大学物理-质点和质点系的动量定理.ppt

1/14,力的累积效应 The accumulative effects of forces,对时间积累 The time accumulative effects of forces,对空间积累 The space accumulative effects of forces,动量(Momentum)冲量 (Impulse) 动量定理 (Theorem of momentum) 动量守恒 (Momentum conservation),动能(Kinetic energy) 功(Work) 动能定理 (Theorem of kinetic energy) 机械能守恒 (Mechanical energy conservation),2/14,一 动量(Momentum)冲量(Impulse) 质点的动量定理(Theorem of momentum of amass point),牛顿第二定律:动量为p的质点,在和外里F的作用下,其动量随时间的变化率等于作用在质点上的合外力.,Newtons Second law :When a mass point with momentum p is under the effect of the combined external force F, the time variation of the momentum is equal to the external force acting on the particle.,3/14,动量(Momentum),冲量(Impluse) (矢量Vector),力对时间的累积效应,物体由于运动具有的机械效果,Objects with the mechanical effect because of moving,The time accumulation effects of forces,4/14,质点的动量定理:在给定的时间间隔内,外力作用在质点上的冲量,等于质点在此时间内动量的增量.,Theorem of momentum of a mass point: In the given time interval, the impulse on a mass point by a external force is equal to the increment in the momentum of that mass point in the same time interval,5/14,在直角坐标系分量表示 In cartesian coordinate componet equations,质点在某方向上的动量增量,仅与质点在该方向上所受外力的冲量有关,说明(Description),The momentum increment of the mass point on some direction only depends on the impulse of the extermal force on the same direction,6/14,二 质点系的动量定理 (Theorem of momentum of the mass point system),对两质点分别应用质点动量定理,因内力,7/14,质点系动量定理:作用于系统的合外力的冲量等于系统动量的增量.,The theorem of momentum of the mass point system: The impulse of the combined external forces acting on the mass point system equals the increment in momentum of that system.,8/14,作用于质点系的合外力等于质点系动量随时间的变化率.,The combined external force acting on the mass point system is equal to the momentum variation rate of the mass point system with respect to time.,对于无限小的时间间隔内,质点系动量定理.,9/14,区分外力和内力,内力仅能改变系统内某个物体的动量，但不能改变系统的总动量.,注意,10/14,1、F 为恒力,2、F 为变力,讨 论,动量定理常应用于碰撞问题,注意,在p一定时, t 越小,则F越大,11/14,例 1 一质量为0.05kg、速率为10m/s的刚球,以与钢板法线呈45角的方向撞击在钢板上,并以相同的速率和角度弹回来.设碰撞时间为0.05s.求在此时间内钢板所受到的平均冲力,建立如图坐标系,方向沿x轴反向,解:由动量定理得,12/14,例 2 一柔软链条长为l,单位长度的质量为.链条放在桌上,桌上有一小孔,链条一端由小孔稍伸下,其余部分堆在小孔周围.由于某种扰动,链条因自身重量开始落下.求链条下落速度与落下距离之间的关系.设链与各处的摩擦均略去不计,且认为链条软得可以自由伸开 .,解 以竖直悬挂的链条和桌面上的链条为一系统,建立如图坐标,由质点系动量定