First slide

Work

Question

A bullet moving with a speed of $150 m s^{- 1}$ can just penetrate into two planks of equal thickness. Then the number of such planks required, if speed is tripled will be

Moderate

A

9
B

18
C

27
D

6

Solution

$\begin{array}{l} W = ΔK . E \\ F . S = 0 - K . E \\ \frac{F \times N t}{F \times 2 t} = \frac{0 - \frac{1}{2} m \times (3 \times 150)^{2}}{0 - \frac{1}{2} m (150)^{2}} \\ N = 2 \times 9 = 18 \end{array}$

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