🧭 Plot Summary
So far the carts have been polite. One moves, one waits, and they stick or bounce. Real situations are messier. Both objects are moving. One of the masses is unknown. A cart is already rolling when it splits in two. A crash leaves nothing but a skid mark on the road.
The rule hasn’t changed: the total momentum before equals the total after. What changes is how you use it. You’ll solve harder before-and-after problems, identify a hidden mass with only a cart collision, and learn a two-stage method that is the backbone of crash reconstruction: momentum for the crash, then work and energy for the skid.
What you will do in this activity
- Solve collisions where both objects are moving, whether head-on or one chasing the other.
- Find an unknown mass or velocity using nothing but conservation of momentum.
- Handle systems that split apart or throw something while already moving.
- Identify a hidden “mystery mass” with a cart collision, without ever putting it on a scale until the end.
- Work backward from a skid mark to the speed before a crash, a preview of Problem 5-3-1.
Why it matters
Investigators can’t ask the cars how fast they were going. They have to work backward from what was left behind. This activity builds the exact toolkit you’ll use in Problem 5-3-1, and sharpens the data-handling you’ll need for Project 5-2-3.
✅ Self-Check Before You Roll On
Check off each item as you get there. These are not grades — they are your own signal.