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 "bouncy" (elastic) and "sticky" (inelastic) collisions?
Marc Reif - February 2011
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