Day 16: Starting the Momentum Model

If you've been following me and read this tweet, you know that I'm going to be defining momentum in a different way this year. We're defining force as the slope of the momentum-time graph. This means we have to define momentum too, which is the hand-wavy part of this unit. Why even come up with this concept? Luckily, we define momentum as "how hard it is to stop something" and "mass times velocity" before anyone was too awake.

Then we went to lab and had a cart bounce off a force probe. We changed the mass of the cart and the spring on the force probe. We graphed momentum-time and force-time graphs. Here's what students noticed in lab:

• The momentum of the cart stayed constant except when it is touching the force probe.
• The force was negative because it pushed in the opposite direction of the initial motion of the cart.
• The momentum of the cart started positive and went negative.
• The force was the largest when the momentum of the cart was zero.
• When we used the springs, the momentum after hitting the spring was the opposite of the momentum before hitting the spring. The two momenta had the same magnitudes.
• When we used the clay, the momentum after hitting the spring was less than the momentum before hitting the spring.
• We could never get the final momentum to be larger than the initial momentum except that one time when the plunger exploded and hit the force probe, but we don't think that counts.