动量守恒是有条件的,动量守恒条件分为三个层次:(1)系统所受合外力为零;(2)系统所受合外力虽然不为零,但在某方向合外力为零,则系统在该方向动量守恒;(3)系统所受合外力远远小于内力,则系统动量近似守恒.对于不同情况,应根据不同的条件去分析.在上述三种情况下,都可以应用动量守恒定律求解相应物理量.解题时,我们如果不注意这几点很容易导致错误,下面举例分析.例1一绳跨过定滑轮,两端分别栓有质 Momentum conservation is conditional, and the conditions of conservation of momentum are divided into three levels: (1) the external force exerted by the system is zero; (2) the external force exerted by the system is not zero, but the external force is zero in a certain direction; The momentum of the system in the direction of conservation; (3) the system suffered external force is far less than the internal force, the system is approximately conservation of momentum for different situations, should be based on different conditions to analyze in all three cases, the momentum conservation can be applied Law solves the corresponding physical quantity. Problem solving, if we do not pay attention to these points can easily lead to errors, the following example analysis. Example 1 A rope across the fixed pulley, both ends were tied quality