First slide

Concept of potential energy

Question

A 2kg body is projected vertically upwards with a velocity of $100 m s^{- 1}$ from ground .The ratio of potential energies 4s and 2s after projection is $(g = 10 m s^{- 2})$

Moderate

A

9:16
B

4:1
C

16:9
D

1:4

Solution

$\begin{array}{l} U = m g = m g (u t - \frac{1}{2} g t^{2}) \\ \frac{U_{2}}{U_{1}} = \frac{m g [100 \times 4 - \frac{1}{2} \times 10 \times 16]}{m g [100 \times 2 - \frac{1}{2} \times 10 \times 4]} \\ \frac{U_{2}}{U_{1}} = \frac{320}{180} = \frac{16}{9} \end{array}$

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