NCERT Curiosity · Class 9 · Science
Work, Energy, and Simple Machines
Two children of different masses, the same slide, the same drop — energy conservation decides what happens at the bottom.
Work Kinetic & Potential Energy Power Simple Machines
Key idea
W = F × s
Force times displacement in the force's direction. The SI unit of work is the joule (J) — lifting 3 bags does 3× the work of lifting 1.
Key idea
When Work Isn't Simply “Effort”
Zero work

Carrying a box while walking — the force is perpendicular to displacement, however tiring it feels.

Positive or negative

Pushing forward is positive work; a goalkeeper stopping a ball does negative work.

Key idea
Work done on an object equals its change in energy.
A thrown ball gains energy from the work done throwing it, then transfers that energy, knocking over the wicket it hits.
Key idea
Forms of Energy

Mechanical

Motion or position.

Thermal

Makes things hot.

Sound

Vibrations of matter.

Electrical

Motion of charges.

Nuclear

Stored in atomic nuclei.

Chemical

Stored in fuels and food.

Key idea
K = ½mv²
Doubling velocity quadruples kinetic energy. A 0.2 kg cricket ball at 43 m/s carries about 185 J.
Key idea
U = mgh
Stored by position or deformation — a stretched slingshot, a bent bow, or a raised ball. A 200 g ball 10 m up stores 20 J.
Key idea
Conservation of Mechanical Energy
Released
Max potential, zero kinetic
→
Bottom
Zero potential, max kinetic
→
Far side
Max potential again
Key idea
P = W ÷ t
Running up the stairs and walking up slowly do the same work — but running needs far more power. SI unit: watt.
Key idea
Simple Machines

Pulley

Changes the direction of effort, not always its size.

Inclined plane

A longer, gentler ramp needs less force, over more distance.

Lever

A longer effort arm lets a small force move a bigger load.

Chapter · key terms to remember
Key Terms

Work (W = F × s)

Force times displacement in the force's direction.

Work-energy theorem

Work done on an object equals its change in energy.

Kinetic energy (K = ½mv²)

The energy of motion.

Chapter · key terms to remember
More Key Terms

Potential energy (U = mgh)

Energy of position or deformation.

Mechanical energy

Kinetic energy + potential energy.

Power (P = W ÷ t)

The rate at which work is done.

Chapter · key terms to remember
More Key Terms

Simple machine

A device changing the size/direction of a force.

Mechanical advantage

Load ÷ effort — how much a machine multiplies force.

Opening

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