1. MOMENTUM Expectations
Title of your notes:
MOMENTUM Expectations
Calculate the impulse applied to, and momentum of a physical system
Demonstrate and apply the laws of conservation of momentum in one dimension
When you see this star…write the information down!

2. Stand in front of a 1000 kg truck moving at 1 m/s
Suppose you were captured by an evil, crazy physics teacher. She gave you the following choice:
Stand in front of a 1000 kg truck moving at 1 m/s Or
Stand in front of a 1kg meatball moving 1000 m/s

3. What do you choose? What do you choose?
The meatball is very dangerous! Beware! It isn’t very massive, but it’s moving very fast! Think about it; we’ll come back to it later…

4. Think in terms of sports…
What is momentum?
What is momentum?
Think in terms of sports…
Momentum can be thought of as “how difficult it is to stop a moving object”

5. p = mv Momentum depends on… Momentum depends on… Equation Alert:
The little “p” is from the term progress defined as “the quantity of motion with which a body proceeds in a certain direction.”
Momentum=(mass)(velocity)

6. Mrs. Lessner…you’re nuts! Wouldn’t their momentum be the same?
Let’s compare the kinetic energy and the momentum of both objects… hmmm…which is worse? The meatball or the truck? Clearly KE and momentum are not the same!
Write down the example I do on the board comparing the momentum of the two objects

7. Units for momentum problems
Kg•m/s
mass
velocity

8. Conservation of Momentum
Momentum isn’t created or destroyed in a given system. It may be transferred from one object to another. Only an outside or external force will change the total momentum

9. Conservation of Momentum
What does that mean to me? The total momentum before a collision = the total momentum after a collision m1v1i = m2v2f The same applies for a “recoil” velocity

10. Conservation of Momentum
Vocab Alert:
Closed system: one not affected by external forces
Isolated system: completely isolated from environment
System: Portion of the universe focused on for study.

11. Conservation of Momentum
Example: Conservation of Momentum
Calculate the recoil velocity of a 4.0kg rifle that shoots a 0.05kg bullet at a speed of 280 m/s. Vfr = -3.5 m/s Because the rifle has a much larger mass, it’s velocity will be much smaller than the bullet

12. Impulse
Impulse
Product of net force and time
Vector in direction of force
An impulse causes a change in momentum
Units: -N•s
Why is there a negative sign?

13. Impulse Impulse Impulse = (Ft) Equation Alert:

14. Impulse-Momentum Theorem
Equation Alert:
Ft = p
Impulse = change in momentum – so a change in force over time, creates an impulse

15. Impulse-Momentum Theorem
Ft = mv
Impulse
Momentum

16. Coming to a stop sign Coming to a stop sign
Change momentum of car by applying brakes
More force = less time
More time = less force
How does my change in momentum change in the two situations above? Or doesn’t it?

17. Increase time, thereby reducing force
How do Airbags work?
How do Airbags work?
Increase time, thereby reducing force

18. Sports Science

19. Demonstration: Impulse-Momentum
Two identical balls are dropped from the same height onto the floor. In case 1 the ball bounces back up, and in case 2 the ball sticks to the floor without bouncing. In which case is the impulse given to the ball by the floor the biggest? 1. Case 1 2. Case 2 3. The same
The impulse-momentum theory says that the impulse that acts on an object is given by the change in the momentum of the object, and this change is proportional to the change in velocity. The ball that sticks has a velocity of downward to zero, but the velocity of the ball that bounces goes downward then upward. This change in momentum is greater and therefore has a greater impulse on it.

20. Example 1: Impulse Impulse
A soccer ball (mass kg), rolls toward an attacking midfielder at 6 m/s. The ball is shot toward the goal at a speed of 26 m/s. Given that the kick’s impact lasted for 0.008s, what was the average force on the ball?

21. Example 2: Impulse
Impulse
What average force must act on a 7.8 x 104 kg Boeing 737 to bring it from rest to a takeoff speed of 67 m/s in 40 s?

22. Skip #1 on your Guided Notes page for now. We’ll come back to it.
Collisions
Collisions
Skip #1 on your Guided Notes page for now. We’ll come back to it.

23. Elastic Collisions Elastic Collisions
An elastic collision is one where:
Momentum is conserved.
Kinetic energy is conserved
The objects colliding aren’t deformed or smashed (Thus no kinetic energy is lost)
  Ex: billiard ball collisions
Make sure everyone understands what “conserved” means

24. Inelastic Collisions Inelastic Collisions
An inelastic collision is one where:
Momentum is still conserved
Kinetic energy is lost.
The lost kinetic energy will be transformed into other energy types
The objects often interlock and stick together
May be deformed and mangled
Main indicator is the lack of conservation of KE.
Ex: car crash
Make sure everyone understands what “conserved” means

25. Problem Solving Steps Go back to “#1…” in your Guided Notes.
Draw a picture for the “initial” scenario
Before the collision
Draw a picture for the “final” scenario
After the collision
Write your Conservation of Momentum Equation based on your initial and final scenario
Isolate the unknown & substitute values

26. Example: Collisions
A kg bullet is fired at a velocity of 450 m/s from a 2.0 kg gun. What is the recoil velocity of the gun? (explosions are reverse collisions)
Elastic or Inelastic?
What is conserved: Momentum, KE or both?

27. 𝑚 𝑏 𝑣 𝑏𝑖 + 𝑚 𝑔 𝑣 𝑔𝑖 = 𝑚 𝑏 𝑣 𝑏𝑓 + 𝑚 𝑔 𝑣 𝑔𝑓
A kg bullet is fired at a velocity of 450 m/s from a 2.0 kg gun. What is the recoil velocity of the gun? (explosions are reverse collisions)
mb=0.0050kg
vbf=450m/s
mg=2.0kg
vbi=0m/s
vgi=0m/s
Vgf in m/s
Initial Final
𝑝 𝑖 = 𝑝 𝑓
𝑚 𝑏 𝑣 𝑏𝑖 + 𝑚 𝑔 𝑣 𝑔𝑖 = 𝑚 𝑏 𝑣 𝑏𝑓 + 𝑚 𝑔 𝑣 𝑔𝑓
0= 𝑚 𝑏 𝑣 𝑏𝑓 + 𝑚 𝑔 𝑣 𝑔𝑓
𝑣 𝑔𝑓 = − 𝑚 𝑏 𝑣 𝑏𝑓 𝑚 𝑔
𝑣 𝑔𝑓 = −0.0050𝑘𝑔×450 𝑚 𝑠 2.0𝑘𝑔
=-1.125m/s
It’s not moving!

28. Example: Collisions
A 1200 kg car is stopped at a traffic light when a 3500 kg truck moving at 8.4 m/s hits it from behind. The bumpers lock, and after the collision they both move at 4 m/s. Is this an elastic or inelastic collision?