9th Class Physics Chapter 5 Notes PDF 2026 – Work, Energy and Power

$W = F \times d$ — work is done only when a force moves its point of application in the direction of the force.

Work is maximum when the force and displacement are in the same direction ( $0^{\circ}$ ); zero when they are perpendicular ( $9 0^{\circ}$ ).

Example 1 — work done

A force of $20$ N moves an object $5$ m. Find the work done.

$W = F \times d = 20 \times 5$ .

Answer: $100$ J

Remember

Holding a heavy object stationary does zero work in the physics sense — there's no displacement, however tired your arms get.

Kinetic energy: $K E = \frac{1}{2} m v^{2}$ — energy due to motion.

Potential energy: $P E = m g h$ — energy due to position (height above a reference level).

Example 2 — kinetic energy

Find the kinetic energy of a $2$ kg body moving at $10$ m/s.

$K E = \frac{1}{2} m v^{2} = \frac{1}{2} (2) (10)^{2}$ .

Answer: $100$ J

Remember

Renewable energy resources (solar, wind, hydro) replenish naturally; non-renewable ones (coal, oil, gas) don't, on any human timescale.

$P = \frac{W}{t}$ — power is the rate of doing work, SI unit watt (W) $=$ J/s.

Example 3 — power

A machine does $500$ J of work in $10$ s. Find its power.

$P = \frac{W}{t} = \frac{500}{10}$ .

Answer: $50$ W

Efficiency $= \frac{useful energy output}{total energy input} \times 100%$ — always less than $100%$ , since some energy is always lost (usually as heat).

Quick Recap

● $W = F \times d$ ; maximum when force and displacement align, zero when perpendicular
● $K E = \frac{1}{2} m v^{2}$ ; $P E = m g h$
● $P = \frac{W}{t}$ , unit watt
●Efficiency $= \frac{output}{input} \times 100%$ , always under $100%$

9th Class Physics Chapter 5 Notes

Work

Energy: Kinetic and Potential

Power

Efficiency

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