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系统不受外力或所受外力的合力为零.这个系统的动量就保持不变.这个结论叫做动量守恒定律.它的数学表达式为这个等式左边是系统的物体在相互作用前的总动量,右边是系统的物体在相互作用后的总动量,表达式是表示系统的动量保持不变,而不是组成系统的各个物;体的动量在互相作用前后保持不变.下面着重阐述在理解和运用动量守恒定律时应注意的几个问题.1.条件性.如同一切定律、定理都有适用条件一样,动量守恒定律也有适用条件:系统不受外力或所受外力的合力为零,系统的动量守恒,在实际应用中,当物体系所受外力之和远小于内力大小的情况下,也可近似认为总动量守恒;在所受外力之和不为零,但在某个方向上分力之和为零,或分力之和远小于内力大小,则总动量在该方向的分量保持不变.例 1.一门大炮质量为M,由静止沿倾角为θ的光滑斜面滑下L米后,沿水平方向发射质量为m的炮弹,开炮瞬间炮身停下来,求炮弹速度.析与解:此题多数学生都知道炮弹和炮所组成的系统为研究对象,选地为参照物;选炮弹开始运动到离开炮膛时为作用时间,利用动量守恒定律求解,问题在于什么方向守恒.
The system is free from external forces or the resultant force of external forces. The momentum of this system remains unchanged. This conclusion is called the law of conservation of momentum. Its mathematical expression is that the left side of this equation is the total momentum of the system’s objects before the interaction. The right side is the total momentum of the system’s objects after the interaction. The expression is that the momentum of the system remains unchanged, not the various components of the system; the momentum of the body remains constant before and after interaction. The following focuses on understanding and Several issues that should be noted when using the law of conservation of momentum. 1. Conditionality. Like all laws and theorems have applicable conditions, the law of conservation of momentum has applicable conditions: the system is free from external force or the resultant force of the external force is zero, the system’s Conservation of momentum, in practical applications, when the sum of the external forces received by the object system is much smaller than the magnitude of internal forces, it can be approximately considered that the total momentum is conserved; the sum of external forces received is not zero, but the force component is in a certain direction. The sum is zero, or the sum of the force components is much smaller than the magnitude of the internal force, then the total momentum in this direction remains the same. Example 1. Mass of a cannon is M, and the L is slid down by a smooth slope with the inclination θ at rest. Rear In the horizontal direction, artillery shells with a mass of m are fired, and the artillery body is stopped instantly to find the speed of artillery shells. Analysis and Solution: This question most students know that the system of artillery shells and guns is the object of study and the place of selection is the reference object; The time from the start of the movement of the projectile to the time it left the artillery shell is solved using the law of conservation of momentum. The question is what direction conservation.