Question

A bullet of mass 0.1 kg is fired on a wooden block to pierce through it, but it stops after moving a distance of 50 cm into it. If the velocity of bullet before hitting the wood is 10 m/s and it slows down with uniform deceleration, then the magnitude of effective retarding force on the bullet is 'x' N. The value of 'x' to the nearest integer is __________.

Hint:

Retarding force $F = \frac{mv^2}{2d}$. It's often faster than using kinematic equations for acceleration first.

Answer 1

Correct Answer: 10

Step 1: Work-Energy Theorem: $W_{all} = \Delta K.E$.
Step 2: $-F \times d = 0 - \frac{1}{2}mv^2$.
Step 3: $F \times 0.5 = \frac{1}{2} \times 0.1 \times (10)^2 = 0.5 \times 10 = 5$.
Step 4: $F = 5 / 0.5 = 10 \text{ N}$.