Question

A man is standing at a spring platform. Reading of spring balance is. If man jumps outside platform, then reading of spring balance:
(a) first increases and then decreases to zero
(b) decreases
(c) increases
(d) remains the same

Answer 1

Hint: In this question, we will analyse the reading of a spring balance on which a man is standing as the man jumps outside the platform. We will follow Newton's third law of motion in the solution to explain the consequence as the man jumps out of the platform of the spring balance. We will also discuss the frictional force that helps the man to stand on the spring balance.

Complete answer:
The man standing on the spring balance exerts a force equal to his body weight on the spring balance. Hence, in accordance with Newton's third law of motion, the spring balance also applies a force equal in magnitude at the man in the opposite direction. Therefore, the net force at the man is zero as the gravity and the force applied by the spring balance operate in opposite direction and cancel each other.
But when the man tries to jump outside of the platform, he applies force over the spring balance and hence the reading of the spring balance increases initially. But as the man’s feet leave the contact of the spring balance, the reading of the spring balance decreases to zero.

Option (a) first increases and then decreases to zero is the right answer.

Note:
The momentary increase in the reading of the spring balance is due to the extra force applied by the man’s legs over the platform of the spring balance. Newton's third law of motion also holds true in this situation as well because the platform also applies a force over the man that pushes him upwards to jump.