Question

if a mass of 4 kg is dropped from a height of 100 meters, What is its velocity when it touches to the ground

Answer 1

conservation of energy concept is used

Answer 2

Potential Energy = mass * height * acceleration due to gravity
= 4* 100 * 10 [ g ]
= 4000 J.
Now, 
The potential energy of the mass at rest at 100 metre height will be the same as the kinetic energy of the mass when it will touch the ground.

So, Kinetic energy = Potential Energy 
Kinetic energy = 4000 J.
1/2 * mass * velocity^2= 4000 J
1/2 * 4 * velocity^2 = 4000 J
Velocity^2 = 2000 J
Velocity = 20 root 5 m/s
Velocity of the mass when it touches the surface will be 20 root 5 = 44.6 m/s.