Chapter 7
Work and Kinetic Energy
(Continued)
Dr. Jie Zou PHY 1151G
Department of Physics
1
Outline
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Kinetic energy
Work-energy theorem
Power
Examples
Dr. Jie Zou PHY 1151G
Department of Physics
2
Kinetic Energy
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Kinetic energy K: The energy of motion.
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K = (1/2)mv2, where v is the speed.
SI units for kinetic energy: joule (J).
Kinetic energy is always greater than or equal to
zero.
Example: Consider your kinetic energy when
jogging. Assuming a mass of about 62 kg and a
speed of 2.5 m/s, your kinetic energy is K =
(1/2)mv2 = (1/2)(62 kg)(2.5 m/s)2 = 190 J.
Dr. Jie Zou PHY 1151G
Department of Physics
3
Work-Energy Theorem
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Work-energy theorem: The total
(net) work done on an object is equal
to the change in its kinetic energy.
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Wtotal = K = (1/2)mvf2 - (1/2)mvi2
Here vf and vi are the final and initial speed
of the object, respectively.
Dr. Jie Zou PHY 1151G
Department of Physics
4
Example 7-5
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A 4.1-kg box of books is lifted
vertically from rest a distance
of 1.6 m by an upward applied
force of 60.0 N. Find
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(a) The work done by the applied
force.
(b) The work done by the force of
gravity.
(c) Use work-energy theorem and
find the final speed of the box.
Can you use kinematics equations
to solve (c)?
Dr. Jie Zou PHY 1151G
Department of Physics
5
Example 2
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A boy exerts a force of
11.0 N at 29.0 above the
horizontal on a 6.40-kg
sled. Find the work done
by the boy and the final
speed of the sled after it
moves 2.00 m, assuming
the sled starts with an
initial speed of 0.500 m/s
and slides horizontally
without friction.
Dr. Jie Zou PHY 1151G
Department of Physics
6
Power
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Power: A measure of how quickly work
is done.
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Average power = W / t.
W is the work done in time t.
SI units: J/s = watt (W).
1 horsepower (hp) = 746 W.
Dr. Jie Zou PHY 1151G
Department of Physics
7
Example 7-8
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To pass a slowmoving truck, you
want your 1.30103
kg-car to accelerate
from 13.4 m/s (30.0
mph) to 17.9 m/s
(40.0 mph) in 3.00 s.
What is the minimum
power required for
this pass?
Dr. Jie Zou PHY 1151G
Department of Physics
8
Homework
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See online homework assignment at
www.masteringphysics.com
Dr. Jie Zou PHY 1151G
Department of Physics
9