1. Momentum and its Conservation
Chapter 9

2. Momentum p = m v vector An object’s mass times its velocity
Moving inertia
Inertia is an object’s ability to oppose a change in its motion (Newton’s 1st Law)
Mass is most common way to measure inertia, and velocity is the most common way of measuring motion
No motion, no momentum
Momentum
p = m v
p : momentum (kgm/s)
m : mass (kg)
v : velocity (m/s)
vector

3. Impulse-Momentum Theorem
Recall Newton’s Second Law:
F = ma = m Dv/Dt
Newton actually wrote it as F = Dp/Dt, where Dp = pf – pi
Rearranging, Dp = FDt = I (Impulse)
Impulse is defined as the change in momentum of an object (same units as momentum)

4. Clips Golfball : ~http://www.youtube.com/watch?v=2Y57pw_iWlk
Billiards :
Water balloon :

5. Conservation of Momentum
Momentum must be conserved in a closed, isolated system (perfect physics world).
Closed: nothing gets out (no loss of mass)
Isolated: nothing gets in (no external forces)
Initial momentum must equal the final momentum.
pi = pf

6. Impact Lab Ball 1 Mass of 1 Ball 2 Mass of 2 Predict Observe
Procedures
Set one end of the ramp on a book.
Place one ball 30 cm from the end of the ramp on the floor.
Place another one at the top of the ramp. Predict the outcome.
Release the ball and observe the collision.
Repeat for other collisions.
Record each mass.
Impact Lab
Materials
Ramp
Soft, tennis, golf, and/or practice ball
Scale
Objective
Observe collisions between two objects
Ball 1
Mass of 1
Ball 2
Mass of 2
Predict
Observe

7. Angular Momentum Conservation of Angular Momentum: Li = Lf
Just like linear momentum, angular momentum must also be conserved.
p = m v  L = I w
Notice: same basic equation
L : angular momentum (kgm2/s)
I : moment of inertia (kgm2)
w : angular velocity (rad/s)
Conservation of Angular Momentum: Li = Lf

8. Ch9 HW Answers s
kgm/s, 31,521 N
kgm/s, 10 m/s N
m/s
kgm/s; -100 N; -10 N
x10-23 kgm/s; 7.8 N
70. 1,300 s or 22 min
m/s
m/s
x104 kgm/s; 312 N
m/s or 5 m/s west
Note: signs may be different!