1. Unit 08 “Impulse and Momentum”
Test Review

2. Momentum is a vector because
Definition
The product of the mass of an object and its velocity
Equation Units Vector
p = mv
(momentum = mass x velocity)
Momentum is a vector because
velocity is a vector.
+p east/north
-p west/south
kg.m/s

3. Momentum is a “quantity of motion”
Momentum is a “quantity of motion”. Changing an objects momentum requires a force.
In general …
An object with a smaller mass is easier to stop.
An object with a smaller velocity is easier to stop!
It is harder to stop a large object.
It is harder to stop a fast moving object.

4. Which ball is easier to stop? Why?
The 1st rock, because it has less velocity, so it has less momentum. Less momentum means easier to stop!
The rock on the small hill because it has less velocity.
(Notice the masses are the same)

5. Impulse Δp = pf - pi Δp = mvf - mvi Δp = FΔ t
Definition
Equation
Definition
Equation
Impulse is the amount of change in an objects momentum.
Impulse is the product of the force applied to an object and the amount of time applied
Δp = pf - pi
Δp = mvf - mvi
Δp = FΔ t

6. Impulse-Momentum Theorem: For a given change in momentum…
The more time the object has to stop, the less force needed to stop it.
The less time the object has to stop, the more force needed to stop it.

7. State the Impulse – Momentum Theorem:
For a given change in momentum, the more time a force is applied to stop an object the less force needed to stop it.
List the three types of protection and how they help protect an object when the object comes to a stop.
Types of Protection
When the object hits …
Cushion
“Sinks in”
“Stretches out”
Suspension
Crumple Zone
“bends” or “breaks”

8. What type of protection do each of these offer?
Elastic bands in an egg-drop project.
Bumpers that collapse of the front of a car.
Helmets – for sports, biking, motorcycles etc…
Highway guardrails.
Spiderman’s web.
Cell phone covers.
Suspension
Crumple Zone
Cushion
Crumple Zone
Suspension
Cushion

9. Briefly explain why an Egg Drop apparatus that falls more slowly (a smaller initial velocity) helps keep the egg safe.
Briefly explain how the seatbelt gives you more time to stop.
Briefly explain how the airbag gives you more time to stop.
A smaller velocity will mean a smaller change in momentum, and therefor Impulse on the egg as it stops.
The seatbelt acts like a suspension. It stretches out giving the person more time to stop, so there is less force on the person.
The airbag acts like a cushion. The person sinks in giving the person more time to stop, so there is less force on the person.

10. A. E. S. An egg was dropped on various surfaces from the same height
A.E.S. An egg was dropped on various surfaces from the same height. The amount of time and the force applied to stop the egg was recorded below. Does changing the amount of time to stop the egg change the force applied to the egg?. A.E.S.
Time (s)
Force (N)
Concrete
0.8 25
Sand
1.2 17
Pillow
3.5
5.8
Memory Foam
5.0
4.0
Yes, changing the amount of time to stop the egg does change the amount of force applied to the egg.
According to the Impulse-Momentum theorem, for a given change in momentum, the more time applied to stop an object the less the force needed to stop the object.
For example, the memory foam acts like a cushion allowing the egg to sink in over more time, 5.0seconds and therefor have a smaller force of only 4.0N. Compared to the hard concrete which stops the egg in only 0.8seconds and applies a large force of 25N.

11. Medium Problem Solving
How long does it take a trampoline to stop a 60kg boy if it puts a 250N force upward on the boy when lands on it with downward velocity of -9m/s.
FΔt = mvf - mvi
Δt = ?
Vf = 0m/s
m =60kg
F = 250N
Vi = -9m/s
250N(Δ t)= (60kg)(0m/s) – (60kg)(-9m/s)
250N(Δ t)= 0kgm/s + 540kgm/s
250N(Δt)= 540kgm/s
Δt = 2.16s