Physics · Unit 5: Momentum ·  Project 5-2-3

Collision Lab

Roll carts into each other, measure every speed, and decide whether momentum really was conserved. It’s the exact check you’ll use to reconstruct a real crash  ·  3 class days

Starringp_before = p_after ± δpgap ≤ δ_before + δ_after
Unit 5: Momentum

🔁 Review the Concepts

This project reuses the cart-and-video setup from several earlier lessons. Skim whichever one feels shaky before you start.

Activity 5-1-3
Conservation of Momentum in One Dimension
The rule you’re testing, and a first cart-collision lab with the same track and timing zones. Start here.
Review activity →Deep dive →
Activity 5-2-1
Elastic vs. Inelastic Collisions
The three bumper types, and how to classify a collision from its kinetic energy.
Review activity →Deep dive →
Activity 5-2-2
Conservation of Momentum — Two Objects
Solving for unknowns, both objects moving, and why a small final speed makes measurements less reliable.
Review activity →Deep dive →
Project 4-1-4
Pendulum Energy Swap
Where you learned to measure speed from video frames, and how a one-frame error moves your answer.
Review project →Deep dive →

🧭 Plot Summary

You’ve been told that momentum is conserved. In this project you’ll find out whether you can show it. That’s harder than it sounds. Real carts roll on real tracks with real friction, and you measure their speeds from video that can only be read to the nearest frame. Your before-and-after numbers will never match exactly.

So “verify” has to mean something more careful: do the two totals agree within the uncertainty of your measurements? And if they don’t, is it random scatter or a systematic error you can find? You’ll build and level a track, run a coast test to measure friction, collect a full set of collisions, and write a verification you could defend to a skeptic.

What you will do in this project

  • Build and level a collision track, then run a coast test to see how much friction alone changes a cart’s speed.
  • Plan a set of collisions with different bumpers and different mass ratios, and run each three times.
  • Measure every speed before and after from slow-motion video, with an uncertainty on each.
  • Calculate the total momentum before and after, and test whether the two agree within their uncertainty.
  • Hunt for a systematic error, then write a verification claim you could defend to a skeptic.

Why it matters

A crash reconstruction is only as trustworthy as the physics under it. Before you lean on conservation of momentum in Problem 5-3-1, you need to know how well it holds, and how to say so honestly.

✅ Self-Check Before You Roll On

Check off each item as you get there. These are not grades — they are your own signal.