Question

子弹的速度为v时，打穿一块木板后速度为零，木板对子弹的阻力恒定，那么子弹射入木板一半时的速度为多少

A.v/2 B.v/24 C.v/3 D.根号2v/2

Answer 1

设木板厚度为D,木板对子弹的摩擦阻力为f
设所求速度为V1
(1/2)mV^2=fD
(1/2)mV^2-(1/2)mV1^2=f(D/2)=(1/2)fD=(1/4)mV^2
(1/2)V1^2=(1/2)V^2-(1/4)V^2=(1/4)V^2
V=V/(根号2)=(根号2)V/2
选D

Answer 2

D
fs=0.5mv^2
f0.5s=0.5m根号2v/2 ^2