Understanding Motion, forces and energy: IGCSE Physics Explained

Where motion, forces and energy sits within IGCSE Physics

The motion, forces and energy section constitutes the single largest block of content in the Cambridge IGCSE Physics syllabus. It spans physical quantities and measurement, kinematics, dynamics, momentum, energy, work, power, and pressure. Candidates who sit either the Core or Extended tier will encounter questions drawn from this section on every paper they attempt. The section also supplies the mathematical and conceptual vocabulary that underpins later topics in waves, electricity, and nuclear physics, making a secure understanding here the foundation on which the rest of the course is built.

The material divides naturally into three strands. The first strand establishes the language of physics: quantities, units, scalars, and vectors. The second strand addresses how objects move and why they change their motion (kinematics and dynamics). The third strand connects force and motion to energy, work, power, and pressure. Treating them in this order mirrors the way exam questions escalate from definition recall through graph interpretation to multi-step calculation.

Physical quantities and measurement

Every measurement in physics consists of a numerical value and a unit. The IGCSE syllabus expects candidates to use SI base units (metre, kilogram, second, ampere, kelvin, mole) and to convert between common multiples such as kilometres to metres or grams to kilograms. Precision in unit handling is not merely a formality; examiners frequently withhold marks when a final answer carries no unit or an incorrect one.

Measurement techniques

• Length: rulers (resolution typically 1 mm), vernier callipers, micrometers. Always record to the smallest division of the instrument.
• Volume: measuring cylinders read at the bottom of the meniscus for liquids; displacement method for irregular solids.
• Time: digital stopwatches (resolution 0.01 s). For short or repeating intervals, measure multiple cycles and divide. The classic example is timing 20 swings of a pendulum and dividing by 20 to find the period.

Motion: speed, velocity and acceleration

Definitions and equations

Speed and velocity share the same unit but differ conceptually. A car driving in a circle at a steady 30 m/s has constant speed but continuously changing velocity because its direction changes. This distinction matters whenever a question uses the word "velocity" rather than "speed" - the examiner is signalling that direction is relevant.

Distance-time and speed-time graphs

Graph interpretation is one of the most frequently examined skills in this section. The two graph types encode different information, and candidates must be fluent in reading both.

A horizontal line on a distance-time graph indicates the object is stationary. A straight line with a positive gradient indicates constant speed. A curve whose gradient increases indicates acceleration. On a speed-time graph, a horizontal line indicates constant speed (zero acceleration), an upward slope indicates positive acceleration, and a downward slope indicates deceleration.

Worked example 1: Speed-time graph

A cyclist accelerates uniformly from rest to 12 m/s in 8 seconds, then travels at constant speed for 10 seconds, then decelerates uniformly to rest in 6 seconds. Find the total distance travelled.

1. Phase 1 (triangle): area = 0.5 x 8 x 12 = 48 m
2. Phase 2 (rectangle): area = 10 x 12 = 120 m
3. Phase 3 (triangle): area = 0.5 x 6 x 12 = 36 m
4. Total distance = 48 + 120 + 36 = 204 m

Mass, weight and density

Mass is a measure of the quantity of matter in an object, measured in kilograms. It does not change with location. Weight is the gravitational force acting on that mass, calculated by W = mg, where g is the gravitational field strength (approximately 9.8 N/kg on Earth's surface, often rounded to 10 N/kg in IGCSE calculations). Weight is measured in newtons and, unlike mass, varies with location.

Density connects mass and volume through the relationship rho = m / V. The density of water (1000 kg/m³ or equivalently 1.0 g/cm³) serves as a useful benchmark: objects with density greater than this value sink in water; those with lower density float.

Worked example 2: Density calculation

A metal block has mass 540 g and dimensions 10 cm x 6 cm x 3 cm. Find its density in kg/m³ and identify the likely metal.

1. Volume = 10 x 6 x 3 = 180 cm³ = 180 x 10^-6 m³ = 1.80 x 10^-4 m³
2. Mass = 540 g = 0.540 kg
3. Density = 0.540 / (1.80 x 10^-4) = 3000 kg/m³
4. This value matches aluminium (2700 kg/m³ is the textbook value; the slight discrepancy suggests impurities or measurement uncertainty, but aluminium is the closest standard metal)

Forces and Newton's laws

A force is a push or pull that can change the shape, speed, or direction of motion of an object. The resultant force on an object is the single force that has the same effect as all the individual forces acting together. When forces act along the same line, finding the resultant is a matter of addition (same direction) or subtraction (opposite directions).

Newton's three laws

Friction, drag and terminal velocity

Friction acts between surfaces in contact to oppose relative motion. Air resistance (drag) acts on objects moving through air and increases with speed. When a skydiver jumps from an aircraft, gravitational force initially exceeds drag, producing a net downward force and acceleration. As speed increases, drag grows until it equals weight. At that point the resultant force is zero, acceleration ceases, and the skydiver falls at a constant speed called terminal velocity. Opening the parachute dramatically increases drag, reducing terminal velocity to a safe landing speed.

Worked example 3: Newton's second law

A car of mass 1200 kg accelerates from rest to 20 m/s in 10 seconds. Calculate the resultant force, assuming constant acceleration.

1. Acceleration: a = (v - u) / t = (20 - 0) / 10 = 2.0 m/s²
2. Resultant force: F = ma = 1200 x 2.0 = 2400 N

Momentum

Momentum is defined as the product of an object's mass and its velocity: p = mv. Its SI unit is kg m/s. Momentum is a vector quantity; direction matters. The principle of conservation of momentum states that in a closed system (no external resultant force), the total momentum before an event equals the total momentum after. This principle applies to collisions and explosions alike.

Worked example 4: Conservation of momentum

A 2.0 kg trolley moving at 3.0 m/s collides with a stationary 1.0 kg trolley. After the collision, they stick together. Find the velocity of the combined trolley.

1. Total momentum before: (2.0 x 3.0) + (1.0 x 0) = 6.0 kg m/s
2. Total momentum after: (2.0 + 1.0) x v = 3.0v
3. By conservation: 3.0v = 6.0, so v = 2.0 m/s

The Extended tier also requires understanding of impulse: the change in momentum equals the resultant force multiplied by the time for which it acts (F x t = change in mv). This relationship explains why crumple zones in cars reduce injury; they extend the collision time, thereby reducing the force experienced by passengers for the same change in momentum.

Energy, work and power

Energy stores and transfers

Energy exists in various stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, and electromagnetic. The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. In any real transfer, some energy is dissipated (usually as thermal energy to the surroundings), reducing the useful output.

Key equations

Worked example 5: Energy conversion

A ball of mass 0.50 kg is dropped from a height of 20 m. Assuming no air resistance, find its speed just before hitting the ground. Take g = 10 m/s².

1. GPE at the top = mgh = 0.50 x 10 x 20 = 100 J
2. All GPE converts to KE at the bottom: KE = 100 J
3. 0.5 x 0.50 x v² = 100
4. 0.25v² = 100
5. v² = 400
6. v = 20 m/s

Pressure

Pressure is defined as force per unit area: p = F / A, measured in pascals (Pa), where 1 Pa = 1 N/m². This relationship explains why sharp objects cut more easily than blunt ones: the same force is concentrated over a smaller area, producing higher pressure.

In a liquid, pressure increases with depth according to p = rho x g x h, where rho is the liquid's density, g is gravitational field strength, and h is the depth below the surface. This pressure acts equally in all directions at a given depth and does not depend on the shape of the container. A practical consequence is that dams are built thicker at the base, where the water pressure is greatest.

Worked example 6: Liquid pressure

Calculate the pressure due to water at a depth of 15 m. Take the density of water as 1000 kg/m³ and g = 10 m/s².

1. p = rho x g x h = 1000 x 10 x 15 = 150 000 Pa = 150 kPa

Note that this is the pressure due to the water column alone. The total pressure at that depth would include atmospheric pressure (approximately 100 kPa) added to this value, giving roughly 250 kPa.

Common errors and how to avoid them

Self-check questions

1. A runner completes a 400 m lap in 50 seconds. Calculate the runner's average speed. If the runner finishes at the starting point, what is the displacement and the average velocity?
2. From a speed-time graph, a car accelerates from 0 to 15 m/s in 5 s, then maintains 15 m/s for 20 s. Sketch the graph and find the total distance.
3. An object of mass 5.0 kg rests on a table. Calculate its weight (g = 10 N/kg) and the normal contact force exerted by the table.
4. A 0.030 kg bullet travelling at 400 m/s embeds in a 2.0 kg wooden block at rest. Find the velocity of the block and bullet immediately after impact.
5. A crane lifts a 500 kg load through 12 m in 30 seconds. Calculate the work done and the power output of the crane. Take g = 10 m/s².

Exam strategy for motion, forces and energy questions

Questions on this section appear across Papers 1 through 6 and range from one-mark definitions to extended calculations worth six or more marks. The paper-specific demands differ. Multiple-choice papers (Papers 1 and 2) frequently test graph interpretation and conceptual understanding of Newton's laws. Theory papers (Papers 3 and 4) demand structured calculations with clear working, and the Extended paper often combines several equations in a single problem, such as using conservation of energy followed by F = ma to find an acceleration. Practical papers (5 and 6) test measurement skills directly: reading instruments, recording data, and analysing results graphically.

A reliable approach to multi-step calculations is to write down the relevant equation, list the known quantities with their units, substitute, and solve. Presenting working in this format not only reduces errors but also earns method marks even if the final numerical answer contains an arithmetic slip. For graph questions, always label both axes, plot points with small, precise crosses, and draw a smooth best-fit line (or curve). When asked to find a gradient, choose two points that are far apart on the line, not data points from the table, and show the calculation explicitly.

Candidates sitting the Extended tier should pay particular attention to momentum, impulse, and the interplay between kinetic energy and gravitational potential energy in problems involving slopes, pendulums, and projectiles. These topics carry high mark allocations and reward systematic, step-by-step problem solving of exactly the kind that Cambridge mark schemes are designed to credit.