Question

A bullet of mass 200 g moving with a velocity of 100 ms-, penetrates into a
number of wooden planks each of thickness 2 cm and offering constant
retardation. Bullet comes to rest when it just enters the 25th plank. The
magnitude of resistive force offered by the plank is about

Answer 1

Explanation:

Mass (m) = 50 g = 0.05 kg

Initial velocity (u) = 100 m/s

Final velocity (v) = 0 m/s

Distance penetrated (s) = 0.02 m

[i] Bullet's Initial momentum :

mu = 0.05 × 100 = 5 kg.m/s

[ii] Bullet's Final momentum :

mv = 0.05 × 0 = 0 kg.m/s

[iii] Retardation caused by the wooden block :

We know, v² - u² = 2as

Or, (0)² - (100)² = 2×a×0.02

Or, - 10000 = 0.04a

Or, a = - 250,000

Here, negative sign (-) actually indicates negative acceleration or retardation which actually says that there is a decrease in velocity for which it undergoes retardation.

Numerically, we can say, retardation caused by the wooden block is 2.5×10^5 m/s²

[iv] Resistive force (F) = ma

Just change the values according to your question:)