PHY008 · Mechanics · Classes 11–12
Centripetal force
Objective: To investigate the centripetal force needed to keep an object moving in a circle.
🧰 Apparatus
- Rubber bung on a string
- Glass or plastic tube
- Hanging masses (to provide the force)
- Stopwatch
- Metre scale
📐 Formulas and SI units
F = mv²/r (N)
a = v²/r (m/s²)
v = 2πr/T
💡 Theory
An object moving in a circle at constant speed is always changing direction, so it accelerates towards the centre: a = v²/r. A net inward force F = mv²/r is needed; here the string tension provides it.
🪜 Procedure
- Whirl the bung in a horizontal circle with the string through the tube.
- Keep the radius fixed using a marker on the string.
- Time 20 revolutions and calculate v = 2πr/T.
- Change the hanging mass (the force) and repeat.
- Plot F against v²: it is a straight line.
📋 Observations Simulated values
✅ Result and conclusion
A 0.5 kg mass at 4 m/s on a 2 m radius needs a 4 N centripetal force.
🔬 In a real lab
The string is never exactly horizontal and friction at the tube adds error; keeping the radius constant by eye is hard.
⚠️ Safety
Whirl only soft bungs, wear goggles, and keep a clear space around you.
❓ Quick check
1. Doubling the speed makes the centripetal force…
2. If the string breaks, the object moves…
3. What provides the centripetal force for the Moon?
🏋️ Practice
Easym = 1 kg, v = 2 m/s, r = 1 m. F?
Answer: 4 N
Mediuma for v = 10 m/s, r = 50 m?
Answer: 2 m/s²
ChallengeHalving r at the same speed changes F by…?
Answer: Doubles it