A rifle bullet loses 1 25 t h of its velocity in passing through a plank. The least number of such planks required just to stop the bullet is

Step-by-step solution

If u   is the initial velocity of bullet then after passing through one plank the velocity became v = u - u 25 = 24 u 25 Using kinematic equation for uniform acceleration motion, v 2 - u 2 = 2 a s 24 u 25 2 - u 2 = 2 a s ⇒ 2 a s = u 2 - 625 + 576 625 ⇒ 2 a s = - u 2 49 625 If n   number of planks are required to bring the bullet at rest then, v = 0 ⇒ 0 - u 2 = 2 a n s ⇒ - u 2 = n - 49 u 2 625 ⇒ n = 625 49 = 12 . 75  or  13 Thus, 13   planks will be required in