[力學]
看板TransPhys作者simcity2013 (ONLY THE STRONG SURVIVE)時間10年前 (2014/01/23 19:56)推噓2(2推 0噓 3→)留言5則, 2人參與討論串1/2 (看更多)
[單元] 剛體動力學
[來源] 劉宗儒精要
[題目]
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]
自由體圖中有子彈給wheel的撞擊力和wheel給子彈的反作用
力,而由於摩擦力相對撞擊力來說微不足道,所以可忽略.因此系
統(子彈+wheel)所受合力為0,此時系統線動量守恆.
0.004 ×330 = 0.004 ×220 + 2 ×V(CM)
=> V(CM) = 0.22m/s
而由於為純滾動所以ω=V(CM)/R = 0.22/0.2 = 1.09 rad/s
但答案不對.
不知是否是題目沒說均質所以我擅自把它當均質來算導致錯誤
所以此題只能用角動量守恆得解.
不過在此發現有些題目沒說它均不均質但它卻是當均質來算,這要如何判斷阿??
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