Class 9 — Work and Energy

Chapter Overview & Weightage

Work and Energy is the second-most-scored chapter in Class 9 Physics after Motion. It introduces the language of work, energy, and power — concepts that recur in every higher class right up to JEE. The CBSE board exam typically pulls 5 to 7 marks from this chapter.

Most questions are formula-based numericals. Conceptual questions appear as 1- or 2-mark “what is the difference between…” style asks. The good news — no calculus, no vectors, just clean SI-unit arithmetic.

Year	CBSE Weightage
2024	6 marks
2023	7 marks
2022	5 marks
2021	6 marks
2020	7 marks

Key Concepts You Must Know

Work is done only when a force produces displacement in its direction. No displacement, no work — even if you push a wall for hours.
Kinetic energy (KE) is energy of motion. Doubling speed quadruples KE.
Potential energy (PE) depends on position relative to a reference (usually ground).
The law of conservation of energy is the single biggest idea in this chapter — energy converts from one form to another but is never created or destroyed.
Power is the rate of doing work. Two students lifting the same weight to the same height do equal work, but the faster one delivers more power.
The 1 kWh unit (used in electricity bills) equals $3.6 \times 10^6$ J. This conversion comes up every year.

Important Formulas

$W = F \times s \times \cos\theta$

When force and displacement are in the same direction, $\cos\theta = 1$ and $W = Fs$ . Use this for almost every Class 9 numerical.

$KE = \frac{1}{2}mv^2$

The factor of $\frac{1}{2}$ is non-negotiable — students forget this in 30% of attempts.

$PE = mgh$

Always with respect to a reference level (usually the ground).

$P = \frac{W}{t}$

Units: 1 watt = 1 joule per second. 1 kilowatt-hour = $3.6 \times 10^6$ J.

$W_{net} = \Delta KE = \frac{1}{2}m v_f^2 - \frac{1}{2}m v_i^2$

Net work on a body equals its change in kinetic energy.

Solved Previous Year Questions

PYQ 1 (CBSE 2023, 3 marks)

A 10 kg object is lifted to a height of 2 m. Calculate the work done against gravity. If the same object is then dropped, find its KE just before hitting the ground. ( $g = 10\,\text{m/s}^2$ )

Solution:

Work done against gravity: $W = mgh = 10 \times 10 \times 2 = 200\,\text{J}$ .

When dropped, all this PE converts to KE (no friction). So $KE = 200\,\text{J}$ at the ground.

Cross-check: $v = \sqrt{2gh} = \sqrt{40} = 6.32\,\text{m/s}$ . $KE = \frac{1}{2}(10)(6.32)^2 \approx 200\,\text{J}$ . Consistent.

PYQ 2 (CBSE 2022, 5 marks)

A pump can lift 600 kg of water per minute to a height of 25 m. Calculate the power of the pump in watts and in horsepower (1 HP = 746 W).

Solution:

Work per minute: $W = mgh = 600 \times 10 \times 25 = 150{,}000\,\text{J}$ .

Time = 60 s. Power: $P = W/t = 150000/60 = 2500\,\text{W}$ .

In HP: $2500/746 \approx 3.35\,\text{HP}$ .

PYQ 3 (CBSE 2021, 2 marks)

State the law of conservation of energy. A ball falls from a height of 5 m. Show that its mechanical energy at the top, midway, and just before hitting the ground is the same.

Solution:

Law: Energy can neither be created nor destroyed; it can only be transformed from one form to another. The total mechanical energy of an isolated system is conserved.

At the top ( $h = 5\,\text{m}$ , $v = 0$ ): $E = mgh = 5mg$ .

Midway ( $h = 2.5\,\text{m}$ ): $v^2 = 2g \cdot 2.5 = 5g$ . $E = mg(2.5) + \frac{1}{2}m(5g) = 2.5mg + 2.5mg = 5mg$ .

Just before ground ( $h = 0$ ): $v^2 = 2g \cdot 5 = 10g$ . $E = \frac{1}{2}m(10g) = 5mg$ .

All three values are equal — energy is conserved.

Difficulty Distribution

For Class 9 Work and Energy, expect roughly:

Easy (definition + formula): 2 marks
Medium (numerical): 3 marks
Hard (conceptual + multi-step): 2 marks

The chapter is friendlier than Motion or Sound because the formulas are short and reusable.

Expert Strategy

Always start a numerical by writing down what is given (with units) and what is asked. Half the marks in CBSE Physics come from showing the formula and substituting values cleanly — even if the final number is wrong, partial credit is generous.

For “lift then drop” problems, energy conservation is faster than kinematics. Skip the $v^2 = 2gh$ step entirely if the question asks for KE at impact.

Memorise the unit conversions:

$1\,\text{kWh} = 3.6 \times 10^6\,\text{J}$
$1\,\text{HP} = 746\,\text{W}$
$1\,\text{calorie} = 4.18\,\text{J}$ (rare in Class 9 but possible)

Common Traps

Trap 1 — Forgetting the $\frac{1}{2}$ in kinetic energy. Students write $KE = mv^2$ instead of $\frac{1}{2}mv^2$ . This loses 1 mark every time.

Trap 2 — Using grams instead of kilograms. SI formula $KE = \frac{1}{2}mv^2$ requires $m$ in kg. If the problem gives 500 g, convert to 0.5 kg first.

Trap 3 — Confusing power and work. Two questions can have the same setup but different answers depending on whether they ask for work (joules) or power (watts).

Trap 4 — Forgetting the cosine in the work formula. When a force is at an angle to displacement, only the component along displacement does work. CBSE rarely tests this in Class 9 but JEE-style coaching books do.

Trap 5 — Using “weight” interchangeably with “mass.” Weight is in newtons ( $mg$ ). Mass is in kg. Always check the unit before plugging in.