Originally Posted by

**litterboy**
In perfectly elastic collisions, we know that when an object with mass m1 and speed u collides with another object with mass m2 which is stationary(where m1 is much larger than m2), m1 will continue moving at u while m2 will move with a speed of 2u.

We can see that the total momentum before collision in the system is m1*u+ m2(0)=m1*u

However, the total momentum after collision is m1*u + m2(2u)

They are totally inequivalent and there's no external participated in the collision!!

How does it obey the conservation of momentum???