Question

A bullet moving with a velocity of 100 m/s can just penetrate two planks of equal thickness. The number of such planks penetrated by the same bullet, when the velocity is doubled, will be –

• A. 4
• B. 6
• C. 8
• D. 10

Answer 1

Answer: (C)

8

Answer 2

Let the thickness of each plank = x then from third equation of motion,

0 = u² – 2a (2x)

 u² = 4ax ...(i)

If velocity is doubled, let n planks are required

then,

0 = (2u)² – 2a(hx)

 n = $\frac{4u^{2}}{2ax}$ = $\frac{4(4ax)}{2ax}$ = 8