Question

A block is suspended by a spring initially in natural length $l_0$. Take initial position of block as zero reference level for gravitational potential energy of block. The block is released from rest at its initial position. Now as the block comes down till the maximum downward displacement for the first time, the sum of the elastic potential energy of the spring and the gravitational potential energy of the block:

• A. increases
• B. decreases
• C. can either increase or decrease depending on the spring constant
• D. decreases first then increases

Answer 1

The correct option is D decreases first then increases

Total energy of the block is constant.

Hence, $E=\frac{1}{2}{mv}^2+U=constant$

As the block descends, the velocity will first increase and then decrease to zero. Hence, $U$ first decreases and then increases as shown in the diagram.