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Question

# (a) Define the term 'energy' of a body. What is the SI unit of energy. (b) What are the various forms of energy? (c) Two bodies having equal masses are moving with uniform speeds of v and 2v respectively. Find the ratio of their kinetic energies.

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Solution

## (a) Energy of a body is defined as the capacity of the body to do work. Its SI unit is Joule and it is represented as 'J'. (b) Following are some of the forms of energy: Thermal energy, chemical energy, electric energy, electromagnetic energy, nuclear energy, magnetic energy, mechanical energy, sound energy, etc. (c) The kinetic energy of a body of mass 'm', moving with a velocity 'v' is given by $\frac{1}{2}m{v}^{2}$. As both the bodies have equal masses, let's say 'M', then the ratio of their kinetic energies = $\frac{{\left(K.E.\right)}_{1}}{{\left(K.E.\right)}_{2}}=\frac{\frac{1}{2}M\left(v{\right)}^{2}}{\frac{1}{2}M\left(2v{\right)}^{2}}=\frac{v{}^{2}}{4{v}^{2}}=\frac{1}{4}$ Hence, the ratio of kinetic energies of both bodies is 1:4.

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