PHY080 · Energy & Renewable · Classes 9–11
Energy conservation
Objective: To show that potential energy changes into kinetic energy while the total energy stays constant (when friction is negligible).
🧰 Apparatus
- Curved track (or a model roller coaster)
- Trolley or ball
- Light gates or video analysis
- Metre rule, balance
📐 Formulas and SI units
PE = mgh; KE = ½mv² (J)
v = √(2gh) with no friction
💡 Theory
At the top the car has potential energy mgh. As it runs down, PE turns into KE = ½mv². With no friction the total stays mgh, so at the bottom v = √(2gh) whatever the mass or the shape of the track. Friction turns some energy into heat.
🪜 Procedure
- Measure the mass and the starting height.
- Release the car from rest.
- Measure its speed at the lowest point with a light gate.
- Compare ½mv² with mgh.
- Repeat from other heights and with a rougher track.
📋 Observations Simulated values
✅ Result and conclusion
A 1 kg car from 5 m has 49 J and reaches 9.9 m/s at the bottom; with 20% friction loss it reaches 8.85 m/s.
🔬 In a real lab
Friction and air resistance always remove some energy, so the car cannot climb back to its starting height.
⚠️ Safety
Keep the track stable and clear; do not stand in the path of a released car.
❓ Quick check
1. At the lowest point the car has the most…
2. Doubling the mass makes v at the bottom…
3. Where does 'lost' energy go?
🏋️ Practice
Easyv from 20 m with no friction?
Answer: ≈ 19.8 m/s
MediumPE of 2 kg at 3 m?
Answer: 58.8 J
ChallengeWhy must each hill be lower than the first?
Answer: Friction removes energy