Question

A rifle bullet loses (1/10)th of its velocity in passing through a plank. The least number of such planks required just to stop the bullet is 

(A) 5 

(B) 10 

(C) 11 

(D) 20

Answer 1

Correct option: (A) 5

Explanation:

Let thickness of one plank be s and such n planks are arranged to stop the bullet.

hence V = 0 here V² = V₀² + 2as×n i.e. 0 = V₀² + 2ans i.e.

n = {(– V₀²) / (2as)} ______ (1)

bullet loses (1/10)th of velocity

 ∴        V = (9 / 10)V₀ --- velocity in passing through plank

∴        {(9 / 10)V₀}² = V₀² + 2as

{(– 19) / (100)}V₀² = 2as

 pulling in eq. (1)

n = [{– V₀²} / {– V₀²(19 / 100)}] = {(100) / (19)} = 5