Question

a bullet is moving at 20m/s strikes a wooden plank and penetrates 4 cm before coming to stop the time taken to stop is

Answer 1

Speed of bullet at the start= u = 20 m/s

Penetrated into Plank = 4 cm

Speed of bullet after penetrating the plank = v = 0

${v}^{2} - {u}^{2} = 2as \\ 0 - 400 = \frac{2 \times 4 \times a}{100} \\ a = - 5000 \: {ms}^{ - 2}$

$t = \frac{v -u}{a} \\ t = \frac{ - 20}{ - 5000} \: sec \\ t = \frac{1}{250} sec$

Answer 2

Answer: t = 0.004 s.

Explanation: Here, u = 20 m/s, v = 0 m/s [rest] and S = 4 cm or 1/25 m. We have to find out 't'.

We know,

2aS = v^2 - u^2

→ a = - u^2/2S

→ a = - (20 m/s)^2/(2 × 1/25 m)

→ a = - (400 m^2/s^2)/(2/25 m)

→ a = - 5000 m/s^2

→ (v - u)/t = - 5000 m/s^2

→ (- 20 m/s)/t = - 5000 m/s^2

→ t = 20/5000 s or 0.004 s.