Work, Energy and Power: Class 9 Study Guide
Work is done when a force moves an object. Energy is the ability to do work.
Work is done when a force moves an object in the direction of the force: W = F × s, measured in joules (J). No work is done if there is no movement, so pushing a wall that does not move is no work in physics. The energy of a body is its capacity to do work, also in joules.
Power is the rate of doing work: P = W ÷ t, in watts (W). One watt is one joule per second. A kilowatt-hour is an energy unit used on electricity bills: 1 kWh = 3.6 × 10⁶ J.
Two kinds of mechanical energy
Kinetic energy is the energy of motion: KE = ½mv². Potential energy near the Earth is the energy of position: PE = mgh. A 2 kg object lifted 5 m (g = 10) has PE = 2 × 10 × 5 = 100 J. A 2 kg object moving at 3 m/s has KE = ½ × 2 × 9 = 9 J.
| Quantity | Formula | Unit |
|---|---|---|
| Work | F × s | joule |
| Kinetic energy | ½mv² | joule |
| Potential energy | mgh | joule |
| Power | W ÷ t | watt |
Conservation of energy
Energy cannot be created or destroyed. It changes from one form to another. A falling ball loses PE and gains KE, and the total stays the same if there is no air resistance. A ball dropped from 5 m has, just before it lands, ½mv² = mgh, so v² = 2gh = 100 and v = 10 m/s.
PE at the top.
KE + PE.
Total is conserved.
Check the units.
Power in daily life
A 60 W bulb uses 60 J every second. Used for 5 hours, it uses 0.06 kW × 5 h = 0.3 kWh. Efficient appliances do the same job with less power. Switching off unused lights saves energy and money.
Worked examples and common mistakes
Example: A force of 20 N moves a box 4 m. W = 80 J. Example: A machine does 600 J of work in 30 s. P = 20 W. Common mistakes: using the mass in grams, using the distance when the force is at right angles to the movement (no work is done), and saying that energy is 'used up'. It changes form.
Check your understanding: model answers
1. 90 J. 2. ½ × 4 × 25 = 50 J. 3. 0.1 kW × 2 h = 0.2 kWh.
✨ Key Takeaways
- Work = force × displacement.
- KE = ½mv² and PE = mgh.
- Power = work ÷ time.
- Energy is conserved but changes form.
💬 Questions for Discussion & Reflection
- Find the work done by 15 N over 6 m.
- Find the KE of a 4 kg body at 5 m/s.
- How much energy does a 100 W bulb use in 2 hours, in kWh?