Physics · 1.6 Momentum · Conservation laws
Momentum. Collide it.
Collide two trolleys and verify that total momentum (p = mv) is conserved. Switch between elastic and inelastic (sticking) collisions and watch the before/after momentum bars balance.
0625 Topic 1.6.1 — Momentum p = mv
1.6.2 — Conservation of momentum
1.6.3 — Impulse F = Δp/Δt
Trolley A (red)
2.0
2.0
Trolley B (blue)
1.0
-1.0
Momentum & energy
Total p before
3.0 kg·m/s
Total p after
3.0 kg·m/s
v₁ after
— m/s
v₂ after
— m/s
KE before
5.0 J
KE after
5.0 J
Momentum is always conserved. In an inelastic collision, kinetic energy is not — some becomes heat/sound.
📋 Method (Cambridge ATP procedure)
- Place two trolleys on a friction-compensated runway; use light gates to measure velocity before and after collision.
- For inelastic collisions, fit pins and cork (or Velcro) so the trolleys stick.
- Measure m₁, m₂ on a balance; measure u₁, u₂ before and v₁, v₂ after.
- Calculate total momentum before (m₁u₁ + m₂u₂) and after (m₁v₁ + m₂v₂) and compare.
Conclusion: within experimental accuracy, total momentum before = total momentum after.
⚠ Sources of error & precautions
- Friction — compensate by tilting the runway so a trolley moves at constant velocity when pushed.
- Velocity measurement — use light gates with a known card length to reduce timing error.
- Straight-line collision — ensure the trolleys collide head-on along the runway.
- Mass — measure trolley masses accurately on a balance.
🎯 Syllabus reference (0625)
- 1.6.1 — define momentum p = mv; define impulse = FΔt = Δp.
- 1.6.2 — apply the principle of the conservation of momentum to one-dimensional collisions.