[理学]12质点动力学.ppt

1.2质点动力学 牛顿三个基本运动定律是整个动力学的基础 l复习牛顿三定律 l非惯性系与惯性力 l牛顿三定律的应用 1.2质点动力学 牛顿运动定律1. Newtons First Law, the Law of Inertia When there is no net force acting on an object, that object maintains its motion with a constant velocity. No net force doesnt means no force at all, just that whatever forces are presented add up to zero Constant velocity means both magnitude and direction Zero velocity is a special case of constant velocity This works both ways: if velocity is constant, then net force must be zero and if the net force is zero, then the velocity must be constant. 1.2质点动力学 Inertial Frames of Reference A frame of reference in which Newtons first law is valid is called an If V=Constant or V=0 They are inertial frames of reference non-inertial 1.2质点动力学 Some Demonstrations of First Law Flick the card! Hit a nail on head 1.2质点动力学 牛顿运动定律2. Newtons Second Law of Motion u外力作用下质点获得加速度An object acted upon by a net force accelerates 力是一个矢量Force add as vectors 合力的方向与加速度的方 向一致The net force points in the direction of the acceleration The force and accele- ration are related by m called inertial mass 1.2质点动力学 Units of force Acceleration is m/s2 Mass is in kg So, force is kg m/s2=Newton In term of the velocity, Newtons second law becomes 1.2质点动力学 Components of Newtons second law in Cartesian coordinate system in natural coordinate system Centripetal force tangential force 1.2质点动力学 牛顿运动定律3. Newtons third law(作用力与反作用力定律) Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the one it exerts. 3rd定律定义了相互作用的性质! 除上述性质外 作用力与反作用力本质相同 The tow forces are called an “action-reaction” pair. 作用力与反作用力（成对出现）大小相等、方向相反， 作用在不同物体上（不能平衡或抵消）。 1.2质点动力学 Some Demonstrations of third Law 1.2质点动力学 Inertial frame Very low speed, v m m 2 2 ）。求：（）。求：（1 1）物体）物体 相对于电梯的加速度。（相对于电梯的加速度。（2 2）绳子的张力。）绳子的张力。 解：解： 消去消去T T m1 m2 T T 惯性力 1.2质点动力学 例：水桶以 旋转，求水面形状？ 解：水面 z 轴对称，选柱坐标系。 任选水面一小质元，在切线 方向静止，在旋转参考系 r z mg mr2 1.2质点动力学 作业: 1-17,1-18,1-20, 1-22, 1-24,1-25,1-26 1.2质点动力学 课堂测试课堂测试1 1： 升降电梯相对于地面以加速度升降电梯相对于地面以加速度a a 沿铅直向上运沿铅直向上运 动。电梯中有一轻滑轮绕一轻绳，绳两端悬挂动。电梯中有一轻滑轮绕一轻绳，绳两端悬挂 质量分