C. Sometimes
D. Not enough information
Comment:
If the car were twice as heavy, its increase in potential energy would be
twice as great.
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Potential Energy
• Example: Potential energy of 10-N ball is the
same in all 3 cases because work done in
elevating it is the same.
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Kinetic Energy
• Energy of motion
• Depends on the mass of the object and square
of its speed
• Include the proportional constant 1/2 and kinetic
energy = 1/2 x mass x speed x speed
• If object speed is doubled kinetic energy is
quadrupled.
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Kinetic Energy
CHECK YOUR NEIGHBOR
Must a car with momentum have kinetic energy?
A. Yes, due to motion alone
B. Yes, when motion is nonaccelerated
C. Yes, because speed is a scalar and velocity is
a vector quantity
D. No
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Kinetic Energy
CHECK YOUR ANSWER
Must a car with momentum have kinetic energy?
A. Yes, due to motion alone
B. Yes, when motion is nonaccelerated
C. Yes, because speed is a scalar and velocity is
a vector quantity
D. No
Explanation:
Acceleration, speed being a scalar, and velocity being a vector quantity
are irrelevant. Any moving object has both momentum and kinetic
energy.
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Kinetic Energy
• Kinetic energy and work of a moving object
– Equal to the work required to bring it from rest
to that speed, or the work the object can do
while being brought to rest
– In equation form: net force x distance =
kinetic energy, or Fd = 1/2 mv2
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Work-Energy Theorem
• Work-energy theorem
– Gain or reduction of energy is the result of
work.
– In equation form: work = change in kinetic
energy (W = ∆KE).
– Doubling speed of an object requires 4 times
the work.
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Work-Energy Theorem
• Applies to decreasing speed:
– reducing the speed of an object or bringing it
to a halt
• Example: Applying the
brakes to slow a moving
car, work is done on it
(the friction force supplied
by the brakes x distance).
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Work-Energy Theorem
CHECK YOUR NEIGHBOR
Consider a problem that asks for the distance of a
fast-moving crate sliding across a factory floor and then
coming to a stop. The most useful equation for solving this
problem is
A. F = ma.
B. Ft = ∆mv.
C. KE = 1/2mv2.
D. Fd = ∆1/2mv2.
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Work-Energy Theorem
CHECK YOUR ANSWER
Consider a problem that asks for the distance of a
fast-moving crate sliding across a factory floor and then
coming to a stop. The most useful equation for solving this
problem is
A. F = ma.
B. Ft = ∆mv.
C. KE = 1/2mv2.
D. Fd = ∆1/2mv2.
Comment:
The work-energy theorem is the physicist's favorite starting point for
solving many motion-related problems.
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Work-Energy Theorem
CHECK YOUR NEIGHBOR
The work done in bringing a moving car to a stop is the
force of tire friction x stopping distance. If the initial speed
of the car is doubled, the stopping distance is
A. actually less.
B. about the same.
C. twice.
D. None of the above.
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Work-Energy Theorem
CHECK YOUR ANSWER
The work done in bringing a moving car to a stop is the
force of tire friction x stopping distance. If the initial speed
of the car is doubled, the stopping distance is
A. actually less.
B. about the same.
C. twice.
D. None of the above.
Explanation:
Twice the speed means four times the kinetic energy and
four times the stopping distance.
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Conservation of Energy
• Law of conservation of energy
– Energy cannot be created or destroyed; it
may be transformed from one form into
another, but the total amount of energy never
changes.
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Conservation of Energy
• Example: Energy transforms without net loss or
net gain in the operation of a pile driver.
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Conservation of Energy
A situation to ponder…
• Consider the system of a bow and arrow. In
drawing the bow, we do work on the system and
give it potential energy. When the bowstring is
released, most of the potential energy is
transferred to the arrow as kinetic energy and
some as heat to the bow.
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A situation to ponder…
CHECK YOUR NEIGHBOR
Suppose the potential energy of a drawn bow is 50 joules
and the kinetic energy of the shot arrow is 40 joules. Then
A. energy is not conserved.
B. 10 joules go to warming the bow.
C. 10 joules go to warming the target.
D. 10 joules are mysteriously missing.
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A situation to ponder…
CHECK YOUR ANSWER
Suppose the potential energy of a drawn bow is 50 joules
and the kinetic energy of the shot arrow is 40 joules. Then
A. energy is not conserved.
B. 10 joules go to warming the bow.
C. 10 joules go to warming the target.
D. 10 joules are mysteriously missing.
Explanation:
The total energy of the drawn bow, which includes
the poised arrow, is 50 joules. The arrow gets 40
joules and the remaining 10 joules warms the
bow—still in the initial system.
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Skateboard Physics
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