1. Momentum, Force and Impulse
CHAPTER 15

2. Momentum
Momentum is a measure of an object’s motion, and how likely it is that the object will remain in motion
Objects with a lot of momentum will take more force and longer to stop.
You slow momentum by applying friction (2 objects pressing against each other)

3. momentum = velocity * mass
Calculating Momentum
momentum = velocity * mass
momentum (p) → kg*m/s
mass (m) → kg
velocity (v) → m/s
momentum will increase as the velocity gets bigger or the mass gets bigger

4. Practice A 60kg girl jogs at a pace of 4m/s. What is her momentum?
Formula:
Variables

5. Practice
An adult, female fly has a mass of kg. While flying, her velocity is 5 km/hour (or 1.5 m/s). What is her momentum?
Formula:
Variables

6. Bill Nye Time!
Complete your questions while you watch.

7. Changing Momentum is Related to Force
15.2

8. Force Force is any push or pull on an object.
The size of the force required to move an object depends on how long you apply force, the mass of the object, and how fast you want to make the object go.

9. Force Forces can be large or small
They can also act for a short time or a long time.
Ex. If you brake gradually for a long time (small force X long time), or if your brake quickly ( large force X short time), the result is the same.
The important measurement is the IMPULSE→ the combination of the size of the force and the time the force acts

10. force = mass *change in velocity
Calculating Force
force = mass *change in velocity
time
mass (m) → kg
velocity (v) → m/s
time (t) → s
force (f) → newtons (N)
changing speed in a longer time → reduces the force!

12. Practice
What force is involved in stopping a 1200 kg car travelling at 11 km/h (28m/s) in 25 seconds?
Formula:
Variables

13. Impulse
15.2

14. impulse = force *time = change in momentum
Calculating Impulse
impulse = force *time = change in momentum
impulse (I) → kg*m/s
time (t) → s
force (f) → newtons (N)
Impulse will increase as force is applied for a longer time or of the force gets bigger

15. Increase time → decrease Force!
Why Care?
In a collision the change in momentum can not be changed therefore the impulse can not be changed.
However the idea of impulse can be used to reduce the force upon impact.
Increase time → decrease Force!

17. Practice
A car collides with another car. The force of the collision is 2000 kg∙m/s2. The collision lasts for 5 seconds. What is the impulse?
Formula:
Variables
What is the relationship between the force of impact and the impulse number?

18. Conservation of Momentum
15.2
image from:

19. What happens to the momentum in each of the following collisions:
COLLISION ONE (Head- on)
m = 20 kg
v = 3 m/s
p = +60kgm/s
m = 20 kg
v = 0 m/s
p = 0kgm/s
m = 20 kg
v = 0 m/s
p = 0 kgm/s
m = 20 kg
v = - 3 m/s
p = -60kgm/s
Total p before = (+60 kgm/s) + (-60 kgm/s) = 0 kgm/s
Total p after = 0 kgm/s

20. What happens to the momentum in each of the following collisions:
COLLISION TWO (Rear - end)
m = 10 kg
v = 5 m/s
p = +50kgm/s
m = 10 kg
v = - 5m/s
p = - 50kgm/s
m = 20 kg
v = 2 m/s
p = +40kgm/s
m = 20 kg
v = + 7 m/s
p = + 140kgm/s
Total p before = (+50 kgm/s )+ (+40 kgm/s) = +90 kgm/s
Total p after = (- 50kgm/s) + (+ 140kgm/s = +90kgm/s