Mechanics
Elastic collision
Watch total momentum stay the same as the balls bounce apart.
Study elastic collisions and momentum conservation with interactive colliding bodies. Observe how kinetic energy and momentum are shared when objects bounce apart.
Conservation of momentum
In a collision with no external forces, total momentum is conserved. In an elastic collision, kinetic energy is also conserved.
When two balls collide on a frictionless surface, their individual momenta change, but the system's total momentum stays the same. In a nearly elastic collision, they bounce apart with kinetic energy nearly unchanged — fast balls in, fast balls out.
- p = mv → Σp_before = Σp_after
- KE = ½mv² (conserved in elastic collisions)
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Two identical balls roll toward each other at the same speed and collide elastically. After the collision, how do their velocities compare to before?
Predictions to weigh
- 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
Variable roles
What you set:
- Mass m (each ball) (kg)
- Approach speed v (m/s)
What you measure:
- Ball A momentum p = mv (kg·m/s)
- Ball A KE = ½mv² (J)
- Ball A momentum after — ideal prediction (kg·m/s)
- Total momentum after collision (A + B) (kg·m/s)
How the investigation runs
- 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.
Governing equation
Momentum — p = m·v
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 — KE = ½·m·v²
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.
What the printable worksheet asks students to work out
- 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?
Where this shows up beyond the lab
- 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.
- AP Physics 1 — Unit 4: Linear Momentum
- AP Physics C: Mechanics — Unit 4: Linear Momentum
- IB Physics — A.2 Forces and momentum
- General High School Physics — Momentum & collisions
- NGSS High School Physics — Conservation of momentum
- Middle School Physical Science — Newton's third law and collisions
- Middle School Physical Science — Kinetic energy
- Welcome to elastic collisions
- Press Play
- Momentum exchange
- Select a ball
- Open the data
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.