**Tip 4: Problems on collisions in 1-D**
**At first, we determine the system under observation. **

For any collision, the net momentum of the system (consisting of colliding particles) is always conserved if the net external forces are zero.

$P_{i}=P_{f}$

For elastic collisions, the coefficient of restitution $e=1.$

So we apply:

- Momentum conservation before and after the collision
- Kinetic Energy conservation before and after the collision. $K_{i}=K_{f}$

For completely inelastic collisions, the coefficient of restitution $e=0.$

- Momentum conservation before and after the collision is still valid
- The final velocity of both the colliding masses is same.