作者simcity2013 (ONLY THE STRONG SURVIVE)
看板TransPhys
标题[力学]
时间Thu Jan 23 19:56:12 2014
[单元] 刚体动力学
[来源] 刘宗儒精要
[题目]
A bullet of mass 4.0g and velocity 330m/s passes
through a wheel at rest on a rough floor.The wheel is a
solid disk of mass 2.0kg and radius 20cm.The bullet passes
through the wheel at a perpendicular distance of 19cm from
the center,and the bullet's final velocity is 220m/s.If the
wheel rolls without slipping,find the wheel's angular velocity.
[想法]
自由体图中有子弹给wheel的撞击力和wheel给子弹的反作用
力,而由於摩擦力相对撞击力来说微不足道,所以可忽略.因此系
统(子弹+wheel)所受合力为0,此时系统线动量守恒.
0.004 ×330 = 0.004 ×220 + 2 ×V(CM)
=> V(CM) = 0.22m/s
而由於为纯滚动所以ω=V(CM)/R = 0.22/0.2 = 1.09 rad/s
但答案不对.
不知是否是题目没说均质所以我擅自把它当均质来算导致错误
所以此题只能用角动量守恒得解.
不过在此发现有些题目没说它均不均质但它却是当均质来算,这要如何判断阿??
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1F：推 youtuuube000:为了要纯滚动地板的摩擦力似乎不能忽略 01/23 21:14
2F：→ youtuuube000:你对地面设轮子跟子弹的角动量守恒试试看答案对不对 01/23 21:16
3F：→ simcity2013:角动量守恒是Ok的 01/23 21:22
4F：推 youtuuube000:那看来是因为静摩擦力大到不可忽略的程度才能够构成 01/23 21:47
5F：→ youtuuube000:纯滚动 不然轮子底部会跟地板有相对移动 01/23 21:48