Ever wondered why you feel more tired after climbing stairs than after walking on flat ground? The answer lies in work, energy and power.
💡 In Simple Words: Work is what you get when a force moves something. Energy is the “fuel” that lets you do work. Power tells you how fast you use that fuel.
What is Work? (work definition)
When you push a box across the floor, you are doing work. In physics, work is the product of a force that actually moves something and the distance it moves in the direction of that force.
Formula: Work (W) = Force (F) × Distance (d) × cos‑θ, where θ is the angle between the force and the direction of motion. If the force is straight along the motion, cos‑θ = 1, so W = F d.
Key point: If you push but the box doesn’t move, no work is done. The force must cause displacement.
Example of work: A 10 N push that slides a book 2 m across a table does 20 J of work (10 × 2 = 20). The unit joule (J) equals one newton‑meter.
Energy: Types and How It Changes (energy types)
Energy is the ability to do work. Think of it as a “fuel tank” that powers everything from a rolling ball to a light bulb.
Two main kinds appear in the ICSE syllabus:
- Kinetic energy – energy of motion. Calculated by ½ m v² (half the mass times speed squared).
- Potential energy – stored energy due to position or condition, like a ball held up on a shelf. The common form is gravitational potential energy = m g h (mass × gravity × height).
Energy can change from one form to another, but the total amount stays the same – that’s the law of conservation of energy.
Example: A roller‑coaster car at the top of a hill has lots of gravitational potential energy. As it rolls down, that energy turns into kinetic energy, making the car speed up.
Power: Doing Work Faster (power formula)
Power tells us how quickly work gets done or energy gets transferred. If you finish a task in half the time, you’re using twice the power.
Formula: Power (P) = Work (W) / Time (t). The unit is watt (W), equal to one joule per second.
Another handy form uses force and velocity: P = F × v (when force and motion are in the same direction).
Example: Lifting a 5 kg book onto a shelf 0.5 m high takes about 25 J of work (m g h = 5 × 9.8 × 0.5). If you do it in 2 seconds, the power is 12.5 W (25 ÷ 2).
Quick Comparison
| Concept | What it measures | Unit | Key formula | Everyday example |
|---|---|---|---|---|
| Work | Force × distance in direction of force | Joule (J) | W = F‑d‑cosθ | Pushing a shopping cart 3 m |
| Energy | Ability to do work | Joule (J) | Various: KE = ½‑mv², PE = mgh | Battery storing energy for a flashlight |
| Power | Rate of doing work | Watt (W) | P = W/t = F‑v | Microwave heating food faster than a stove |
Common Mistakes to Avoid
- Mixing up energy and power – energy is “how much”, power is “how fast”.
- Forgetting the cosine factor when force isn’t aligned with motion.
- Assuming work is done whenever you push – if there’s no displacement, work is zero.
- Confusing the difference between work and energy; work is a transfer of energy.
📝 Likely Exam Questions
- Define work and write its SI unit.
Work is the product of the component of force along the direction of displacement and the distance moved. Unit: joule (J). - A 15 N force pulls a crate 4 m horizontally. How much work is done?
W = F d = 15 × 4 = 60 J. - State the difference between kinetic and potential energy with examples.
Kinetic energy is energy of motion (e.g., a moving car). Potential energy is stored due to position (e.g., a book on a shelf). - Calculate the power developed when a 200 J work is done in 5 s.
P = W/t = 200 ÷ 5 = 40 W. - Explain why a person carrying a heavy bag up stairs does more work than walking on a level road.
Going up stairs adds a vertical displacement, so gravitational potential energy increases (m g h). On level ground, height doesn’t change, so only horizontal work is done.