# week5-3 - A lever is used to lift a rock. Will the work...

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A lever is used to lift a rock. Will the work done by the person on the lever be greater than, less than, or equal to the work done by the lever on the rock? (assume no dissipative force, e.g. friction, in action). a) Greater than b) Less than c) Equal to d) Unable to tell from this graph The work done by the person can never be less than the work done by the lever on the rock. If there are no dissipative forces they will be equal. This is a consequence of the conservation of energy.

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2/11/2011 2 Potential Energy and the Reference Point Unlike kinetic energy for Potential energy we have to define where zero is. A block is at a height h above the floor and d above the desk. Potential energy is mgh with respect to the floor but mgd with respect to the desk. If we dropped block it would have more kinetic energy hiiting the floor than hitting the desk h d
Conservative forces are forces for which the sum of kinetic and potential energy is conserved. Gravity and elastic forces are conservative. The energy due to the work done by these forces change between kinetic and potential energy, but the sum of the two is constant . Friction is not conservative. work done by friction force change to heat. Conservative forces

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Any work done by frictional forces is negative . That work removes mechanical energy from the system. And change it to heat. Friction forces do negative works
2/11/2011 5 2/11/2011 Physics 214 Fall 2010 5 Power It is not only important how much work is done but also how quick, i.e. the rate, at which work is done So the quantity Power = P = W/t (unit is a watt) is very important. Generally energy supplies, motors etc are rated by power and one can determine how much work can be done by multiplying by time. W = Pt (joules).

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6 Examples of Watt and Joules The unit for electrical usage is the kilowatt – hour. A kilowatt – hour is the energy used by a 1000 watt device for 3600 seconds 1kWHr = 1000*3600 = 3.6 million joules In order to lift an elevator with a mass of 1000kg to 100 meters requires 1000*9.8*100 joules but we need to do it in 20 seconds. The power we need is
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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 University-West Lafayette.

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week5-3 - A lever is used to lift a rock. Will the work...

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