20.A bullet loses 1/20 of its velocity on passing through a plank. The least number of planks required to stop the bullet is
solution: velocity of bullet after passing from plank ,its velocity decreases . initial velocity(u)=u final velocity(v)=initial velocity-1/20 of initial velocity v=u-u/20 v=19u/20 we have formula for one plank, (v)2-(u)2=2as, (19u/20)2-(u)2=2as 39u2/400=2as ……….(1) for n planks: then width of one plank will be ‘s’ width of n planks will be ns after passing from n planks …