# Question #22770

##### 1 Answer

Nov 18, 2016

Let

If the average resisting force acting within plank be F then by law of conservation of energy we can write

If n such planks are required to stop the bullet (that means to gain zero kinetic energy) then

Dividing (2) by (1) we get

As n represents number of planks required to stop the bullet , it should be a whole number just greater than 20.25.

So n should be