1. Section 9.2 Conservation of Momentum
 Objectives
Relate Newton’s third law to conservation of momentum.
Recognize the conditions under which momentum is conserved.
Solve conservation of momentum problems.

2. TWO PARTICLE COLLISIONS
I suggest you read chapter 9 from the old book.
The forces that 2 objects exert on each other are equal but opposite in direction according to Newton’s 3rd Law.
So with figure 9-6 FDonC = -FConD
That leads to
pCF – pCI = -(pDF – pDI)
Or pCF + pDF = pCI + pDI
This tells us that the sum of the momenta of the balls is the same before and after the collision.
Conserved Properties – properties that are the same before and after an interaction. Examples are energy and momentum.

3. MOMENTUM IN A CLOSED, ISOLATED SYSTEM
Closed System – a system that does not gain or lose mass.
Isolated System – a closed system on which the net external force is zero.
Closed Isolated System – collection of objects such that neither matter nor energy can enter or leave the collection.
Law of Conservation of Momentum – states that the momentum of any closed isolated system does not change.
pAI + pBI = pAF + pBF
mAvAI + mBvBI = mAvAF + mBvBF

4. MOMENTUM IN A CLOSED, ISOLATED SYSTEM
Example Problem 2 p. 237
mAvAI + mBvBI = mAvAF + mBvBF
1875(23) (17) = 1875vF vF
= 2900vF
60550 = 2900vF
20.88 m/s = vF
Do Practice Problems p. 238 # 13-18

5. RECOIL
The momenta of the skaters after the push are equal and opposite in direction.

6. PROPULSION IN SPACE Example Problem 3 p. 240
mAvAI + mBvBI = mAvAF + mBvBF
84(0) (0) = 84vAF (-875) since gas shoots
-84vAF = backwards sending
vAF = .365 m/s Astronaut forward
Do Practice Problems p. 240 # 19-21

7. TWO DIMENSIONAL COLLISIONS
Skip Example 4
Skip Practice Problems p. 243 # 22-25

8. CONSERVATION OF ANGULAR MOMENTUM
Angular Impulse – the product of the torque and the time interval.
Angular Momentum – product of the moment of inertia and the angular velocity.
Angular Impulse Angular Momentum Theorem – states that the angular impulse is equal to the object’s change in angular momentum.
The Law of Conservation of Angular Momentum – states that if no net external Torque acts on an object, then its angular momentum does not change. An object’s initial angular momentum is equal to its final angular momentum L1 = L2
Don’t worry about these formulas to much.
LI = LF
IIωI = IFωF
ωF / ωI = II / IF  
f = ω / 2Π
fF / fI = II / IF

9. TOPS AND GYROSCOPES
Because of the conservation of angular momentum, the direction of rotation of a spinning object can be changed only by applying a torque.
SKIP 9.2 Section Review p. 245 # 26-31