Question

A bullet travelling horizontally looses 1 / 20^th of its velocity while piercing a wooden plank. Then the number of such planks required to stop the bullet is 

1) 6 

2) 9 

3) 11 

4) 13

Answer 1

Correct option is 3) 11 

After piercing it is decelerating so acceleration is negative.

So, the equation of motion:

v² = u² - 2as

⇒ \(\frac {19^2 u ^2}{20 ^2} = u^2 - 2 \,as\)

or, \(2as=\frac {39}{400} u^2\)

Now let's assume the bullet will pass through n number of blocks. So the net distance traveled by the bullet will be ns and finally, it will come to rest.

So, using the equation of motion:

\(v^2=u^2−2as\)

⇒ 0 = u² − 2ans

Now, we have

\(n = \frac {u^2}{2as} ≈11\).