1. What is Momentum?
An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg∙m/s m=mass v=velocity Momentum is also a vector (it has direction)

2. How hard is it to stop a moving object?
To stop an object, a force must be applied for a period of time.
This is called Impulse (J)
J = FΔt Units: N∙s
F = force (N)
Δt = time elapsed (s)

3. How hard is it to stop a moving object?
Using Newton’s 2nd Law we get
FΔt= mΔv
Which means
Impulse = change in momentum

4. Why does an egg break? or not break?
An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why?
FΔt= mΔv
In both cases, m and Δv are the same.
If time (Δt) increases, what happens to the amount of force (F)?
Answer: Force decreases! When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time needed to stop the egg; therefore, less force is applied to the egg.

5. Stand in front of a 1000 kg. truck moving at 1 m/s, or
A Thought Experiment:
Suppose that you were captured by an
evil physicist who gave you the following choice:
You must either:
Stand in front of a 1000 kg. truck moving at 1 m/s, or
Stand in front of a 1 kg. frozen meatball moving at 1000 m/s.
…think…

6. Truck:
Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s
Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules
Meatball:
Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s
Meatball's kinetic energy = 0.5 mv2 = (0.5)(1kg) (1000 m/s)2 = Joules

7. Using vectors to understand
A 120 kg lineman moving west at 2 m/s tackles an 80 kg fullback moving east at 8 m/s.
After the collision, both players move east at 2 m/s.
Draw vector diagrams for the before AND after situations; draw two diagrams.

8. Answer to previous slide question

9. impulse = change in momentum

10. Bouncing increases impulse explain...

11. Impulse= change in momentum

12. Elastic collisions: momentum is redistributed or transferred without loss.
m1v1o + m2v2o = m1 v1f +m2v2f

13. Inelastic collisions: when two objects become entangled
m1v1o + m2 v2o = (m1+m2)vf

14. mredvred + mbluevblue = m(red +blue) vf
Inelastic Collisions
vi = 10m/s
vi = 0 (at rest) M M
Before Collision
p = mv
vf = 5m/s M
After Collision
p = 2mv
mredvred + mbluevblue = m(red +blue) vf

15. Granny & Little Timmy: calculate the final velocity.

16. Law of Conservation of Momentum
In the absence of an external force, the momentum of a system does not change.

17. Stopping a Truck
Suppose a large truck and a small truck are moving with equal momentum. Which will require more work to stop? Which will require more force to stop in a given distance?

18. More Force to Stop the Little Truck!??
Since the little truck is moving substantially faster it will have more Kinetic Energy!

19. Impulse

20. YIKES!
If the boulder and the boy have the same momentum, will the boulder crush the boy?
…do not try this at home!

21. Truck Collision In a head-on collision:
Which truck will experience the greatest force?
Which truck will experience the greatest impulse?
Which truck will experience the greatest change in momentum?
Which truck will experience the greatest change in velocity?
Which truck will experience the greatest acceleration?
Which truck would you rather be in during the collision?

22. Ride in the BIG truck! 