Question

An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

Answer 1

Potential energy of the object = m × g × h

= 40 kg × 10 ms^-2 × 5 m

= 40 × 10 × 5 kg m^-2m

= 2000 J

Let v be the velocity of the object at half the height, i.e., after travelling 2.5 m.

Then v² - u² = 2as

v² - 0² = 2 × 10 ms^-2 × 2.5 m

So, v² = 50 m² s^-2

Then, 

Kinetic energy `1/2 mv^2`

= `1/2 xx 40  kg xx 50  m^2 s^-2`

= 1000 J     ...(∵ 1 J = 1 kg m² s^-2)