Chapter 10 · NCERT Class 9 Science

Work and Energy

Carrying a bag across a room can feel exhausting — yet in physics you might be doing no work at all. Tap each term to see what work, energy and power really mean, and the formulas that tie them together.

⚡ 3 topics⏱ ~25 min📝 12-question quiz

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Six words that power physics

Work, energy and power have exact meanings here — not the everyday ones. Tap each term to see its definition, its formula and its unit.

Explore · Work, energy & powertap a term

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The three big ideas

Work is done only when a force moves an object: W = F × s (force × displacement in the direction of the force). Unit: joule (J); 1 J = 1 N × 1 m.
Zero work, case 1 — no displacement. If the object does not move (s = 0), then W = F × 0 = 0, no matter how hard you push.
Zero work, case 2 — force perpendicular to motion. Carrying a bag while walking on level ground: the support force is vertical, the motion is horizontal, so that force does no work.

Worked example. A force of 10 N moves a box 5 m in the direction of the force. Find the work done.

W = F × s = 10 N × 5 m = 50 J.

Common mistake: thinking that holding a heavy bag perfectly still does work. In physics it does no work — there is no displacement (s = 0), so W = 0, even though your arms get tired.

Energy is the capacity to do work. Unit: joule (J), the same as work.
Kinetic energy — the energy of a moving object: KE = ½ m v².
Potential energy — energy stored because of height/position: PE = m g h.
Conservation of energy: energy cannot be created or destroyed, only changed from one form to another. A ball dropped from a height converts its PE → KE as it falls — the total stays the same (ignoring air resistance).

Power is the rate of doing work: P = W ÷ t. Unit: watt (W); 1 W = 1 J/s, and 1 kW = 1000 W.
The commercial unit of energy is the kilowatt-hour (kWh) — the energy used by a 1 kW device in 1 hour. 1 kWh = 3.6 × 10⁶ J.
Electricity bills count “units”, where 1 unit = 1 kWh.

Where you'll meet it

Energy, put to work

Hydroelectric power

Water held high behind a dam stores potential energy (PE = m g h). As it falls, that PE becomes kinetic energy, which spins turbines and is converted into electricity — a chain of energy transformations, with the total conserved.

Your electricity bill

The meter adds up energy in kilowatt-hours (kWh), the commercial unit. A 1 kW heater for 2 hours uses 2 kWh = 2 “units” — power × time is exactly what you pay for.