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# Momentum - PowerPoint PPT Presentation

Momentum. the quantity of motion. § 8.1–8.2. What’s the point?. Nature keeps careful account of momentum. Think Question. Which process will require more time ?. Pulling as hard as you can to accelerate a little red wagon from rest to a speed of 1 m/s.

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### Momentum

the quantity of motion

§ 8.1–8.2

• Nature keeps careful account of momentum.

Which process will require more time?

• Pulling as hard as you can to accelerate a little red wagon from rest to a speed of 1 m/s.

• Pulling as hard as you can to accelerate a horse trailer from rest to a speed of 1 m/s.

• The two take the same amount of time.

Which process will require more force?

• Accelerating a little red wagon from rest to a speed of 1m/s in ten seconds

• Accelerating a horse trailer from rest to a speed of 1m/s in ten seconds.

• The two take the same force.

Impulse = FDt

where

F Is net force

Dt is duration

Impulse

Effect of a force over time

Impulse is a vector.

Two objects of different masses move at the same velocity. Which one is harder to stop?

The light object.

The massive object.

Two objects with the same mass move different speeds. Which one is harder to stop?

The fast object.

The slow object.

p = mv

Momentum

Formula

momentum is a vector.

Prove that when a net force F is applied to an object of mass m for a time Dt, its change in momentum is

Dp = FDt

• Impulse = FDt.

• FDt

= mDv

• So FDt = D(mv) = change in momentum

F =

dt

Newton’s second law

Which changes its momentum the most?

• A moving object that stops when it hits a barrier.

• A moving object that bounces back from a barrier.

Hints:

Momentum is a vector.

final p – initial p

final p – initial p

initial p

initial p

Dp

Dp

final p

final p= 0

Rebound and Momentum

Which changes its kinetic energy the most?

• A moving object that stops when it hits a barrier.

• A moving object that bounces back from a barrier.

Hints:

Kinetic energy is a scalar.

Which has the greater net work done?

### Conservation of Momentum

Here today, somewhere tomorrow

§ 8.2

If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest impulse in the collision?

The insect.

The car.

It’s a tie.

Hint: Remember Newton’s third law.

If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest momentum change in the collision?

The insect.

The car.

It’s a tie.

Hint:Remember the impulse-momentum theorem.

• Newton’s first law:

• no outside force

• no change in v

• thus no change in p

• So an isolated object’s momentum never changes.

Conservation of Momentum

• Newton’s third law:

• interacting objects apply equal and opposite impulses to each other

• they experience equal and opposite momentum changes

• So the sum of their momenta remains the same.

The total momentum of an isolated system never changes.