Elastic Collisions & Momentum Conservation
1 · Predict
Two identical balls roll toward each other at the same speed and collide elastically. After the collision, how do their velocities compare to before?
- Each ball bounces straight back at the same speed it came in with
- Both balls stop dead at the point of collision
- Both balls continue on unchanged, as if they passed through each other
2 · Set Up
- Open the elastic-collision simulation and press Reset.
- Confirm both balls have equal mass and set the approach speed for each trial below (Ball A moves right, Ball B moves left at the same speed).
- Press Play, watch the momentum probes on both balls, and record Ball A's momentum reading right after the balls separate.
3 · Collect Data
| Mass m (each ball) (kg) | Approach speed v (m/s) | Ball A momentum p = mv (kg·m/s) | Ball A KE = ½mv² (J) | Ball A momentum after collision (kg·m/s) |
|---|---|---|---|---|
| 1 | 6 | |||
| 1.5 | 8 | |||
| 2 | 5 |
Plot approach speed v (x-axis) against Ball A's kinetic energy before the collision (y-axis).
4 · Analyze
- Show your work computing Ball A's momentum before the collision (p = mv) for one trial and compare it to the simulation's momentum probe reading.
- Compare Ball A's momentum before and after the collision. What does the sign change tell you about its direction? Is the total momentum of Ball A + Ball B conserved?
5 · Extend
- The simulation uses a restitution of 0.98, not a perfect 1.0. How would you expect the rebound momentum to differ from your idealized prediction, and why?
- If Ball B were twice as heavy as Ball A, would Ball A still bounce straight back? Use the general elastic-collision formulas to reason about it.
The Physics Behind This Experiment
Momentum
Each ball's momentum before impact, from the mass and approach speed you set for each trial. Total momentum (Ball A + Ball B) is conserved through the collision.
Kinetic Energy
Ball A's energy of motion before impact. In a perfectly elastic collision this is fully conserved; the live sim's 0.98 restitution keeps only e² of it, which your Elaborate answers should account for.