A.3 · IB Physics
Energy conservation on a track
Release a ball on a curved track and watch energy move between kinetic and potential. Switch on friction and see it drain into thermal energy while the total stays fixed.
Predict first
Start from the defaults (no friction, no hill). If you change the mass from 1.0 kg to 2.0 kg, what happens to the ball's speed at the bottom of the track?
Gravitational PE mgh becomes KE ½mv², so v = √(2gh) and the mass cancels. Both KE and PE double, but the speed does not change.
Check it in the model: Move the Mass slider from 1.0 to 2.0 kg and compare the Speed readout as the ball passes the bottom.
kinetic potential (and track) thermal
friction force F = μN, N = normal force from the track
thermal = total work done against friction
KE + PE + thermal = E₀ (constant)
Try this
- Frictionless, no hill: release from 3.0 m. Check that the ball rises back to 3.0 m on the far side and the total stays at E₀.
- Tick Friction on and set μ = 0.12. Watch the thermal bar grow while KE + PE shrinks. Does the total change?
- Add the hill (top at 1.3 m). Release from 1.1 m, then from 1.5 m. Explain both results using energy only.
Exam tip: with no friction KE + PE is constant; with friction the work done against it becomes thermal energy, so the total is still conserved. Compare release height with the top of any hill: if the ball starts lower, it cannot cross, however long you wait.