1. MOMENTUM & IMPULSE

2. Momentum Often misused word, though most have the right idea.
4/23/2008
Momentum
Often misused word, though most have the right idea.
Momentum, denoted p, is mass times velocity
p = m·v
Momentum is a conserved quantity (and a vector)
Often relevant in collisions (watch out for linebackers!)
News headline: Snowball Hits Unsuspecting Sled
1 kg snowball strikes 5 kg sled at 12 m/s and sticks
Momentum before collision: (1 kg)(12 m/s) = 12 kg*m/s
Lecture 9

3. Bowling ball or basketball?
Picture two lanes at a bowling alley, one with a bowling ball the other with a basketball going at the same speed. Which will exert more force on the pins? Why?

4. Example
A 2250 kg pickup truck has a velocity of 25 m/s to the east. What is the momentum of the truck?

5. Solution
p = mv
(2250 kg)(25 m/s east)
5.6 x 104 kg x m/s to the east

6. PRACTICE
A 21 kg child on a 5.9 kg bike is riding with a velocity of 4.5 m/s to the northwest.
What is the total momentum of the child and the bike together?
What is the momentum of the child?
What is the momentum of the bike?

7. LINEAR MOMENTUM Impulse F∆t = ∆p = mv – mu
The product of the force and the time of a collision
The impulse-momentum theorem states that when a net force is applied to an object over a certain time interval, the force will cause a change in the object’s momentum
[ F∆t = m∆v ]
F∆t = ∆p = mv – mu

8. LINEAR MOMENTUM
Stopping times and distances depend on the impulse-momentum theorem.
Force is reduced when the time interval of an impact is increased.

9. EXAMPLE
A 57 gram tennis ball falls on a tile floor. The ball changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball?

10. SOLUTION
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N

11. PRACTICE 2
An 82 kg man drops from rest on a diving board 3.0 m above the surface of the water and comes to rest 0.55 s after reaching the water. What is the net force on the diver as he is brought to rest?

12. PRACTICE 3
A 2240 kg car traveling to the west slows down uniformly from m/s to 5.00 m/s. How long does it take the car to decelerate if the force on the car is 8410 N to the east? How far does the car travel during deceleration?

13. Solution ∆t = ∆p / F ∆t = mv – mu / F Givens: Mass = 2240 kg
u = 20.0 m/s to the west = m/s
v = 5.00 m/s to the west = m/s
F = 8410 N to the east
Equation to use: F∆t = ∆p
∆t = ∆p / F
∆t = mv – mu / F

14. CAR CRASH
Would you rather be in a head on collision with an identical car, traveling at the same speed as you, or a brick wall?
Assume in both situations you come to a complete stop.

15. CAR CRASH Everyone should vote now
Raise one finger if you think it is better to hit another car, two if it’s better to hit a wall and three if it doesn't’t matter.

16. CAR CRASH The answer is… It doesn’t matter! Look at FΔt= mΔv
In both situations, Δt, m, and Δv are the same! The time it takes you to stop depends on your car, m is the mass of your car, and Δv depends on how fast you were initially traveling.

17. DISCUSSION
Force is reduced when the time interval of an impact is increased.
Imagine a person being tossed in the air and caught in a blanket.
The blanket “gives way” and extends the time of collision to change the momentum over a longer period of time.

18. TASK
Using concepts we’ve covered, explain what is happening in the following graphic.