Cart collisions: elastic and inelastic
What students will learn
- Test whether momentum is conserved in a collision
- Contrast elastic and inelastic collisions with the same equipment
What you'll need
- Two carts with magnetic and hook-and-loop bumpers
- Track
- Two motion sensors, one per cart
- Extra masses
Setup
Level the track carefully; a slow drift ruins momentum numbers. Put a motion sensor on each cart. Magnetic bumpers face each other for elastic runs; turn the carts around to the sticky bumpers for inelastic runs.
What students do
- Push one cart toward the stationary other and record both velocities through the bounce.
- Compute total momentum before and after. Compare.
- Load one cart with extra mass and repeat.
- Turn the bumpers around so the carts stick, and run it again.
- Compute kinetic energy before and after for both kinds of collision.
What to expect
Momentum comes out conserved within a few percent in every run. Kinetic energy survives the magnetic bounce but drops sharply when the carts stick. That asymmetry, momentum always, energy sometimes, is the whole point of the unit.
Teaching notes
A favorite exam setup in AP and IB. If time is short, do elastic as a class demo and let groups run the inelastic case; the sticky collision has the more surprising numbers.