Lesson_Text_3.2

# Lesson_Text_3.2 - 1 Lesson 3.2 Energy 1 Energy The concept...

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1 Lesson 3.2 Energy 1. Energy : The concept of energy is linked closely with the concept of work. Energy is required to do work. For example, you will not be able to lift a book, if you do not have energy. Thus we can give a qualitative definition to energy as Energy is the ability to do work . When you do work, energy is used up . What happens to the energy that is used up in doing work? For example when you lift a book, you do work against gravity. You spent energy to do this work. As a result of this work, the book is now placed at a position from where if you allow it to fall, it will do work on other objects. In other words, the book now has energy and can do work. Where did the book get its energy. The energy you spent in lifting the book has been transferred to the book, now the book has energy. In other words, energy does not get simply used up, it gets transferred from one source to another. When you do work to compress a spring, the energy you spent is transferred to the spring. A compressed spring has energy, it can do work when released. When you kick a ball, you do work on the ball. The energy you spent in doing this work has been transferred to the ball. This energy appears in the form of speed in the ball. A moving object has energy because it can do work on other objects in the process of coming to rest. For example, a moving rock will do work on you when you stop it. (Try it if you dare!). Since energy is the ability to do work, it is measured in the same unit as work. The unit of energy is the joule (J) Mechanical energy is classified into two main categories, namely potential energy and kinetic energy. 2. Potential energy : A raised book has energy because it is placed at a higher position relative to the ground. A compressed or a stretched spring has energy because its natural state has been altered. A raised book and a compressed or stretched spring both have potential energy .

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2 Potential energy is a measure of the energy possessed by an object by virtue of its position or state. It is a measure of the work done in placing the object in that position or state. You saw in lesson 7 that a book of mass m kg can be raised without producing an acceleration by applying an upward force equal to its weight, namely mg N. When you raise the book by a height h m, the work done by this force = m g h J. This is a measure of the gravitational potential energy of the book placed at that position. We will use U to represent potential energy. Gravitational Potential energy of an object of mass m kg placed at a height h above ground level is given by U = m g h Here ground level is chosen as the reference point where the potential energy is zero because it is from there that we lift the book. We are free to choose any other point as the reference point for the purpose of defining potential energy. 3.
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Lesson_Text_3.2 - 1 Lesson 3.2 Energy 1 Energy The concept...

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