Chapter 8

What is momentum? Momentum can be defined as inertia in motion. Another way to think of it is to consider how hard it is to stop something that is moving.

What is the formula? The formula is Momentum = mass x velocity or M=mv. M = momentum m = mass v = velocity The units are kg-m/s

Examples? An example of momentum is seen when we consider a bike and a freight train moving at the same speed. Which one is harder to stop? Why?

What is impulse? A change in momentum is brought about by applying a force for a certain time interval. We call this an impulse, thus Impulse = Force x time.

Impulse = change in momentum Setting impulse equal to change in momentum gives us: Ft = mv A large momentum can be achieved by applying a large force, applying a force over a long time or both.

How can collisions be safer? To avoid being injured, we like to extend the time of contact when we encounter large momentums.

Why airbags? Examples include using airbags, making cars with padded dashboards, bending our knees when we jump from large heights, etc.

Bouncing Bouncing requires more impulse since an object must be quickly stopped and then it must be “thrown” back upward.

Bouncing hurts more If an object falls and bounces off your head instead of breaking when it hits, it will hurt more.

Total momentum doesn’t change The momentum of all closed systems remains the same even though individual parts may undergo a change.

Equal and opposite An example is a rifle bullet system. The bullet undergoes a large change in momentum, but so does the gun and it is equal and opposite. This is recoil

Conservation of Momentum The net momentum hasn’t changed and thus we say it has been conserved. One of the central laws in physics is the Law of conservation of momentum.

Total momentum doesn’t change In the absence of an external force the momentum of a system remains unchanged.

Momentum is conserved When moving objects collide in the absence of external forces net momentum (before) = net momentum (after)

Elastic Collisions When objects collide without being permanently deformed and without generating heat, the collision is said to be an elastic collision. Colliding objects bounce perfectly in elastic collisions.

Inelastic Collisions Whenever colliding objects become tangled or couple together, they are said to undergo an inelastic collision.

Types of collisions Most collisions in the everyday world are inelastic. Even when billiard balls collide on a pool table, they don’t bounce off with the same impact as they had before impact.

But they do slow down. Why? Heat and sound are generated when the billiard balls collide and “steal” some of the momentum, thus they eventually slow down.

Even fireworks? Yes even fireworks Even the fragments that are produced by a firecracker follow the rules of vector addition. The vector sum after explosion equals that of before explosion.