1. Conservation of Momentum
6-2

4. Momentum is conserved
The total momentum before a collision is equal to the total momentum after the collision.

6. Momentum is conserved during collisions
Unless stated in a problem, disregard friction
Momentum is conserved for objects pushing away from each other.

8. Answers to Practice 6D, p. 219

9. Newton’s third law leads to the conservation of momentum
During collision for car one 
During collision for car two 

10. This equation means that if one object gains momentum the other must lose momentum.
Rearrangement of the equation gives the law of conservation of momentum.

11. Forces in real collisions are not constant.
When solving impulse problems use the average force.