Explain how inertia relates to mass Day 1 Opening Activity: Today I will: Define momentum Explain momentum in terms of inertia and velocity and change in momentum Define inertia Explain how inertia relates to mass

Chapter 7 - Momentum

Inertia – resistance to a change in motion Momentum – inertia in motion (a moving object’s resistance to any change in its motion) Momentum = mass X velocity p = mv

How much momentum does a car sitting at a stop light have? Zero!! None!! It is at rest so v = 0; p=mv

Inertia of car vs. truck truck – more massive; more inertia car – small mass, less inertia

Which one has the greater momentum (car or truck)? Can’t say! How is it possible for the car to have more momentum then the truck? (what would we need to know)

We would have to look at the mass and the velocity of both!

What if we have a change in mass or in velocity? If the mass or velocity changes (or both), so does the momentum When we have a change in velocity over a time period what does that tell us? accelerating (gaining or losing velocity over time) What is the cause of an acceleration? a net force Start here 8th

Acceleration is greater Recall from Newton’s Second Law the relationship between force and acceleration Greater force Acceleration is greater means the object undergoes a greater change in velocity momentum = mass X velocity; a greater change in velocity will give a greater change in momentum

Change in p and change in v Δp = Δ(mv) also can be written Δp = (Δm)(∆v) or Δp = m(Δv) Start here with 3rd and 6th

Increasing Momentum (start with small p end up with a bigger p) To increase the p – apply the greatest force for as long as possible (greatest impulse—increasing both force and impact time) When would this be something you would want?

Decreasing Momentum (big p to small p) When would we want to quickly decrease momentum?

7.2 Impulse changes momentum Force: needed to cause a change in momentum length of time the force is applied is also important; A large force over a large time interval will also give a great amount of change in momentum This change in momentum is called the impulse

Impulse – the change in momentum Impulse is the applied force that causes a change in velocity and the time interval that force is applied Impulse = force X time interval I = FΔt The greater the impulse the greater the change in momentum

Impulse = change in momentum I = FΔt Δp = Δ(mv) Since I = Δp… FΔt = Δ(mv)

vocab… Impact force – applied force (golf club) The unit for impact force is the Newton (N) Impact time – how long the force is applied (units = s) Impulse = impact force X time Units for impulse = Ns Start with 3rd

Page 100 RQ 1-7

Why is the impulse the same for a car with airbags and a car without? Why have airbags if the impulse is the same?

What does the following picture have to do with impulse?

Ft = Ft Car with air bags Impulse = Ft Small impact on person force; large time Car without air bags Impulse = Ft Large impact on person force; small time Ft = Ft

Momentum Problem Packet: yellow

7.3 Bouncing Object comes into contact with another object and bounces off In a bounce the impulse is greater, why??

Has a greater change in velocity!! NOTE: if the object did not bounce it would hit the surface and stop

Opening Question: Today I will: Explain the conservation of momentum Explain what it means to say that momentum is conserved Identify different types of collisions Change in momentum = impulse, how is it possible for the momentum to decrease but the impulse remains the same? The answer is simple. The impulse = change in momentum. So the impulse equals the difference in the momentum from before the collision to after (pafter- pbefore). Turn in impulse and momentum problem sheet from break!!!

7.4 Conservation of Momentum Forces causes acceleration Impulse causes a change in momentum An impulse is formed when an external (outside) force acts on an object The external force causes the object to accelerate and to change momentum (Internal forces can’t do this…remember the horse and cart problem) Start with 3rd 6th period

Example: Soccer ball Ball has air inside, air pushes outward on ball and ball inward on air; this is an internal force Will the air cause the ball to move? No…some outside force must be applied

Newton’s First Law – If an outside (external) force is not present, an object will NOT accelerate. (it’s velocity will remain constant) If the object has the same velocity, there will not be any change in momentum.

Law of conservation of momentum Conserved – when something remains the same; unchanged In the absence of an external force (net force), the momentum of a system remains unchanged. The net momentum can not be gained or lost.

Cannon and Cannonball Cannon exerts a forward force on cannonball The cannonball exerts an equal force backwards on the cannon Cannonball accelerates more because it has less mass (gains speed more quickly) Cannon also accelerates but a lot less than the cannonball and in the opposite direction Start here 3rd

In terms of impulse FΔt = m (Δv) F and t is the same for both cannon and cannonball; so impulse is the same The change in momentum each undergoes is the same (note: difference in mass and velocity)

Cannon and Cannonball in terms of momentum… Before the cannon fires…the system is at rest…momentum = zero Cannonball is then fired… Cannonball has a positive momentum (moving forward) Cannon has a negative momentum (moving backwards) These momenta are equal but opposite Overall momentum after firing = Zero!

Pc + PCB = PC’ + PCB’ 0 + 0 = PC’ + PCB’ 0 = PC’ + PCB’ -PCB’ = PC’

7.5 Collisions law of conservation of momentum: net pbefore collision = net pafter collision

2 types of collisions Elastic collision Inelastic collision

Elastic Collision Objects collide without being permanently deformed and without generating heat Perfect elastic collision, colliding objects bounce perfectly Conservation of momentum holds true Turn to page 94

Inelastic Collision Colliding objects become tangles or couple together Conservation of momentum holds true even when colliding objects become distorted and generate heat Freight train page 94 Start with 6th period on Thursday, April 5

Take out yellow problem packet Read each problem and identify as elastic or inelastic for all conservation of momentum solved and practice exercises Start w 8th

Opening Question: Today I will: Explain what it means to say that momentum is conserved Identify different types of collisions Solve for conservation of momentum problems What does it mean to say that momentum is conserved? Have out yellow packets!!! If you were not here yesterday, turn in your impulse and momentum worksheet!

Math Equations Psystem before = Psystem after Example for two objects Before After P1i + P2i = P1f + P2f m1v1i + m2v2i = m1v1f + m2v2f Note: in an inelastic collision the final velocities are the same Also possible for objects to start out together with same velocity and then to separate

Practice Exercises 7-10

Examples of Collisions Crashes

Think and Solve A) 20, 000 Ns B) can’t solve without t 2000N A) 16 Ns B) -32 Ns C) 64 N D) double the F 4 km/h Case 1: 0.83 m/s case 2: 0.16 m/s 100,000 m/s (that would be 224, 000 mi/h)

Problems on page 669 -1320N 0.5 N A) 500 Ns B) 500 Ns C) 500 N Same as before explosion so 3 X 4 = 12 units