week5-2

# week5-2 - If work is done but no kinetic energy is gained...

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Potential Energy If work is done but no kinetic energy is gained, we say that the potential energy has increased. For example, if a force is applied to lift a crate, the gravitational potential energy of the crate has increased. The work done is equal to the force (mg) times the distance lifted (height). The gravitational potential energy equals mg X h. potential energy implies storing energy to use later for other purposes. For example, the gravitational potential energy of the crate can be converted to kinetic energy and used for other purposes After releasing the string, it reach the ground with higher speed, i.e. large kinetic energy, if it’s positioned higher, i.e. higher potential energy at the beginning.

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2/9/2011 2 5.0 kg box lifted (without acceleration) thru height of 2.0 m What is increase in potential energy and how much work I Is needed ? PE = mgh = (5.0 kg)(9.8 m/s 2 )(2.0m) = 98J Ch 6 E 8 A). 5000 J, 5000 J B). 490 J, 490 J C). 98 J, 98 J D). 196 J, 196 J E). 49 J, 49 J b) F = ma = 0 = F lift – mg F lift = mg = (5.0kg)(9.8m/s 2 ) = 49N W = Fd = (49N)(2.0m) = 98J
2/9/2011 3 Conversion Between Potential and Kinetic energy If we raise an object a height h so that it starts and finishes at rest then the average force = mg and the work done = mgh . This energy is stored as potential energy. if the mass is allowed to fall back to it’s original point then v 2 = v 0 2 + 2gh ½ m v 2 = ½ mv 0 2 + mgh, assume v 0 = 0 mgh = 1/2mv 2 = KE So the original work in lifting is stored and then returned as kinetic energy F = mg h g

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## This note was uploaded on 04/23/2011 for the course PHYS 214 taught by Professor Staff during the Spring '08 term at Purdue.

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week5-2 - If work is done but no kinetic energy is gained...

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