Chap 10 Solutions copy

# Chap 10 Solutions copy - ENERGY AND WORK 10 Q10.1. Reason:...

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10-1 E NERGY AND W ORK 10 Q10.1. Reason: The brakes in a car slow down the car by converting its kinetic energy to thermal energy in the brake shoes through friction. Cars have large kinetic energies, and all of that energy is converted to thermal energy in the brake shoes, which causes their temperature to increase greatly. Therefore they must be made of material that can tolerate very high temperatures without being damaged. Assess: This is an example of an energy conversion. All of the car’s kinetic energy is converted to thermal energy through friction. To get an appreciation of how much kinetic energy is absorbed by the brake shoes, consider instead the energy explicit in stopping the car by hitting a stationary object instead! Q10.2. Reason: When you hit a nail with a hammer to pound it into some object, many processes are at work. For example, some small amount of energy goes into temporarily increasing the nail’s kinetic energy as it moves into the object. Part of the energy goes into permanently deforming the object to accept the nail. An appreciable portion of the initial kinetic energy of the hammer is converted to thermal energy through, for example, friction between the nail and the object as the nail moves into the object. Some gets directly converted to kinetic energy of the molecules that make up the nail (see section 11.4 for an atomic view of thermal energy and temperature) from the collision between the nail and the hammer. Assess: If you ever try hammering nails, the thermal energy generated is appreciable. Note that energy can be transformed directly into kinetic energy of atoms or molecules that make up an object. As a simpler example, banging a hammer on a solid object directly will increase the temperature of the both the hammer and the object. Q10.3. Reason: We must think of a process that increases an object’s kinetic energy without increasing any potential energy. Consider pulling an object across a level floor with a constant force. The force does work on the object, which will increase the object’s kinetic energy. Since the floor is level the gravitational potential energy does not change. The other form of potential energy possible is that stored in a spring, which is also zero here. Assess: For there to be no potential energy change, the object in question must remain at the same height. Q10.4. Reason: Here we must increase potential energy without increasing kinetic energy. Consider lifting an object at constant speed. Consider the object plus the earth as the system. The force does work that increases the gravitational potential energy of the object, while the kinetic energy does not increase because the velocity of the object remains the same. Another possibility is the compression of a spring by an applied force at constant velocity. Note that constant velocity is not necessary for the change in kinetic energy between the beginning and end of a process to be zero. Lifting an object or compressing a spring in any way, as long as the initial velocity is equal to the final velocity at the end of the process leads to no change in net kinetic energy. Any kinetic energy

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## This note was uploaded on 11/30/2011 for the course PHYSICS 121 taught by Professor Shawhan during the Fall '10 term at Maryland.

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Chap 10 Solutions copy - ENERGY AND WORK 10 Q10.1. Reason:...

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