Work, Power & Energy – ICSE Class 10 Physics

1) Work — What Does It Actually Mean?

So here's the thing. In our daily life, "work" means anything that requires effort — studying, lifting bags, even just thinking about what to eat. But in Physics? Nope. Physics has a very specific definition.

Work is said to be done only when a force applied on a body produces displacement in the body.

I remember in class my teacher kept saying: "No displacement, no work!" And honestly, that's the golden rule. If you push a wall until you're sweating but the wall doesn't move — in Physics terms, zero work. Disappointing, right? 😅

The Formula (This is your bread and butter)

Special Cases of θ — Memorise These!

2) Energy — The Ability to Do Work

Energy is basically the capacity to do work. If something has energy, it can do work. And here's the cool part — when you do work on an object, its energy changes. They're two sides of the same coin.

SI unit of energy: joule (J). Same as work. Makes life easier.

3) Kinetic Energy (KE) — Energy of Motion

Any moving object has kinetic energy. A speeding car, a flying bird, a ball you just threw — they all have KE. The faster it moves and the heavier it is, the more KE it has.

Derivation of KE — Don't Skip This!

Look, I know derivations feel boring at first. But in ICSE boards, the 2-mark "derive the expression for kinetic energy" question is guaranteed. It appears almost every year. Here's a really simple way to remember it:

Step 1: Start with the work done to bring a body from rest to velocity v.

Step 2: Work done = Force × displacement.

Step 3: Use F = ma and Newton's third equation of motion.

See how clean that is? Write this 2-3 times and you'll never forget it. The trick is in those two equations — F = ma and v² = 2as. Join them and boom, you're done.

4) Potential Energy (PE) — Energy of Position

Potential energy is the energy stored in a body because of its position or configuration. There are different types, but for ICSE Class 10, we focus on gravitational potential energy.

Derivation of PE — Another Free 2 Marks!

This one's even simpler. I used to mess it up until a friend showed me this trick. Just think: when you lift something up, you're doing work against gravity. That work gets stored as potential energy. That's literally it.

5) The Law of Conservation of Energy

"Energy can neither be created nor destroyed. It can only be transformed from one form to another."

This is one of those statements that sounds simple but is SO important. What it means is that the total energy in an isolated system always stays constant. It just changes forms.

The Classic Example — A Ball Dropped from Height

This is the most common question in ICSE. At any point during the fall:

• At the top (height h): PE = mgh, KE = 0. Total = mgh.
• Midway (height h/2): PE = mg(h/2), KE = ½ mv² = mg(h/2). Total = mgh.
• Just before hitting ground (height 0): PE = 0, KE = ½ mv² = mgh. Total = mgh.

This formula v = √(2gh) is incredibly useful. ICSE asks this in so many forms — finding speed just before hitting ground, finding height from speed, etc. If you memorize just this one relationship, half the numericals are done.

6) Power — How Fast Work is Done

Work and energy tell us how much work is done, but they don't tell us how quickly. That's where power comes in.

Power is the rate of doing work. Or in simpler words — how much work gets done per second.

Also, since W = F × s:

This one formula — Power = Force × velocity — is a lifesaver in numericals. If a question gives you force and speed, you can directly find power in one step. No need to find work and time separately.

SI Unit of Power

• 1 watt = 1 joule / 1 second. So 1 W = 1 J/s.
• Larger units: 1 kilowatt (kW) = 1000 W. 1 megawatt (MW) = 10⁶ W.
• Horsepower (hp): An older unit. 1 hp = 746 W. ICSE sometimes uses this in problems.

7) Commercial Unit of Energy — kilowatt-hour (kWh)

In our electricity bills, we don't pay in joules — we'd need HUGE numbers for that. Instead, we use kilowatt-hour (kWh).

1 kWh is the energy consumed when a device of power 1 kW runs for 1 hour.

So if you see a 100 W bulb running for 10 hours: energy consumed = 0.1 kW × 10 h = 1 kWh = 1 "unit" on your electricity bill.

8) Difference Between Work, Energy and Power — Tabular Form

This is a VERY common 2-mark question. Write it in table format in your exam for extra marks. I wrote exactly this in my board exam.

9) Solved Numericals — The Fun Part 💪

Alright, let's actually solve some real ICSE-level problems. These are the types that actually come in the exam. I've included a mix of easy and tricky ones.

Numerical 1 — Basic Work Calculation

A force of 50 N acts on a body and displaces it by 5 m. Calculate the work done when the force is: (a) in the direction of displacement (b) at 60° to the displacement

See how the work is less when the force is at an angle? That's why pulling something at an angle is "easier" in terms of force, but less work goes into horizontal displacement.

Numerical 2 — Kinetic Energy

A car of mass 800 kg is moving with a speed of 72 km/h. Find its kinetic energy. If the driver applies brakes and the car stops after 50 m, find the force applied by the brakes.

Numerical 3 — Potential Energy + Conservation of Energy

A stone of mass 2 kg is dropped from a height of 20 m. Find its potential energy at the top, kinetic energy just before hitting the ground, and its velocity just before impact. (Take g = 10 m/s²)

That v = √(2gh) is honestly the most repeated formula in ICSE energy chapters. If you see "dropped from height", this is your go-to.

Numerical 4 — Power Calculation

A pump lifts 1000 kg of water per minute from a well 20 m deep. Calculate the power of the pump. (Take g = 10 m/s²)

Numerical 5 — Energy in kWh (Electricity Bill Style)

A household uses four 60 W bulbs for 5 hours a day, a 100 W refrigerator running 24 hours a day, and a 2 kW heater running 2 hours a day. Calculate the total energy consumed in one day in kWh.

10) Quick Revision — What to Remember the Night Before the Exam

If you're reading this the night before your exam (no judgement, I've been there 😅), here's the absolute minimum you need:

• Work = Fs cos θ — and the three special cases (0°, 90°, 180°)
• KE = ½ mv² with derivation (F = ma + v² = 2as)
• PE = mgh with derivation (F = mg, W = mgh)
• Conservation of energy: PE + KE = constant. v = √(2gh)
• Power = W/t = Fv — SI unit: watt (W)
• 1 kWh = 3.6 × 10⁶ J — commercial unit of energy
• Coolie example — work done is zero (θ = 90°)

Common Mistakes That Cost Marks (I made them so you don't have to)

• ❌ Writing W = Fs without considering θ. Add cos θ always.
• ❌ Forgetting to convert km/h to m/s.
• ❌ Mixing up kW (power) and kWh (energy). kW = rate, kWh = total.
• ❌ Not writing units in the final answer. J for work/energy, W for power.
• ❌ Using g = 9.8 when not specified. Safer to use g = 10 m/s² in numericals unless the question specifically says 9.8.
• ❌ Writing derivation without stating which equations you used. Show every step for full marks.

11) Important Formulas — All in One Place