1. Chapter 8
Impulse and Momentum

2. Momentum and Collisions
This chapter is concerned with inertia and motion. Momentum helps us understand collisions.
Elastic Collisions - objects rebound
Inelastic Collisions - object stick together an usually become distorted and generate heat

3. Momentum Momentum = mass ´ velocity p = mv
Momentum is a vector quantity.

4. Large Momentum Examples
Huge ship moving at a small velocity
High velocity bullet
P = Mv
P = mv

5. Momentum Examples
A large truck has more momentum than a car moving at the same speed because it has a greater mass.
Which is more difficult to slow down? The car or the large truck?

6. Impulse Newton’s Second Law can read SF = ma = m(Dv/Dt) = (Dmv)/(Dt)
= (Dp/ Dt)
Rearranging,
Impulse = Dp = FDt

7. When Force is Limited Apply a force for a long time. Examples:
Follow through on a golf swing.
Pushing a car.
FDt

8. Make it Bounce
Dp = p2 - p1 = -p1 - p1
= -2p1 p1
p2 = -p1

9. Minimize the Force
Increase Dt
Catching a ball
Bungee jumping
FDt

10. Maximize Momentum Change
Apply a force for a short time.
Examples:
Boxing
Karate
FDt

11. Conservation of Momentum
This means that the momentum doesn’t change.
Recall that SF t = D(mv), so SF = 0
In this equation, F is the "external force."
Internal forces cannot cause a change in momentum.

12. Examples Example 1: a bullet fired from a rifle
Example 2: a rocket in space

13. Collisions
Before m1 m2
After m1 m2

14. Inelastic Collisions v = 10 v = 0 M M Before Collision p = Mv v’ = 5 M
After Collision
p = 2Mv’
Mv = 2Mv’
v’ = ½ v

15. Elastic Collisions Conserve Energy and Momentum Before Collision
Equal masses
Case 1:
Case 2:
M > M
Case 3:
M < M

16. Coefficient of Restitution
For perfectly elastic collisions e = 1.
If the two object stick together, e = 0.
Otherwise 0 < e < 1.

17. Center of Mass

18. On to problems...