1. Momentum and Impulse

3. Let’s start with everyday language
What do you say when a sports team is on a roll?
They may not have the lead but they may have ___________
MOMENTUM
A team that has momentum is hard to stop.

4. What is Momentum?
An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg∙m/s^2 m=mass v=velocity Momentum is also a vector (it has direction)

5. Let’s practice
A 1200 kg car drives west at 25 m/s for 3 hours. What is the car’s momentum?
Identify the variables:
1200 kg = mass
25m/s, west = velocity
3 hours = time
P = mv = 1200 x 25 = kg m/s^2, west

6. How hard is it to stop a moving object?
To stop an object, we have to apply a force over a period of time.
This is called Impulse
Impulse = FΔt Units: N∙s
F = force (N)
Δt = time elapsed (s)

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

8. 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 Δt goes up, what happens to F, the force? Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.

9. Practice Problem
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?
Identify the variables:
Mass = 57 g = kg
Δvelocity = +1.2 – (-1.2) = 2.4 m/s
Time = 0.02 s
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N

10. 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.
Videos:
smart car, suburban, bus, old vs. new, smart car 70 mph, egg drop slow motion,
Now take a guess …

11. Car Crash (cont.) 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 matter.
And the answer is…..

12. The answer is… It Does Not Matter! Look at FΔt= mΔv
Car Crash (cont.)
The answer is…
It Does Not 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.

13. FΔt= mΔv Egg Drop connection How did you use this in your egg drop?
Which of these variables did you control?
FΔt= mΔv
Which of them do you want to maximize, which do you want to minimize
Note: we are looking at the force on the egg.
Therefore, m represents the egg mass, not the entire mass of the project

14. Lets look at the calculations for your Egg Drop …

15. ∆v = Vbefore collision – Vafter collision
On Earth:
FE = megg(g)
Impulse Force:
FIE = (megg∆v)/ ∆t
∆v = Vbefore collision – Vafter collision
∆t on Earth for the collision = 0.1 seconds

16. ∆v = Vbefore collision – Vafter collision
On Jupiter:
FJ = megg(j) Fall time is 0.02s at Jupiter's Gravity from 3.2 m j = the acceleration on Jupiter due to gravity
MJ = 318 x ME
Impulse Force:
FIJ (∆t) = (megg∆v)
∆v = Vbefore collision – Vafter collision
Find ∆t with only the FIE and your Egg will survive.

17. HINTS: You need to calculate the following for Earth:
Use only the mass of the Egg now; not the entire system.
FIE = (megg∆v)/ ∆t … using your data.
To find ∆t with only the FIE and on Jupiter:
Find Vbefore collision by using j (acceleration on Jupiter) and the fall time on Jupiter of 0.02 seconds in the Vfinal equation.
Solve FIJ (∆t) = (megg∆v) for ∆t and substiture in the values form above using FIJ = FIE to find ∆t during the collision so that your Egg will survive.