Notes: Momemtum

CW: Acceleration and Momentum

H: The Laws of Motion

Momentum

2-18-09

Objectives

1. describe the relationship between mass, velocity, inertia, and momentum

2. relate conservation of momentum to motion in a collision

What makes an object difficult to stop?

Calculating MomentumThe momentum of an object is

directly related to its mass and its velocity.

MassMass measures inertia. The mass of an object tells you how much the object resists accelerating.

Nothing will change speed unless acted upon by a force. If it’s sitting still, it will keep sitting still. If it’s moving fast, it will keep moving fast.

Only a force can change the velocity of an object.

Velocity

Fast objects are also difficult to stop.

Momentum = (mass)(velocity)

p = mv

Example

A ball of mass 2 kg is moving with a speed of 4 m/s.

What is its momentum?

SolutionMomentum = (mass)(velocity)

p = mv

p = (2 kg)(4 m/s)

p = 8 kg m/s

ExampleA runner has a mass of 75 kg and is running at 6 m/s.

What is his momentum?

SolutionMomentum = (mass)(velocity)

p = mv

p = (75 kg)(6 m/s)

p= 450 kg m/s

Momentum is a vector quantity. Its direction is the same as the direction of the object's velocity.

Elastic collision

Resulting momentum

Two dimensional collision

Knowing the momentum of moving objects helps explain what happens when objects collide with one another.

Although moving objects can collide in many different ways, they have one important thing in common.

Law of Conservation of Momentum

The total momentum before and after the collision is not changed by the collision.