Question

A body of mass 20kg is drop from a height of 20m above the ground .Calculate the kinetic energy at the moment when it strikes the ground.

Answer 1

The velocity by which it strike the ground is (2gh)^1/2

V=(2×10×20)^1/2

V=20m/s

Kinetic energy =1/2 mv^2

1/2.20.20.20

4000J

Answer 2

You can easily calculate it

we know potential energy mgh at 20m above ground will be mgh = 20×10×20 = 4000j

the moment he strike the ground , then it's height will be 0 so , whole Potential will change to kinetic energy

So kinetic energy = 4000j


Another method:-

Final velocity of stone when it reaches the ground =
${v}^{2} - {u}^{2} = 2as = v = 20$
then find kinetic energy
$\frac{1}{2} \times 20 \times 20 \times 20 = 4000j$