momentum

動量(Momentum)不一定會守恒(conserve)。
動量 p = mv，可以表示物體的運動狀況，當有淨力作用於物體時，物體的運動狀況便可以改變。
所以動量守恒定律是，當物體不受外力作用時，碰撞前及碰撞後物體的總動量才會保持不變。


例如運動中的物體，它的動量為 mv，若受力作用而加速或減速，而它的速度改變了，它的動量便改變了。

不一定conserve架,視乎情况而定;
Conservation of momentum required the total initial momentum equal to the final total momentum.
If there exist external force, like friction, the velocity of the objects may change, so momentum may not conserve.

Momentum is conserve if and only if the action is work in a close system and no any other external force is applied.

NO!if there are external forces acting on the system,the total linear momentum DOES NOT CONSERVE.

Principle of Conservation of Linear Momentum:
The total linear moment of A SYSTEM is only conserved when there are no external forces acting on it.