Question

A bullet of mass 5 g travelling at a speed of 120 m s${}^{-1}$ penetrates deeply into a fixed target and is brought to rest in 0.01 s. Calculate the average force exerted on the bullet.

• A. 40 N
• B. 50 N
• C. 60 N
• D. 70 N

Answer 1

The correct option is C 60 N

Mass of the bullet $=$ m
$=5g=5\times {10}^{-3}kg$
Initial velocity of the bullet $=$ u
$=120m{s}^{-1}$
Final velocity of the bullet $=$ v
$=0m{s}^{-1}$
Time taken by the bullet to come to rest
$=t=0.01s$
Force exerted on the bullet $=$ F $=$?
We know, that
F $=$ ma
$\Rightarrow =m\left(\frac{v-u}{t}\right)$ ........... (1)
$[\because a=\frac{v-u}{t}]$
Substituting the above values in (1),
We get,
$F=5\times {10}^{-3}\left(\frac{0-120}{0.01}\right)$
$=(5\times {10}^{-3})\times (-12000)$
$=-5\times 12=-60N$
As 'F' is negative, it indicates that the retarding force is 60 N.
Therefore, the force exerted on the bullet by the target to stop it is 60 N.