Chapter 7

Momentum

Remember: Inertia is the resistance of any moving or nonmoving object to change its state of motion.

Inertia in motion (moving objects) is known as MOMENTUM.

7.1 MomentumMomentum is the product

of the mass and the velocity of an object.

Momentum = mass X velocity

(momentum = mv)

Momentum has both direction and magnitude

Therefore, momentum is a vector quantity.

As mass increases, momentum increases.

For example: getting hit by a tennis ball vs. getting hit by a bowling ball

As velocity increases, momentum increases

For example: getting touched on the face vs. getting punched in the face

Could a roller blade and a 1-ton truck ever have the same momentum?

Yes – if the velocity of the skate was substantially more than the truck since the mass is so much less.

7.2 What changes momentum?

If the velocity or mass change, the momentum changes, since

momentum = velocity X mass

Since mass remains constant, momentum generally changes because of changes in velocity.

What is this called?

Acceleration

What causes acceleration?

Forces!

Therefore… The greater the force acting on an object, the greater the change in velocity, and the greater the change in momentum.

Time is also important…

The same force over more time creates a greater change in momentum.

Therefore…

Time and force are both important in changing momentum.

Force of impact X Time of impact = Impulse

(f X t)

Impulse = any change in momentum

Impact = force (N)

Why do golfers “follow through” when they hit the golf ball?

They are increasing the time of impact which will increase the momentum.

Your car is out of control. You are about to crash.

You have a choice:Do you crash into a brick wall or a nice, soft haystack?

Why do you choose the haystack?

Why will the haystack cause less damage?

Because the time of impact is greater.

Remember: f X t = impulse

As time , force .

It takes longer for the haystack to bring your momentum to zero than it would if you hit the wall.

Why do we have airbags in cars?You are forced to jump off a building.

You can land on a trampoline or the cement ground.

Which do you choose?Why?

The trampoline has more “give”.This means that the time of impact is greater, and, therefore, the force of impact is less.

As time of impact , the force

of impact

For example:A car hitting a brick wall

Breaking blocks of wood with your hands.

The action is sudden and short, and the force is high.

7.3 BouncingImpulses are greater when bounces occur.

Why?

The force needed to cause an object to bounce back is greater than the force needed to merely stop it.

For example:A glass bottle falls on your head.If it breaks when it hits your head, the impulse has ended.

If it bounces off your head, it takes more force for your head to bounce it away.

7.4 Conservation of MomentumRemember Newton’s 2nd Law?

F=maIf you want to accelerate something, you exert a net

force on it

If you want to change momentum, you exert an impulse on it.

Internal forces – forces that act inside a system

External forces – forces that act on the entire system

For example:You are in a car. If you push on the dashboard, the car does not move. You are an internal force.If you get out of the car and push on the trunk, you can now move the car; you are now an external force.

A rifle recoils as it shoots a bullet.The bullet gains momentum as it shoots out.The rifle gains momentum as it recoils.

BUT…The system does not gain or lose momentum.

Momentum of a system does not change unless outside forces act on it.

If momentum does not change, we say it is conserved.

Law of Conservation of Momentum:

In the absence of an external force, the momentum of a system remains unchanged.

Law of Conservation of Momentum

Momentum of A + Momentum of B

= (mass A + mass B)

X new velocity together

For example: (write this one down) A 70000g man is running

with a velocity of 6m/s. A 7000g baby sits in the road moving at 0m/s. What is the velocity of the two after the man picks the baby up?

7.5 CollisionsElastic collisions – collisions that occur when the two colliding objects do not have a lasting deformation or the generation of heat or sound.

Elastic Collisions10 m/s 0 m/s 0 m/s 10 m/s

10 m/s 5 m/s 5 m/s 10 m/s

Inelastic Collisions – Collisions which occur between two objects that become distorted and generate heat.Inelastic objects become entangled or couple together after the moment of impact.

Inelastic Collisions

4 m/s 0 m/s 2 m/s

7.6 Momentum Vectors

Since momentum is a vector quantity, you can represent collisions with vectors – non-linear as well as linear vectors.

Chapter 7 Key Terms•Conserved•Elastic Collision• Impulse• Inelastic collision•Law of Conservation of Momentum•Momentum