Potential Energy Class 9 Physics

Potential Energy

Hey students we already learned that, ‘the energy possessed by a body due to its shape/size/position or mass is known as potential energy’. E.g. stone at rest on the peak of mountain, water stored in reservoir or dams, we possessed potential energy while watching television you are sitting on chair etc. So let’s derive the formula for potential energy of a body

Imagine a boy of mass ‘m’ at rest on the surface of earth.

Let ‘g’ be the acceleration due to gravity, ‘h’ be the height of object above earth’s surface. Then weight of the body is nothing but the gravitational force acting on body at height ‘h above the earth’s surface, then amount of work done required to change the position of body from height ‘h’ to the surface of earth is given as,

W = Weight × height ……………..(1)

But  Weight = gravitational force = m × g

Then equation (1) becomes

∴ W = m × g × h ………………….(3)

We know that this work done is required to overcome the potential energy of body, in other words we can say that,

Amount work done = potential energy of body

∴Potential energy, E = m × g × h

SI unit of kinetic energy is joule (J).

CGS unit of kinetic energy is erg. Remember that 1 joule = 107 erg

Following are some numeric which will give us more details about kinetic energy.

Ex:1) Find the potential energy of stone mass 5 kg at rest on the peak of mountain of height 36 m. (Use g = 10 m/s2)

Ans: Here m = 5 kg

g = 10 m/s2

h = 36 m

The formula for potential energy is

∴ Potential energy, E = m × g × h

∴ E = 5 × 10 × 36

∴ E = 50 × 36

∴ E = 1800 J

Ex:2) Work done of 1200 J is required to lift the object of mass 500 g to height ‘h’. What is the value of ‘h’? (Use g = 10 m/s2)

Ans: Here m = 500 g = 0.5 kg

g = 10 m/s2

h = ?

The formula for potential energy is

∴Potential energy, E = m × g × h

∴ 1200 = 0.5 × 10 × h

∴ 1200 = 5 × h

∴ h = 1200/5

∴ h = 240 m

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