Inelastic Collision & Momentum Conservation
1 · Predict
A moving ball crashes into a heavier stationary ball and they move off together. Is momentum conserved even though kinetic energy is lost?
- Both momentum and kinetic energy are conserved
- Momentum is conserved, but kinetic energy is not
- Neither momentum nor kinetic energy is conserved
2 · Set Up
- Open the inelastic collision simulation and press Reset.
- Enable the KE probes on ball A and ball B.
- Set ball A's initial speed for each trial below, press Play, and record the shared speed both balls move at right after they collide.
3 · Collect Data
| Mass of ball A m₁ (kg) | Mass of ball B m₂ (kg) | Initial speed of ball A v₁ (m/s) | Initial momentum p = m₁v₁ (kg·m/s) | Combined speed after collision (m/s) |
|---|---|---|---|---|
| 1 | 4 | 6 | ||
| 1 | 4 | 10 | ||
| 1 | 4 | 14 |
Plot the combined final speed (y-axis) against the initial momentum p (x-axis).
4 · Analyze
- Show your work computing the initial momentum p = m₁v₁ for one trial and compare it to the simulation's readout at the same trial.
- Using the total mass (m₁ + m₂) and momentum conservation, predict the final speed and compare it to your recorded value. How close is the match?
5 · Extend
- Compute the kinetic energy just before the collision and just after (using the combined final speed and total mass). Where did the missing kinetic energy go?
- This collision isn't perfectly inelastic (restitution ≈ 0.05, not exactly 0). Would you expect the real final speed to be slightly higher or lower than the ideal prediction? Explain.
The Physics Behind This Experiment
Momentum
The quantity conserved in every collision, including this one. Ball B starts at rest, so the system's total momentum before impact is just ball A's momentum m₁v₁.
Kinetic Energy
Unlike momentum, kinetic energy is not conserved in an inelastic collision — comparing the total KE before and after the collision shows how much energy was lost to deformation and heat.