Question

a bullet moving at 20m/sec strikes the wooden plank and penetrates 4cm before coming to stop the time taken to stop is

Answer 1

$s = \frac{1}{2}( u + v ) t$

[tex]t = \frac{2s}{u+v} [/tex]

Putting right things at right places, you get $t = 0.004 s$

Answer 2

u = 20 m/s
v = 0
s = 4cm = 0.04m

v² - u² = 2as
⇒ 0 - 20² = 2×a×0.04
⇒ -400 = 0.08a
⇒ a = -400/0.08 
⇒ a = -5000 m/s²

v = u + at
⇒ 0 = 20 + (-5000)×t
⇒ -5000t + 20 = 0
⇒ -5000t = -20
⇒ t = 20 / 5000 
⇒ t = 0.004 second