A girl of mass 30 kg is playing on a reversible see-saw with a boy of mass 15 kg. When the girl comes down by 3 meters, the boy goes up by 6 meters.

Which of the statements is TRUE?

Potential Energy = Mass x Acceleration due to Gravity x Height above the ground.

As the girl comes down, she loses height, and hence loses Potential Energy.

So, the boy gaining height, and gains Potential Energy.

Gain in the Potential Energy of the boy = Loss in Potential Energy by the girl.

The energy lost by the girl is transferred to the boy and is gained by him.

How to Calculate Potential Energy

A stone of mass 4 kg is kept at a height of 3.5 m above the ground. Find its potential energy.

The Potential Energy of any object U = mgh

Here:

m is the mass of the object,

g is the acceleration due to gravity

h is the height from the ground

Substituting into the equation we get

U = m ✕ g ✕ h

U = 4 ✕ 10 ✕ 3.5

U = 140J