Question

a rifle losses one divided by 20 of its velocity. how many planks just required to stop the bullet

Answer 1

Let the thickness of the plank be s m

Initial velocity = v m/s

Final velocity after one plank = 19v/20 m/s

(19v/20)^2 - v2 = 2as

Let the number of planks required to stop the bullet be n

0 - v2 = 2nas


n = -v2/(19v/20)^2 - v2
= 10.25

The bullet will definitely stop by the eleventh plank

Answer 2

Let the thickness of plank be x.
Then,
initial velocity(u)=u
then final velocity(v) after 1 plank
=u-u/20=19u/20
So,
a=(v^2-u^2)/2x
=(361u^2/400 - u^2)/2x
=(-39u^2)/800x
So,
Distance travelled by bullet till its v=0
s=(0-u^2)/2*(-39u^2)/800x
s=400x/39
So,
no. of panks=500x/(39*x)
=400/39=10.5 (approx)
So,
minimum 11 planks are required..

Plz mark it as brainliest