Motion, Forces and Energy

Motion, Forces and Energy is Topic 1 of the Cambridge IGCSE Physics (0625) syllabus. It is also the biggest. It typically supplies 25-30% of the marks across Paper 1/2 and Paper 3/4, more than any other topic. If you fix one topic before the exam, fix this one.

The topic runs from measurement and motion graphs through forces, moments, momentum, energy, work, power and pressure. It carries more equations than any other 0625 topic: around 15 of the roughly 25 you must recall, including $v = \dfrac{s}{t}$, $a = \dfrac{\Delta v}{\Delta t}$, $F = ma$, $W = mg$, $\rho = \dfrac{m}{V}$, $\text{moment} = Fd$, $p = mv$, $E = \dfrac{1}{2}mv^2$, $\Delta E = mg\Delta h$ and $p = \dfrac{F}{A}$.

Why do students lose marks here?

Three failure patterns repeat every session. First, unit errors: grams left as grams in $\rho = \dfrac{m}{V}$, or km/h mixed with m/s in speed calculations. Second, graph confusion: reading a distance-time gradient as acceleration, or forgetting that the area under a speed-time graph gives distance. Third, half-finished calculations: a correct equation with no rearrangement, or an answer with no unit. Examiner reports for 0625 flag missing units and wrong sig figs almost every series. Use $g = 9.8\ \text{N/kg}$ unless the question states otherwise. Some papers use 10 N/kg, so read the question.

Extended (Supplement) students face extra content: vector resultants, circular motion, momentum and impulse, and gradient analysis on non-linear graphs. Core students stop at grade C, so anyone targeting A* to B must sit Extended and master the Supplement rows in each subtopic.

How should you revise Motion, Forces and Energy?

Work equation-first, then question-second. Write all the topic’s equations from memory, in words and symbols with units. Check them, then drill 10 past-paper calculations per subtopic using the same routine every time: equation → substitute → rearrange → answer with unit. Most students close their main calculation gaps in two to three focused weeks at that pace.

Then attack graphs separately. Sketch the six standard motion graphs (constant speed, acceleration, deceleration and rest, on both axis types) until you can label gradient and area meanings without notes. Paper 6 also loves this topic: measuring-cylinder readings, pendulum timing and density practicals appear regularly.

How Motion, Forces and Energy Is Assessed Across the Papers

This topic is the workhorse of the whole 0625 assessment, so it surfaces on every paper. Papers 1 (Core) and 2 (Extended) carry several multiple-choice items on speed, acceleration, $F = ma$, moments and energy stores, usually the largest single block of MCQs. Papers 3 (Core) and 4 (Extended) build full structured questions: graph interpretation, density, work and power calculations, and pressure. The Core and Extended split matters most here, because the Supplement adds vector resultants, circular motion, momentum and impulse ($p = mv$ and the impulse idea), and the use of gradients on non-linear graphs. Those Extended-only rows can decide an A grade, and the momentum questions in particular separate the top candidates from the rest. Papers 5 (Practical) and 6 (Alternative to Practical) lean on this topic too: timing a trolley or pendulum, measuring density by displacement, and reading a measuring cylinder are all assessed for technique and uncertainty. Because so many marks ride on a handful of equations, a confident command of $F = ma$, $v = \dfrac{s}{t}$ and the two energy equations pays off on every paper. Expect this topic to feel like a quarter of your total marks, and treat any weakness in it as your first revision priority.

A Worked Example That Shows the Method

A car of mass $1200\ \text{kg}$ is travelling at $24\ \text{m/s}$. The driver brakes and the car stops in $6.0\ \text{s}$. Calculate the average braking force. [3]

Worked solution:

$a = \dfrac{\Delta v}{\Delta t} = \dfrac{0 - 24}{6.0} = -4.0\ \text{m/s}^2$

$F = ma = 1200 \times (-4.0) = -4800\ \text{N}$

The negative sign shows the force opposes the motion, so the braking force has magnitude $4800\ \text{N}$ (2 significant figures). Notice the method: find the acceleration first, then feed it into Newton’s second law. A common slip is to forget the minus sign on the change in velocity, which loses the link between deceleration and the backward force. Always quote the unit and round to the precision of the data given, here 2 significant figures.

Mark scheme:

• M1: $a = \dfrac{\Delta v}{\Delta t}$ with correct substitution
• A1: $a = -4.0\ \text{m/s}^2$ (or $4.0\ \text{m/s}^2$ deceleration)
• B1: $F = ma$ giving $4800\ \text{N}$, with the unit stated

Work through each subtopic below, then check every formula against the physics equations list and practise the routine in the calculation questions guide.