1. Journal 10/27/16 Objective Tonight’s Homework
What would you do if a news report suddenly came on saying that the entire human race was about to be wiped out from an asteroid impact? Assume you have only 3 days left.
Objective Tonight’s Homework
To determine the force needed to stop a moving object
178: 1, 2, 3, 4

2. Notes on Momentum and Impulse
A long time ago, we talked about inertia. We said that more massive objects were harder to stop or get going. But we couldn’t measure this at the time.
Now we can. Imagine you wanted to stop an oncoming car by pushing on it. Which is going to be harder?
- Stopping a car at 3 mph, or at 50 mph?
- Stopping a small motorcycle or a big semi?

3. Notes on Momentum and Impulse
A long time ago, we talked about inertia. We said that more massive objects were harder to stop or get going. But we couldn’t measure this at the time.
Now we can. Imagine you wanted to stop an oncoming car by pushing on it. Which is going to be harder?
- Stopping a car at 3 mph, or at 50 mph?
- Stopping a small motorcycle or a big semi?
Stopping the slow car is easier, and so is stopping the lighter vehicle.

4. Notes on Momentum and Impulse
If we combine both of these ideas, we find that they both scale linearly with difficulty. Additionally, there doesn’t seem to be any special constant factor.
How hard it is to stop something depends only on mass and velocity.

5. Equations Momentum E: p = m•v V: p: momentum (kg • m/s)
m: mass (in kg)
v: velocity (in m/s)
S: Momentum roughly tells us how hard it will be to make an object change its acceleration.

6. Notes on Momentum and Impulse
You’ll note that momentum isn’t measured in Newtons. This means we still don’t know how much force it takes to stop a moving object.
But why not? Because we’re still missing a factor. Let’s look at another example.
Let’s say you drive a car at 60 mph straight into a wall. The car will end up stopped and busted. Now let’s say you drive a car at 60 mph into a big pile of hay. The car will stop, but be ok.
If the force is the same, what’s different between hitting a brick wall and hitting hay?

7. Notes on Momentum and Impulse
The difference here is the time factor. If you want to change the momentum of an object, the amount of force you apply matters, but so does the amount of time it takes.
We call this concept “impulse”. Impulse can be thought of as a change in momentum. It can also be thought of as how we spread a stopping force out over time.

8. Equations Impulse E: I = m•Δv OR I = Favg•Δt
V: I: Impulse (kg•m/s) or (N●s)
m: mass (in kg)
Δv: change in velocity (in m/s) Favg: The average force applied (in N) Δt: How long the force is applied ( in s)
S: Impulse tells us how a stopping force is spread out over time, or how a momentum changes over time. We can calculate the force needed to stop an object by using this equation.

9. Notes on Momentum and Impulse
Examples:
Which has more momentum: A 20 kg bowling ball moving at 10 m/s, or a 200 kg car moving at 2 m/s?

10. Notes on Momentum and Impulse
Examples:
Which has more momentum: A 20 kg bowling ball moving at 10 m/s, or a 200 kg car moving at 2 m/s?
Bowling ball: p = mv p = (20 kg)(10 m/s) p = 200 kg m/s
Car:
p = mv p = (200 kg) (2 m/s) p = 400 kg m/s
The car has twice as much momentum as the bowling ball so it will be twice as hard to stop.

11. Notes on Momentum and Impulse
Examples:
Superman is trying to stop a 2,000 kg wrecking ball from destroying a retirement home full of war veterans. If the wrecking ball is moving at 15 m/s, and superman can exert a force of 20,000 N, how many seconds will he have to push on the wrecking ball to stop it?

12. Notes on Momentum and Impulse
Examples:
Superman is trying to stop a 2,000 kg wrecking ball from destroying a retirement home full of war veterans. If the wrecking ball is moving at 15 m/s, and superman can exert a force of 20,000 N, how many seconds will he have to push on the wrecking ball to stop it?
We said impulse equals 2 things:
I = m•Δv OR I = Favg•Δt
We can set these equal to each other.
m•Δv = Favg•Δt rearrange Δt = m•Δv / Favg
Δt = (2000 kg)(15 m/s) / (20,000 N)
Δt = 1.5 s

13. Practice
Pp 193: 1, 2, 3, 4, 5

14. Exit Question
Look at your equation for impulse. When a person uses a karate chop to break wood, they let their fist bounce. This keeps the total impulse the same, but cuts the time it takes to do the actual chop in half. What happens to the force of the chop because of this?
a) It doubles
b) It stays the same
c) It gets cut in half as well
d) Not enough information
e) None of the above