Question

A bullet moving at $20m/sec$. It strikes a wooden plank and penetrates $4cm$ before coming to stop. The time taken to stop is:

• A. $0.004sec$
• B. $0.016sec$
• C. $0.008sec$
• D. $0.002sec$

Answer 1

Answer: (A)

$0.004sec$

Given,

$u=20m/s$

$s=4cm=0.04m$

$v=0m/s$

$t=?$

From 3rd equation of motion,

$2as=v^2-u^2$

$a=\frac{v^2-u^2}{2s}$

$a=\frac{0^2-20\times 20}{2\times 0.04}$

$a=-5000m/s^2$

From 1st equation of motion,

$v=u+at$

$0=20-5000t$

$5000t=20$

$t=\frac{20}{5000}$

$t=0.004sec$

The correct option is A.