1. Impulse & Momentum

2. Have you ever wondered…
Why golfers and bowlers “follow through”?
Why skydivers bend their knees upon impact?
Why falling on a wood floor (or into a swimming pool) hurts less than falling on a cement one?

3. Have you ever wondered…
What’s the best strategy to win a “water balloon toss” or an “egg toss”?
Why we make auto air bags and padded dash boards?
How the police can figure out how fast you were going BEFORE an accident by measuring your skid marks?

4. Impulse and Momentum
To answer all of these questions--you need to understand: Impulse and Momentum
Recall, Newton’s first law…The Law of Inertia
The study of inertia for moving objects is the study of “momentum”

5. Consider the following:
When a baseball bat hits a baseball, a force is applied OVER A SHORT PERIOD OF TIME.

6. Impulse
Defn: A force applied over a period of time is called an IMPULSE.
What are the SI units of Impulse?

7. Impulse and Momentum
If momentum changes, it’s because mass or velocity change.
Most often mass doesn’t change so velocity changes and that is acceleration.
And mass x acceleration = force
Applying a force over a time interval to an object changes the momentum
Force x time interval = Impulse
Impulse = F t or Ft = mv
Ft = mv

8. Example - Impulse
A batter hits a ball with a force of 200N. If the time of contact was seconds, find the impulse of the ball.

9. Example #2
A batter hits a ball with a force of 500N. If the time of contact was seconds, find the impulse of the ball.
0 N·s
102.5 N·s
51250 N·s
N·s
I have no clue…

10. Example #3
A batter hits a ball with a force of 500N. If the impulse is 75 N·s, then what is the contact time between the ball & bat?
0 sec
0.150 sec
75 sec
37500 sec
I have no clue…

11. Answer: (b)
A batter hits a 500g ball with a force of 500N. If the impulse is 75 N·s, then what is the contact time between the ball & bat?

12. Example #4
A batter hits a ball and the impulse given to the ball is 125 N·s. If the ball & bat are in contact for 200 milli-seconds, then what is the force of the bat?
0 N
0.625 N
2.5 N
625 N
I have no clue…

13. A car at rest possesses:
Amount identified by:
Mr. Letkeman Science Foundations

14. A moving car possesses:
Mr. Letkeman Science Foundations

15. A moving car also possesses:
Mr. Letkeman Science Foundations

16. Momentum
Defn: The MOMENTUM of a body is equal to the product of its mass and its velocity.
“mass in motion”
Is momentum a vector or a scalar?
What are the SI units of momentum?

17. So What’s Momentum ? Momentum = mass x velocity
This can be abbreviated to : momentum = mv
Or, if direction is not an important factor : momentum = mass x speed
So, A really slow moving truck and an extremely fast roller skate can have the same momentum.

18. 1 kg
10 m/sec
1000 kg
.01 m/sec
Question :
Under what circumstances would the roller skate and the truck have the same momentum ?
The roller skate and truck can have the same momentum if the ratio of the speed of the skate to the speed of the truck is the same as the ratio of the mass of the truck to the mass of the skate.
A 1000 kg truck moving at 0.01 m/sec has the same momentum as a 1 kg skate moving at 10 m/sec. Both have a momentum of 10 kg m/sec. ( 1000 x = 1 x 10 = 10 )

19. Mr. Letkeman Science Foundations
Moving car has
mass and velocity
Car at rest has mass
velocity = 0
Mr. Letkeman Science Foundations

20. What happens when the car starts from rest and achieves some velocity?
The car’s momentum:
Increases
Mr. Letkeman Science Foundations

21. What happens when the car moving at some velocity comes to rest?
The car’s momentum:
Decreases
Mr. Letkeman Science Foundations

22. What causes the increase or decrease in momentum?
How big is the force?
Mr. Letkeman Science Foundations

23. Mr. Letkeman Science Foundations
hard to stop, small mass
m v
Mr. Letkeman Science Foundations

24. hard to stop, small velocity
M v
Mr. Letkeman Science Foundations

25. MV

26. How far from the shore does an oil tanker shut off its engines to allow water resistance to stop the tanker as it comes into port?
25 km

27. Conservation of Momentum
Momentum is not created or destroyed
It is transferred
The total momentum before equals the total momentum after
Mr. Letkeman Science Foundations

28. Conservation of momentum
A man (m = 75 kg) stands on a frozen pond with no friction and he wants to get to the shore, which is 100 m away.
How can he accomplish that?
He takes off one of his boots (m = 3kg) and throws it with a velocity of 20 m/s in the opposite direction of the shore. What’s his velocity (speed and direction)?
How long will it take him to get to the shore?
Mr. Letkeman Science Foundations

29. MOMENTUM An object at rest has no momentum, why?
Because anything times zero is zero
(the velocity component is zero for an object at rest)
To INCREASE MOMENTUM, apply the greatest force possible for as long as possible.
Examples :
pulling a sling shot
drawing an arrow in a bow all the way back
a long cannon for maximum range
hitting a golf ball or a baseball (follow through is important for these !)
FORCE
TIME

30. MOMENTUM Decreasing Momentum Ft
Which would it be more safe to hit in a car ?
Knowing the physics helps us understand why hitting a soft object is better than hitting a hard one. Ft mv mv Ft

31. MOMENTUM
In each case, the momentum is decreased by the same amount or impulse (force x time)
Hitting the haystack extends the impact time (the time in which the momentum is brought to zero).
The longer impact time reduces the force of impact and decreases the deceleration.
Whenever it is desired to decrease the force of impact, extend the time of impact !

32. DECREASING MOMENTUM
If the time of impact is increased by 100 times (say from .01 sec to 1 sec), then the force of impact is reduced by 100 times (say to something survivable).
EXAMPLES :
Padded dashboards on cars
Airbags in cars or safety nets in circuses
Moving your hand backward as you catch a fast-moving ball with your bare hand or a boxer moving with a punch.
Flexing your knees when jumping from a higher place to the ground or elastic cords for bungee jumping
Using wrestling mats instead of hardwood floors.
Dropping a glass dish onto a carpet instead of a sidewalk.

33. EXAMPLES OF DECREASING MOMENTUM
Bruiser Bruno on boxing …
Increased impact time reduces force of impact
Barney Jervais on bungee Jumping …
Ft = change in
momentum
Ft = change in
momentum
POOF !
CRUNCH !
Ft = Δmv applies here.
mv = the momentum gained before the cord
begins to stretch that we wish to change.
Ft = the impulse the cord supplies to
reduce the momentum to zero.
Because the rubber cord stretches for
a long time the average force on the
jumper is small.

34. CONSERVATION OF MOMENTUM
To accelerate an object, a force must be applied.
The force or impulse on the object must come from outside the object.
EXAMPLES : The air in a basketball, sitting in a car and pushing on the dashboard or sitting in a boat and blowing on the sail don’t create movement.
Internal forces like these are balanced and cancel each other.
If no outside force is present, no change in momentum is possible.

35. MOMENTUM VECTORS Momentum can be analyzed by using vectors
The momentum of a car accident is equal to the vector sum of the momentum of each car A & B before the collision. A B

36. Determine the following momentums…
Ex #1 - A 1200kg car moving at 60kph
Ex #2 - A 5.0g bullet moving at 900m/s
Ex #3 - A 3 ton elephant sitting still
Ex #4 - A 150g hummingbird flying at 40kph
ALL answers need to be in kg m/s

37. Determine the following momentums…Solution
Ex #1 - A 1200lb car moving at 60mph
= kg m /s
Ex #2 - A 5.0g bullet moving at 900m/s
= 4.5 kg m/s
Ex #3 - A 3 ton elephant sitting still
= 0 kg m/s
Ex #4 - A 150g hummingbird flying at 40mph
= 2.68 kg m/s

38. Example #5
A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft.
remains stationary,
moves away from the shore,
moves toward the shore
Not enough information given

39. Answer: (c)
A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft.
Answer: (c) moves toward the shore - Newton’s 3 law

40. Example #6
Which has a greater momentum, a heavy truck at rest or a moving skateboard?
Heavy truck,
Skateboard,
Neither (same momentum)
Not enough information given

41. Answer: (b)
Which has a greater momentum, a heavy truck at rest or a moving skateboard?
Answer: (b) The truck at rest has no speed, hence no momentum. So the moving skateboard has greater momentum.

42. Making the Connection:
Look at the units of Impulse and Momentum…Do you notice anything?
Recall, F = m·a…can you rearrange the equation to say something about Impulse or Momentum?

43. MOMENTUM SOME VOCABULARY :
impulse : impact force X time (newton.sec) Ft = impulse
impact : the force acting on an object (N) usually when it hits something.
impact forces : average force of impact

44. The End...