Conservation of Momentum有邊幾個情況

Basically, there are three kinds of collision.

1. elastic collision
the total kinetic energy of collided objects conserves
2.inelastic collision
the total kinetic energy of collided objects decreases
3.explosion
the total kinetic energy of collided objects increases

However, the total monentum of the collided objects are conserved (unchange) unless there are external forces applied.

When doing calculcation about monentum, you should find out the velocity and mass of the object before or after the collisions respectively.

the general equaion:total mv before collision=total mv after collision

例如, 車a(1000kg, 30m/s) 撞車b(1500kg, at rest) 佢地stick together
搵佢地撞後既速度

1000x30+0x1500=(1000+1500)v

但若果車a 反彈(10m/s)
要搵車b既速度

咁就
1000x30+0x1500=1000x-10+1500v

若果車b係迎頭撞(10m/s) 咁就
1000x30+ -10x1500=1000x-10+1500v

velocity係有方向的

Under all circumstances, momentum MUST be conserved.