Question

A ball falls to the ground as shown below:



(a) What is the kinetic energy of ball when it hits the ground?
(b) What is the potential energy of ball at B?
(c) Which law you have made use of in answering this question?
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Answer 1

(a) In the given case, the mechanical energy of the ball will always remain conserved. Hence, the sum of the potential energy and the kinetic energy will always remain constant.

At the highest point A, total mechanical energy = Potential energy + Kinetic energy = 80 J + 0 J = 80 J

At the lowest point, total mechanical energy = total mechanical energy at the highest point

So, at the lowest point, Potential energy + Kinetic energy = 80 J = 0 + Kinetic energy

Hence, at the lowest point, kinetic energy is 80 J.

(b) As we know that mechanical energy is constant throughout the motion.

So, mechanical energy at point B = Mechanical energy at point A

Mechanical energy at point B = Potential energy at point B + Kinetic energy at point B = 80 J

Potential energy at point B = 80 J $-$ 48 J = 32 J

(c) The law of conservation of mechanical energy has been used while answering the above questions.