1. Impulse-Momentum Theorem
Or Don’t get Stressed
Standards:
2e Students know momentum is a separately conserved quantity from energy.
2f Students know an unbalanced force on an object produces a change in momentum.
2g Students know how to solve problems involving elastic and inelastic collisions in one dimension by using principals of conservation of momentum and energy.
NGSS
HS-PS2-2 Use mathematical representations to support claim that the total momentum of a system of objects is conserved when there is no net force in the system.
HS-PS2-3 Apply science and engineering ideas to design, evaluate and refine a device that minimizes the force on a macroscopic object during a collision.

2. Momentum (p) Quantity used to measure motion
Mass times velocity of object (p=mv)
Inertia of motion
Vector quantity
Units include kg m/s, g cm/s, N s
Question: Define momentum verbally and mathematically. Is momentum a vector or scalar quantity? List some units of momentum.
Activities:
Write the equation for momentum on the board and define terms. Say that this is a new quantity of motion like velocity and acceleration that measures how much motion an object has.
State that the quantity is a vector quantity. This means it can be decomposed into perpendicular and independent components and that two momentas add geometrically (tip to tail).
Show that units include kg m/s, g cm/s and so forth using equation.
State that this quantity is special because it remains the same before and after collisions and explosions. We say momentum is conserved. We’ll talk a lot more about this property of momentum tomorrow.

3. Check Question 1
Under which of the following circumstances will a roller skate have the same momentum as a Mack Truck?
Roller skate moves fast, truck moves slow
Truck moves fast, roller skate moves slow
Truck and roller skate move at same speed

4. Check Question
What is the total momentum of the pictured system of 2 kg masses each with a velocity of 3 m/s? In what direction is this total momentum?

5. Conservation of Momentum
The momentum of a system is conserved if there are no outside net forces.
Conserved means the quantity stays the same
System means that at least two objects are interacting
Outside net force would be force that is not from objects inside the system.
True for collisions and explosions!

6. Impulse (J or I) and Impulse Momentum Theorem
An outside force that causes a change in the momentum of a system.
Impulse is a vector quantity w/ same units as momentum
Impulse = Net Force applied x time applied (J=Fnet Dt)
This is the same relationship that net force has with velocity (net force changes velocity)
J=Fnet Dt=Dp = mvf - mvi
Question: Define impulse verbally and mathematically. What quantities is impulse related to and how? Is impulse a vector or scalar quantity? List some units of impulse. How are impulse and momentum related? What other two quantities that we have studied have this relationship?
Activities:
Discuss how we found out over the last few weeks that an outside force causes a change in velocity. We can apply the same rule here and state that a force will also cause a change in momentum. Such a force is called an impulse.
Go through the punching a car’s dashboard vs pushing against the ground to explain the difference between internal and external forces.
Write equation for impulse and say that this calculates the magnitude of the momentum change. Discuss how this shows why you follow through in racquet sports and in baseball. Swing hard for large force, follow through to apply that force for longest time possible. Both increase impulse and change in momentum.
Point out that impulse is also a vector with the same units as momentum. Show a newton second is a kg m/s.
For physics E write impulse momentum theorem and state that this is true as long as force does not vary too much and as long as the mass of the body stays the same.

7. Two Methods of Getting the Same Momentum Change
Large force over short time
High stress on object
Large amount of deformation
Examples
Small force over long time
Low stress on object
Small amount of deformation
Examples
Question: What are the differences in affecting a momentum change with a large force over a short time versus a small force over a long time. Which would be advisable in the following circumstances: a) Breaking board with Karate Chop b) Boxer absorbing punch c) Restraint system in automobile d) catching softball e) landing on ground after jumping from tree f) starting a long train

8. Check Question 2
Which is a better surface to roller skate on in case of a fall, a wood or concrete rink?
What are the advantages and disadvantages of astro-turf over natural grass as a surface for a sporting event?

9. Momentum Changes in Elastic and Inelastic Collisions
In an elastic collision objects bounce off of each other. In an inelastic collision they stick to one another.
Greater momentum transfer in elastic collision.
Pelton’s Wheel and very sad story
Question: Distinguish between an elastic and inelastic collision. Which causes a greater momentum change an elastic or inelastic collision? Explain your answer.
Activities:
Introduce ideas of elastic and inelastic collisions with the happy and sad balls.
Using bouncing dart demo show that elastic collisions result in greater momenta changes. Reason that in bouncing it takes one impulse to stop the ball and another to start it moving the opposite direction. Draw time lapse pictures of ball contacting wall, being squashed and then reforming and indicate the two impulses if necessary.
Finish with demonstration of Pelton Wheel and tell sad story of James Pelton who was shot in San Francisco for bragging about how much money he had.
Demo: Bouncing Dart
Demo: Pelton wheel

10. Check Question 3
Use the idea of momentum to explain why an ice ball hurts more when it hits you than a snow ball.
a) the mass is larger for the ice ball
b) the snow ball requires a greater momentum change
c) the ice ball requires a greater momentum change

11. The Egg Demo
Sheet allows for momentum to be changed by small force exerted over long time causing little stress on the egg.
Crush zones, seat belts and air bags in car all extend time for you to stop, reducing the force you feel in a crash.
Keep in mind when designing project.
Questions: Explain why the egg did not break when thrown into the sheet. For the egg challenge do you want the egg to stop over a short or long period of time?
Activities:
Demonstrate the sheet saving the egg when thrown into it and ask why. Lead class to answer that egg was stopped over a long period of time and therefore a small force was present to stop the egg,
Point out that above is what you want for the egg device students will be designing and building this week. Lay out challenge is to make a device that will allow the egg to survive the 10 m fall from the top of Clock auditorium w/o using a parachute.
Finish with Red Ribbon week demonstration in which egg is smashed on desk to prove it isn’t hard boiled declaring that this is your brain on drugs. Have first row type cover up with sheet.

12. Changing Momentum Hit with as great a force as possible
Extend time force is applied
Examples
Tennis, golf and baseball exert large force over long time by following through
Bow and Arrow