1.22 use the conservation of momentum to calculate the mass, velocity or momentum of objects

Conservation of momentum:
Total Momentum before Collision = Total Momentum after Collision.

The best way to understand this is through examples.

Example (Collision):

1. So the momentum of A is 2kg x 5m/s = 10 kg m/s

Momentum of B is 3kg x 5m/s = 15 Kg m/s

2.  The total momentum = 25 Kg m/s

3. So to fin out the velocity we first need to find the momentum of B after the impact.

• 3 kg x 7 m/s = 21 Kg m/s 
• 25 Kg m/s - 21 Kg m/s = 4 Kg m/s
• This means that the momentum of A after the impact must be 4 Kg m/s
• So 2 Kg x ?m/s = 4 Kg m/s 
• The answer is 2 m/s

Example (Explosion):

1. A gun's mass is 10 kg and a bullets mass is 0.01 kg

and the velocity and which the bullet travels out of the gun is 400 m/s

2.  Since the gun and bullet are stationary, the momentum for both of them is 0

3. When the bullet is released, it's momentum is 0.01kg x 400 m/s = 4

4. So to fulfil the conservation of momentum the momentum of the gun recoiling backwards has to be     -4
5. So 10kg x ?m/s = -4
6. ? m/s = -0.4m/s
It's fine that this number is a negative. Velocity is a vector quantity so the negative sign jsut means it is moving backwards.