Class 9 Science Chapter 7 Work, Energy and Simple Machines – Revision Notes

Work, Energy and Simple Machines works through the formulas for work, kinetic and potential energy, and power, then explains how pulleys, inclined planes, and levers trade force for distance, and this page maps out each one.

Last Updated: September 23, 2026

Work

• W = F × d (force × displacement in direction of force).
• No displacement or perpendicular displacement = no work.

Energy

• KE = ½mv² (kinetic, motion).
• PE = mgh (gravitational potential).
• Work-energy theorem: net work = change in KE.
• Conservation of energy: total energy constant, only transforms.

Power

• P = W/t; SI unit: watt (W).

Simple Machines

• Mechanical advantage = load/effort.
• Pulleys: change force direction; multiply effort with multiple pulleys.
• Inclined plane: trades distance for reduced force.
• Lever: multiplies force via fulcrum placement.
• Machines don’t reduce total work (ignoring friction) — they trade force for distance.

One-Line Summary

Work (force × displacement) links to kinetic and potential energy through the work-energy theorem and conservation of energy, power measures how fast work is done, and simple machines make tasks easier by trading force for distance without changing total work.

Frequently Asked Questions

What is the difference between work done in the everyday sense and work done in physics?
In physics, work is done only when a force causes an actual displacement of an object in the direction of the force; simply applying force without any movement, like pushing against a wall that does not move, means zero work is done.

What is the law of conservation of energy explained in this chapter?
The law of conservation of energy states that energy can neither be created nor destroyed, only transformed from one form to another, so the total energy of an isolated system always remains constant.

Written by Satish

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