Conceptual Physics – Chapter 7 – Momentum

A truck rolling down a hill has more momentum than a roller skate going the same velocity because it has larger mass.

If the truck is at rest and the roller skate moves, then the roller skate has more momentum.

Momentum is inertia in motion

Momentum = mass x velocity = mv

Question:  Can you think of a case where the roller skate and the truck would have the same momentum?

Force provides a change in velocity.  How long the force acts is important:

Force x time = Change in Momentum

Force x time = Impulse

Impulse = Change in Momentum

F t          =  D mv

Case 1:  Increasing Momentum

To increase momentum, you need to apply the greatest force possible for the longest time.

So, with the golf swing, below,

• Swing as hard as you can
• Follow through with the swing to prolong the time

Case 2: Decreasing Momentum

A longer impact time decreases the Force

A shorter impact time increases the Force.  

On the left, the boxer moves away from the punch, increasing the time of impact & decreasing the force

On the right, the boxer moves toward the punch, reducing the time of impact and increasing the force.

Momentum picked up during the fall must be absorbed by the elastic cord.

The stretching of the cord increases the time and decreases the force.

   7.3    Bouncing 

• Is the karate chop delivered in a short time or in a long time?
• If the hand bounces on impact, is the change in momentum greater?
• Is the impulse greater?

Impulse is greater when bouncing.

The Pelton Wheel’s curved blades cause water to bounce.

Momentum is a vector quantity

If no net force or net impulse acts on a system, the momentum of that system cannot change.

Law of Conservation of Momentum states:

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

Questions:

1. Newton’s second law states that if no net force is exerted on a system, no acceleration occurs.  Does is follow that no change in momentum occurs?

2.  Newton’s third law states that the force a rifle exerts on a bullet is equal and opposite to the force the bullet exerts on the rifle.  Does it follow that the impluse the rifle exerts on the bullet is equal and opposite to the impulse the bullet exerts on the rifle?

In this case, the momentum before the event is zero and must be zero after the event also (Law of Conservation of Momentum).

How can this occur?