1. Collisions Momentum is always conserved in collisions
Elastic Collisions - where kinetic energy before and after collision is the same; objects do not “stick” to each other
examples: billiard balls colliding on a pool table or gliders colliding on a frictionless air track

2. Collisions
Elastic Collisions:

3. Collisions
Inelastic collisions –where kinetic energy before and after the collision is not the same; objects “stick” to each other
examples: golf ball
colliding with a ball
of putty and sticking
together
In these cases, some of the energy is transformed into other types of energy such as thermal, potential, or sound energy

4. Collisions

5. Collisions In all cases, total energy of the system is conserved
Ideally, we can understand this conservation mathematically as:
m1v1i + m2v2i = m1v1f + m2v2f

6. Collisions Helpful Tips:
1. Determine if inelastic or elastic; sticking to each other or not; drawing a picture helps!
2. Determine total momentum before or total momentum after; use m×v for each “object”
3. Find what you’re missing

7. Collisions Practice Problems:
1. A 10 kg block traveling at 5 m/s collides with a 5 kg block traveling at 3 m/s in the same direction. The 10 kg block continues to move at 4 m/s. What is the velocity of the 5 kg block immediately after their elastic collision?
2. Consider a 42,000 kg train car travelling at 10 m/s toward another train car at rest with a mass of 28,000 kg. After the two cars collide, they couple together. What is the final velocity of the two train cars together?
5 m/s m/s