Question

A bullet emerge from a barrel of length $1.2\, m$ with a speed of $640\, ms ^{-1}$. Assuming constant acceleration, the approximate time that it spends in the barrel after the gun is fired is

• A. $4\, ms$
• B. $40\, ms$
• C. $400$ $\mu\,s$
• D. $1\,s$

Answer 1

Answer: (A)

$4\, ms$

Answer 2

Given that, $s=1.2\, m$
and $v=640\, ms ^{-1} a=? ; u=0 ; t=?$
We have the third equation of motion
$2 a s =v^{2}-u^{2}$
$2 a \times 1.2 =640 \times 640$
or $a =\frac{8 \times 64 \times 10^{3}}{3}$
and by first equation of motion
$v =u +a t$
or $t =\frac{v}{a}=\frac{15}{4} \times 10^{-3}$
$=3.75 \times 10^{-3}$
$s=4\, ms$