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Chapter 7. Work and Kinetic Energy (Continued). Outline. Kinetic energy Work-energy theorem Power Examples. Kinetic Energy. Kinetic energy K: The energy of motion. K = (1/2)mv 2 , where v is the speed. SI units for kinetic energy: joule (J). - PowerPoint PPT Presentation
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Dr. Jie Zou PHY 1151G Department of Physics
1
Chapter 7
Work and Kinetic Energy (Continued)
Dr. Jie Zou PHY 1151G Department of Physics
2
Outline Kinetic energy Work-energy theorem Power Examples
Dr. Jie Zou PHY 1151G Department of Physics
3
Kinetic Energy Kinetic energy K: The energy of motion.
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
4
Work-Energy Theorem Work-energy theorem: The total
(net) work done on an object is equal to the change in its kinetic energy. Wtotal = K = (1/2)mvf
2 - (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
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Example 7-5 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 (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
6
Example 2 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
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Power Power: A measure of how quickly
work is done. 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
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Example 7-8 To pass a slow-
moving truck, you want your 1.30103 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
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Homework See online homework assignment
at www.masteringphysics.com