1. 6 – 1 Momentum and Impulse 6 – 2 Conservation of Momentum
Chapter 6
6 – 1 Momentum and Impulse
6 – 2 Conservation of Momentum

2. Momentum: the product of an object’s mass and velocity, a vector
p = mv
SI unit = kgm/s
If the boulder and the boy have the same momentum will the boulder crush the boy?
Hint: Which would have the larger speed?

3. To change an object’s momentum you must apply a force
Example:  Wall exerts a force of 10,000 N. The contact time is 0.01 s.
I = Impulse = F t            = 100 Ns

4. D(p) = Change in momentum
= (mv)final - (mv)initial
 = 0 - mv = - mv
Ignoring negative sign: D(mv) = mv

5. Impulse-momentum theorem: Impulse = change in momentum
FDt = Dp
Only valid when force is constant

6. Impulse and Contact Time

7. Ex: A 0.11 kg softball moving at 12 m/s is hit by the batter; afterward the ball flies off at 15 m/s. If the time interval was 1.3 x10-3 s what force did the batter apply to the ball?
Given: m = 0.11kg, vi = 12 m/s, vf = -15 m/s, Dt = 1.3 x10-3 s F = ?
FDt = Dp
F = Dp/ Dt
F = (pf – pi )/ Dt
F = m(vf – vi )/ Dt
F = 0.11kg (-15 m/s – 12 m/s) / 1.3 x10-3 s
F = x103 N

8. Law of conservation of momentum: the total momentum of all objects interacting with each other is constant provided no external forces are involved
pi = pf (in the absence of friction)

9. For example – firing a rifle
Momentum Before = Momentum After = After firing, the opposite momenta cancel.

10. pi = pf
0 = M v + m V
- M v = m V
v = m V/ - M
v = (0.010 kg)(300 m/s )/ - (4 kg)      
    v = -3 / 4 m/s
= m / s