A bullet moving at  a speed of 20m/sec  strikes a wooden plank and penetrates 4cm before coming to stop. The time taken to stop is

given initial velocity of bullet=u=20 m/sec displacement=    s=4×10−2m Final velocity zero v=0equation of motion is v2−u2=2asacceleration=a=02−u22sa=−20×202×4×10−2a=5000 m/s2 equation of motion    v=u+at0=u+at ⇒t=−ua t=−20−5000 t=0.004 sec=time taken to stop bullet