1. Momentum

2. Can you name 5 safety features found in an automobile?
Seat belts
Air bags
Bumpers
Padded consoles, collapsible steering wheels, padded interior
Crumple zones

3. Momentum, Impulse, and Auto Safety
The concepts of momentum, conservation of momentum, and impulse explain a lot of the science behind car crashes and the safety devices that have been designed because of those crashes.

4. momentum & impulse
Examples of impulses being applied on everyday objects

5. Momentum What is momentum? Anything that is moving has momentum
Depends on what?

6. Momentum Momentum = mass * velocity
In physics, the symbol for momentum is “p”
sssooo…
p = m * v
measured in kg·m/s
Which has more momentum,
a supertanker tied to a dock
or a raindrop falling?

7. A bowling ball and a tennis ball with the same momentum

8. Which has more momentum…
How to increase momentum?
Increase either Mass or velocity
Which has more momentum…
A train standing still
A little car moving at 15 m/h

9. Can the train and the car be made to have the same momentum?
Yes No
We can either stop the car…or we can speed up the train

10. Momentum momentum = mass x velocity p = mv
Which has more momentum: a 300lb football player moving at 5m/s or a 200lb quarterback moving at 10m/s?

11. Answer Momentum of the 300lb player is 300lbs x 5m/s = 1500lb-m/s
Momentum of the quarterback is
200lbs x 10m/s = 2000lb-m/s
The quarterback has a greater momentum!

12. Practice
A 100 kg (220 lbs)football player runs with a velocity of 9 m/s towards the visitors end zone. What is his momentum?
M=100 kg
V=9 m/s
Known:
Unknown:
P=?
P=mv
P=100 kg X 9 m/s
Plug and Chug
P=900 kg * m/s

13. Practice
A 0.5 kg volleyball is spiked at a rate of 10 m/s towards the ground. What is the momentum of the volleyball?
M=0.5 kg
V=10 m/s
Given:
Unknown:
P=?
P=mv
P=0.5 kg X 10 m/s
Plug and Chug
P=5 kg * m/s

14. Momentum Determine the momentum of a ...
60-kg halfback moving eastward at 9 m/s.
60 x 9 = 540 kg·m/s east
1000-kg car moving northward at 20 m/s.
1000 x 20 = 20,000 kg·m/s
40-kg freshman moving southward at 2 m/s.
40 x 2 = 80 kg·m/s

15. IMPULSE

16. Impulse Changes Momentum
The impulse exerted on an object depends on:
the force acting on the object
the time the force acts
Impulse = change in momentum (p)
Impulse = Force x time
Therefore…
Ft = Dp

17. Impulse Changes Momentum
Increasing Momentum
Apply the greatest force possible for the longest time possible. This creates the greatest impulse possible.
Example: Follow through
in sports

18. Impulse Changes Momentum
Decreasing Momentum
Apply a small force over a long time. A longer contact time reduces force.
Example:

19. Impulse Changes Momentum
Decreasing Momentum
Apply a large force over a small time. A shorter contact time increases force.
Example:

20. Impulse Changes Momentum

21. How do the safety features of a car protect you in a collision?
In an accident, the Dp stays the same, but you can change F and t of impact.
When you increase the time that the collision lasts, the force of impact decreases Ft
All safety features in a car are
designed to reduce the force
by increasing the time
you are in contact
with that force.
Remember: Ft = Dp

22. Other examples that decrease the force by increasing the time
Air bags
Catching a baseball with a glove
Fighting with padded gloves v. bare fists
Rolling with a punch
Rocket propulsion
Packing materials

23. Real Life Applications..
Sports…
Follow-through!
Gymnastics Mats
Cars…
Airbags!
Before airbags….
Bumpers
Jumping…
Knees
Shoes

24. Baseball Catching an Egg
Q: Why batters swing when they hit a ball?
A: To increase the time of contact
Catching an Egg
Q: An egg is thrown at you. How can you catch it without breaking it?

25. Momentum Impulse An egg (0.1 kg) is flying through the air at 20. M/s.
How are we going to stop it?

26. BOUNCING
The impulse required to bring an object to a stop and then to “throw it back again” is greater than the impulse required to merely bring the object to a stop.

27. Momentum and Impulse
A tennis ball bouncing off the floor. There is a rapid change in the direction of the velocity when the ball hits the floor.

28. The floor delivers the impulse to the tennis ball.
Bouncing Tennis ball
The floor delivers the impulse to the tennis ball.

29. The Law of Conservation on Momentum
The total momentum of two objects before a collision is equal to the total momentum of the two objects after the collision.

30. What happens in a collision?
Collisions produce a change in momentum (p=mv) on an individual object.
For example, if you are traveling at 60 mi/h (27 m/s) in a 1000 kg car, then you have a momentum of ,000 kg m/s. If you collide with a brick wall, your momentum will be reduced to 0 kg m/s.
[Dp=27,000 kg m/s]

31. Collisions Two types of collisions involving momentum
Elastic-objects bounce off each other
Bowling balls and pins
Inelastic-objects stick together
Football Tackle
The two will continue in the direction of the player with the greatest momentum

32. Elastic Collisions
In an elastic collision, the kinetic energy of the system is unchanged by the collision
Example: In billiards, collisions between balls are almost completely elastic

33. elastic collisions

34. Inelastic Collisions
An inelastic collision results in a decrease in a system’s total kinetic energy.
Kinetic energy is transformed into other types of energy e.g. thermal
Example: When football players hit each other, collisions are inelastic

35. inelastic collisions

36. Happy/Sad Ball Demonstration
Happy ball is made of neoprene rubber. It bounces when it hits the floor—energy is conserved so this is an ELASTIC collision
Sad ball is made of norbornene. It does not bounce when it hits the floor because most of the energy is dissipated in the collision. This is an INELASTIC collision.

37. Football provides many collision examples to think about!

38. Before the collision P1 500 kg m/s 300 kg m/s 200 kg m/s
Momentum of running back is 100kg x 5m/s = 500 kg m/s Momentum of linebacker is 75 kg x (-4 m/s) = -300 kg m/s What is the total momentum before the collision?
500 kg m/s
300 kg m/s
200 kg m/s P1

39. After the collision Momentum of the two players before and after
the collision is the same
(200 kg m/s)
Momentum after (P2) must be 200 kg m/s
P2 = 1.14 m/s x 175kg = 200kg m/s = P1

40. Momentum Practice
2 cars are heading east, car A is traveling 30mi/hr, car B is traveling 60mi/hr. Each car weighs 2000lbs.
What is the momentum of car A?
What is the momentum of car B?
If car B crashes into car A, what is the total momentum?

41. Answer p=mv Car X’s momentum = 30mi/hr x 2000lbs
pX = 60,000 mi-lbs/hr east
Car Y’s momentum = 60mi/hr x 2000lbs
pY = 120,000 mi-lbs/hr west
Total momentum = pY - pX
= 120, ,000
= 60,000 mi-lbs/hr west

42. non-violent collisions
Two stationary ice skaters push off
both skaters exert forces on each other
however, the smaller skater acquires a larger speed than the larger skater.

43. 7.3 Recoil
Q: Why does a shot gun slam against your shoulder when fired?

44. elastic collisions

45. inelastic collisions

46. Two Particle Collisions
Elastic Collisions
p1B p2B = p1A p2A
m1v1B + m2v2B = m1v1A + m2v2A
Inelasic collisions
p1B p2B = p1+2A
m1v1B + m2v2B = m1+2v1+2A