1. Unit 8
Impulse and Momentum

2. Outcomes

3. Part 1 – Forces Change Momentum Part 2 – Momentum is Conserved
Unit 8 - Momentum
Part 1 – Forces Change Momentum
Part 2 – Momentum is Conserved

4. Momentum “mass in motion”
All objects have mass; so if an object is moving, then it has momentum- it has mass in motion.
The amount of momentum an object has depends on two things
How much stuff is moving
How fast the stuff is moving

5. Momentum = Mass x Velocity
Symbol of momentum is “p”
Formula p = m v
Units kg x m/s = kg m /s
A 500 kg car traveling at 2 m/s
p = mv = 500kg x 2 m/s = 1000 kgm/s

6. Momentum = Mass x Velocity
If the velocity is zero (if the object is not moving) then the momentum is zero.
500kg x 0 m/s = 0 kgm/s
Inertia is a property of moving and non-moving objects. Momentum is a property of moving objects only.

7. If Velocity Changes, Momentum Changes
The formula for change in momentum is similar to that for momentum:
Δp = m Δ v
(Sometimes mass changes, but not very often, so we only consider those cases where velocity is changing.)

8. 500 kg car accelerates from 2 m/s to 10 m/s.
Δp = m Δ v
Δp = 500 (10-2) = 500(8) = 4000 kgm/s

9. What causes velocity to change?
A change is velocity is called an acceleration.
Accelerations are caused by a net FORCE.
If you apply a net force to an object, the velocity of the object will change and therefore its momentum will change.

10. F = m a a = Δv / t
The first equation tells us what “causes” acceleration.
The second equation tells us what acceleration “is”.
Together they state
F = m Δv / t

11. F = m Δv / t is messy because it has a fraction on one side.
It is more often stated in its easier form:
F t = m Δv

12. F t = m Δv
F t (the left side of the equation) is called the impulse. It is a force applied for a specific amount of time.
m Δv (the right side of the equation) already has a name – it is called change in momentum.
impulse = change in momentum.
Impulse = Force x time

13. A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s.
Find the impulse.
Impulse = force x time
Impulse = N (3 s) = Ns

14. A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s.
Find the change in momentum.
Impulse = change in momentum
Impulse = Ns
Change in Momentum = kgm/s

15. A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s.vi
Find the new velocity (or vf).
Ft=mΔv Δv = Ft/m
Δv = +1000(3)/ Δv = +6 m/s
Δv = +6 m/s vi = 2 m/s
vf=2+6 = 8 m/s
Δv = vf - vi

16. Change in velocity Remember Δv = vf – vi
If a car speeds up from 2 m/s to 10 m/s
Δv = vf – vi = 10 – 2 = +8 m/s
If a car slow from 10 m/s to 2 m/s
Δv = vf – vi = 2 – 10 = -8 m/s

17. Bouncing – change in velocity is large.
Vi = +10 m/s
Vf = - 6 m/s
Δv = (-6)-(10)
Δv = (-16) m/s

18. Assignment 7a
Due_____