Air track collisions – conservation of momentum
PhysicsEnergy & MomentumAges 15–16Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Energy & Momentum
Two carts of mass m₁ and m₂ move with velocities v₁ and v₂ on an air track, then collide either elastically or inelastically (sticking together). Graphs of p₁, p₂, and the total p over time show that total momentum stays constant, and a table compares momentum and kinetic energy before and after the collision. Drag a cart to change its starting position. The Force sensor screen fits a bumper (soft spring, hard spring, crumple zone or airbag) and plots force against time: the impulse J, the area under the graph, equals the Δp measured with light gates, and a longer contact time gives a smaller peak force.
Lesson: Momentum, conservation of momentum, elastic and inelastic collisions, impulse and force–time graphs
What it shows
Momentum p = mv is a vector, so its sign shows the direction of motion. On a frictionless air track the total momentum m₁v₁ + m₂v₂ is the same before and after a collision; elastic collisions also conserve kinetic energy, while carts that stick together lose some as heat and deformation. During impact the bodies push on each other with equal and opposite forces (Newton's third law). The impulse J = ∫F dt, the area under the force–time graph, equals the change in momentum, so for the same Δp a longer contact time means a smaller peak force: the idea behind crumple zones, airbags and helmets.
How to use
On the air track, set the masses and velocities, drag the carts and press Run, then compare the Before and After rows and press Change collision type. Press Force sensor & crash test, choose Collides with and a Bumper, and press Launch. Each run adds a table row with light-gate times, Δp, J, Δt and peak force. Keep m₁ and u₁ fixed while you compare bumpers, then try the Design challenge.
Parameters you can change
- Mass of cart 1 0.5–5 kg
- Initial velocity of cart 1 -5–5 m/s
- Mass of cart 2 0.5–5 kg
- Initial velocity of cart 2 -5–5 m/s
- Collision type Elastic, Inelastic (sticking together)
- Screen Air track (momentum conservation), Force sensor & crash test
- Bumper Soft spring, Hard spring, Crumple zone, Airbag
- Collides with Wall with force sensor, Cart 2 at rest
Questions to explore
- In the inelastic collision, how can you calculate the common velocity of the carts from their masses and initial velocities?
- With the same change in momentum, why does a crumple zone or airbag give a smaller peak force than a hard spring?
- With equal masses and an elastic collision, what happens to the velocities of the two carts?
Teaching ideas
All guides- How to Use Simulations in PowerPoint and Google SlidesUse simulations in PowerPoint and Google Slides: link a slide to present mode, add a QR code students can scan, and run the demo without leaving your deck.
- Impulse Virtual Lab: Force–Time Graphs and Crash SafetyAn impulse virtual lab: crash a cart into a force sensor with four bumpers, read peak force and contact time, and see why crumple zones and airbags work.
