MomentumMomentum

MomentumMomentum

inertia and motionNewton’s “quantity of motion”helps us understand collisions and

explosions

Momentum and CollisionsMomentum and Collisions Elastic Collisions– objects rebound– e.g. superball

Inelastic Collisions– object stick together an usually

become distorted and generate heat– e.g. clay ball

Momentum

Momentum = mass velocity

p = mv

Momentum is a vector quantity.

A large truck has more momentum than a car moving at the same speed because it has a greater mass.

Which is more difficult to slow down? The car or the large truck?

Large Momentum Examples:

– Huge ship moving at a small velocity

– High velocity bullet

P = mv

P = mv

Impulse

In order to change the momentum of an object you must apply a force over some time interval.

Impulse = Force time interval = F t

Impulse and Momentum

Impulse = Change in Momentum = Final (mv) - Initial (mv)

F t = (mv)

Case 1: Increasing Momentum– Apply a force for a long time. – Examples: • Follow through on a golf swing.

– http://www.youtube.com/watch?v=Xy9GTAENiJA

• Pushing a car.

Ft

Case 2: Decreasing Momentum – Apply a force for a long time.– Examples:• Air bags in cars.

– http://www.youtube.com/watch?v=nlWJayKWvw0– http://www.youtube.com/watch?v=_Av0WGrlTGY – http://www.youtube.com/watch?v=D-ogrMr4lWc

• Catching an egg.• Boxing, Figure 6.6• Soft collisions, Figure 6.3.

Ft

Case 3: Decreasing Momentum– Apply a force for a short time.– Examples: • Boxing• Karate

– http://www.youtube.com/watch?v=jttqJZ9sTBk

Ft

Conservation of MomentumThis means that the momentum doesn’t

change.

Recall that F t = mv)

In this equation, F is the "external force".

Internal forces cannot cause a change in momentum.

ExamplesConservation of Momentum: If

there are no external forces, the total momentum for a system remains unchanged.

Example 1: a person sitting inside a car pushing against the dashboard

Example 2: a bullet fired from a rifleExample 3: a rocket is space

Conservation of momentum

The total momentum of a group of interacting objects remains the same in the absence of external forces

Applications: Collisions, analyzing action/reaction interactions

Question

Which has more momentum?– (A) A parked cruise ship – (B) You in your vehicle driving home

Question

1. A force times time is most appropriately called

(a) weight(b) impulse(c) velocity(d) momentum

Question

2. Suppose two freight cars of equal mass are coupled by a collision. This is an example of

(a) an elastic collision(b) an inelastic collision(c) none of the above

Question

3. Which of the following is true about momentum?

(a) it is a vector(b) it is a product of mass times velocity(c) impulses are required to change it(d) all of the above