First slide

Concept of potential energy

Question

A bolt of mass 0.2 kg falls from the ceiling of an elevator moving down with an uniform speed of 5 ms^-1. It hits the floor of the elevator (length of the elevator = 5 m) and does not rebound. The amount of heat produced by the impact is (Take g = 10 ms^-2)

Moderate

A

5 J
B

10 J
C

15 J
D

20 J

Solution

Here, $m = 0 .2 kg, v = 5 {ms}^{- 1}$
h = length of elevator = 5 m
As relative velocity of the bolt w.r.t. elevator is zero, therefore, in the impact, only potential energy of the bolt is converted into heat energy.
Amount of heat produced
= Potential energy lost by the bolt = mgh
$= (0 .2 kg) (10 {ms}^{- 2}) (5 m) = 10 J$

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