Question

Sir,A bullet loses 1/20th of its velocity while passing through a plank. How many total number of planks are requiredto stop the bullet?

Answer 1

the answer depends upon the mass of the bullet.
the speed n momentum by which the bullet hit the plank.
the air friction caused.
these r the factors required for a exact answer.

well the answer is 20 as if 1/20th part kept on decreasing every plank so
answer is 20

Answer 2

Let the velocity be x
Then loses in velocity while pass through plank=1/20*x
Therefore total no. of plank=x/1/20*x
=20 plank