1. Momentum Conservation

2. Two Balls
Two balls fall at the same rate due to gravity, but with different momenta.
Ball 1 bounces from the ground.
Ball 2 bounces from ball 1.
pf2
pi2
pf1
pi1

3. Two Bodies
When two bodies strike each other they exert a force on each other.
The forces are equal and opposite.
F12
F21

4. Momentum Change
If two forces are equal in magnitude, then the changes in momentum are equal in magnitude.
F12
pf1
pi2
pi1
pf2
F21

5. Internal Forces Two objects can be considered together.
This is called a system.
The internal forces cancel out from the law of reaction.
The external forces remain.
F12
Fg2
F21
Fg1

6. Isolated System A system with no external forces is isolated.
Only internal forces that cancel out
The net change in momentum for all internal forces must be zero.
Total momentum in an isolated system is constant.
Zero change means constant value

7. Law of Reaction Redefined
The law of reaction can be defined in terms of momentum.
In an isolated system the total momentum is conserved.
total momentum of the rocket and gas stays the same
rocket
hot gas is forced out
rocket momentum increases
reaction force acts on the rocket

8. Double Bounce
Initially both balls go down together with the same velocity.
The lower ball gets a force from the floor and changes momentum.
The lower ball hits the upper ball and momentum is conserved between the balls.
pi2
pf2
pi2
pi1
pf1
pm1
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