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A porter carries a heavy bag and calls it hard work. Science counts work a different way. In science, work needs a force and a movement along that force.

This chapter is Work, Energy, and Simple Machines. It is Chapter 7 of your Class 9 book, Exploration.

What you will learn

• When a push does work. A force and a movement along it.
• Two kinds of energy. Kinetic (of motion) and potential (stored).
• Energy changes form. The total stays the same.
• Where it seems to vanish. Friction turns the energy into heat.
• Power. How fast the work is done.
• A machine trades force for distance. Less force needs more distance.
• Three machines. Pulley, lever and ramp.
• Nothing is free. No machine gives out more than it takes in.
• Energy stored in a stretch. A springy object stores energy.
• Work from a graph. Work is the area under the line.

Quick catch-up

These help, though none is required.

• Action and reaction — a force acts between two things.
• Friction, with numbers — friction is a force like any other.
• What a force does — start, stop, speed, turn or squash.
• Heat and temperature — temperature is a number; heat is what moves.
• Which way heat goes — heat moves from hot to cold.
• A rate is a ratio (Maths) — two amounts compared, units kept.

Stay safe

Every lab here is on the screen. Nothing asks you to lift or push anything heavy.