Dr. Jie Zou PHY 1151G Department of Physics

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Chapter 7

Work and Kinetic Energy (Continued)

Dr. Jie Zou PHY 1151G Department of Physics

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Outline Kinetic energy Work-energy theorem Power Examples

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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

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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)?

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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.

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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?

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Homework See online homework assignment

at www.masteringphysics.com