Physis Unit 6: Momentum Transfer Model

The beauty and the utility of the physics that we are learning is that it describes so many things.  Next we apply the techniques and ideas we have studied to objects that interact over relatively brief periods, with forces that change over time.  The collision of a comet with the earth, two football players,  or cars in a parking lot can all be described with the model we will develop.

Paradigm:  We pushed a low-friction lab cart gently down a track and released it.  At the end of the track it ran into a force sensor.  At the other end was a motion detector.

The graphs looked something like this:

From this we determined the area of the F vs. t graph had the same units and was very nearly the same value as the mass times the change in velocity of the lab cart.  We defined two new physical quantities:  momentum (m*delta v) and impulse (F*delta t).  For any system we assume that change in momentum is equal to impulse, based on these results. Momentum is symbolized by the letter "p" and impulse is symbolized by the letter "j".

Then we examined many collisions between two lab carts.  We graphed final system momentum vs. initial system momentum.  The result looked something like this:

From this, we concluded that momentum is a conserved quantity.  For any closed system, the system momentum is a constant, no matter what happens in it.

Remember any collisions that occur in a system with no net external force do not change the total momentum of the system; the system will have the same total momentum before and after the event.

Some collisions have the same total kinetic energy both before and after the event. We call those collisions "elastic", or "perfectly elastic". Any collision with a final kinetic energy less than the initial kinetic energy is "inelastic." Only a few types of real-world collisions come close to elastic, most are inelastic. 

Marc Reif - February 2011