AQA GCSE Physics: Forces and Waves Revision Guide

Forces and Waves are two of the weightiest topics on the AQA GCSE Physics specification. Forces sits on Paper 2 and is widely regarded as the most calculation-heavy section of the entire course, while Waves -- also on Paper 2 -- blends mathematical skills with conceptual understanding of the electromagnetic spectrum and wave behaviour. Together, these two topics routinely account for a large share of the marks on the exam, so thorough preparation here can make a genuine difference to your final grade.

This guide follows the AQA specification closely, covering every key concept, the equations you need, common exam traps, and the revision strategies that will help you answer confidently under timed conditions.

Forces

Scalar and Vector Quantities

Before diving into forces, you need to understand the distinction the specification draws between scalar and vector quantities.

A scalar quantity has magnitude (size) only. Examples include speed, distance, mass, temperature, and time.

A vector quantity has both magnitude and direction. Examples include velocity, displacement, acceleration, and force.

This distinction matters in practice. If two forces act on an object in opposite directions, you cannot simply add the numbers together -- you have to account for direction. Velocity and displacement are the vector equivalents of speed and distance, and examiners will penalise you for using the wrong term. When a question says "velocity," it expects a direction as well as a value.

Contact and Non-Contact Forces

Forces are interactions between two objects. The specification divides them into two categories.

Contact forces require physical contact between the objects. These include friction, air resistance (drag), tension, the normal contact force, and applied forces such as pushes and pulls.

Non-contact forces act at a distance without the objects touching. The three you need to know are gravitational force, electrostatic force, and magnetic force.

Every force interaction involves a pair of objects. When object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. This is Newton's Third Law, which we will return to later.

A force field is the region around an object where a non-contact force acts on other objects. Gravitational fields surround objects with mass, electric fields surround charged objects, and magnetic fields surround magnets and current-carrying conductors.

Gravity and Weight

Weight is the force acting on an object due to gravity. It is measured in newtons (N) and calculated using:

weight = mass x gravitational field strength

W = m g

On Earth, g is approximately 9.8 N/kg (or 10 N/kg in most GCSE calculations). Mass is a measure of how much matter an object contains and is constant wherever you are; weight depends on the gravitational field strength and therefore changes if you travel to a different planet or the Moon.

Weight always acts through an object's centre of mass. A common exam question asks you to identify the centre of mass of an irregular shape -- this can be found experimentally by suspending the object from different points and drawing plumb lines.

Resultant Forces

When multiple forces act on an object, you can replace them with a single resultant force that has the same effect. If forces act along the same line, you add those in the same direction and subtract those in the opposite direction. If forces act at right angles, you can use a scale diagram or calculation to find the resultant (Higher tier students may need to use Pythagoras and trigonometry here).

Free body diagrams are an essential skill. Draw the object as a simple box or dot, then add labelled arrows showing all the forces acting on it. The length of each arrow should represent the magnitude of the force. This approach is worth marks and helps you avoid errors in more complex questions.

If the resultant force on an object is zero, the object is in equilibrium -- it either remains stationary or continues to move at a constant velocity (Newton's First Law). If the resultant force is not zero, the object accelerates in the direction of the resultant force.

Work Done and Energy Transfer

When a force causes an object to move, energy is transferred. The amount of energy transferred is called the work done, calculated using:

work done = force x distance (along the line of action of the force)

W = F s

Work done is measured in joules (J). One joule of work is done when a force of one newton moves an object through a distance of one metre.

This equation links directly to the energy topic. When work is done against friction, energy is transferred to the thermal energy store of the surroundings. When work is done to lift an object, energy is transferred to the gravitational potential energy store.

Forces and Elasticity -- Hooke's Law

When a force is applied to an elastic object such as a spring, the object stretches or compresses. Hooke's law states that the extension of a spring is directly proportional to the force applied, provided the limit of proportionality is not exceeded.

force = spring constant x extension

F = k e

The spring constant (k) is measured in N/m and tells you how stiff the spring is. A higher spring constant means a stiffer spring. Extension (e) is measured in metres, not centimetres -- a very common error in exam calculations.

If you plot a force-extension graph for a spring obeying Hooke's law, you get a straight line through the origin. Beyond the limit of proportionality, the graph curves -- the spring has been permanently deformed and will not return to its original length when the force is removed. The elastic limit is the point beyond which the object no longer returns to its original shape.

The elastic potential energy stored in a stretched spring (up to the limit of proportionality) is:

elastic potential energy = 0.5 x spring constant x extension squared

Ee = 1/2 k e^2

The required practical for this section involves investigating the relationship between force and extension for a spring. You need to know the method, how to take accurate measurements, how to plot and interpret the graph, and how to identify the limit of proportionality from your data.

Speed, Velocity, and Acceleration

Speed is a scalar quantity:

speed = distance / time

The typical speed of a person walking is about 1.5 m/s, running about 3 m/s, and cycling about 6 m/s. Sound travels at approximately 330 m/s in air.

Velocity is a vector quantity -- it is speed in a given direction.

Acceleration is the rate of change of velocity:

acceleration = change in velocity / time

a = (v - u) / t

where v is final velocity, u is initial velocity, and t is time. Acceleration is measured in m/s^2. An object that is slowing down has a negative acceleration (deceleration).

For uniform acceleration, you may also use:

v^2 = u^2 + 2 a s

where s is the distance travelled. This equation is given on the equation sheet but you must be confident rearranging it.

Distance-Time and Velocity-Time Graphs

These graphs are tested almost every year and carry significant marks.

On a distance-time graph, the gradient equals the speed. A steeper gradient means a higher speed. A horizontal line means the object is stationary. A curved line means the speed is changing -- you can find the speed at a particular instant by drawing a tangent to the curve at that point and calculating its gradient.

On a velocity-time graph, the gradient equals the acceleration. A horizontal line means constant velocity. A line sloping upward means acceleration; a line sloping downward means deceleration. Crucially, the area under a velocity-time graph gives the distance travelled (or displacement, if direction is considered). For a straight-line section, this is a simple rectangle or triangle calculation. For a curve, you estimate the area by counting squares.

Newton's Laws of Motion

These are fundamental and heavily examined.

Newton's First Law: An object remains at rest or continues to move at a constant velocity unless acted on by a resultant force. This explains why passengers lurch forward when a car brakes -- their body tends to continue moving at the original speed.

Newton's Second Law: The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.

resultant force = mass x acceleration

F = m a

Force is in newtons, mass in kilograms, acceleration in m/s^2. This is one of the most important equations in GCSE Physics and you must be able to rearrange and apply it in a wide range of contexts.

The required practical here involves investigating the effect of varying force and mass on the acceleration of an object -- typically a trolley on a track.

Newton's Third Law: When two objects interact, they exert equal and opposite forces on each other. These forces act on different objects and are always the same type. For example, when you push a wall, the wall pushes back on you with an equal force. When the Earth pulls you down with gravity, you pull the Earth up with the same gravitational force. A common mistake is confusing Third Law pairs with balanced forces -- balanced forces act on the same object; Third Law pairs act on different objects.

Stopping Distances

The stopping distance of a vehicle is the sum of the thinking distance and the braking distance.

Thinking distance is the distance the car travels during the driver's reaction time (before the brakes are applied). It increases with speed and is also increased by tiredness, distractions, alcohol, or drugs.

Braking distance is the distance the car travels after the brakes are applied until it stops. It increases with speed, and is also increased by poor road conditions (wet, icy), worn tyres, worn brakes, or a heavier vehicle.

When brakes are applied, kinetic energy is transferred to the thermal energy store of the brakes. At very high speeds, the braking force needed to stop in a given distance is much larger, which can cause the brakes to overheat and the car to skid.

Examiners frequently test whether you understand that stopping distance increases with the square of speed -- doubling the speed roughly quadruples the braking distance (because kinetic energy depends on v^2).

Momentum (Higher Tier)

Momentum is defined as:

momentum = mass x velocity

p = m v

Momentum is a vector quantity, measured in kg m/s.

The conservation of momentum states that in a closed system, the total momentum before an event equals the total momentum after. This applies to collisions and explosions. In a collision between two objects, you can set up an equation:

m1 v1 + m2 v2 = m1 v1' + m2 v2'

where the primed values are the velocities after the collision.

Newton's Second Law can also be expressed in terms of momentum. The resultant force equals the rate of change of momentum:

F = change in momentum / time

This explains why safety features such as crumple zones, airbags, and seat belts work. They increase the time over which momentum changes during a collision, reducing the force experienced by the occupants.

Waves

Types of Waves

Waves transfer energy from one place to another without transferring matter. The specification distinguishes between two types.

Transverse waves have oscillations perpendicular to the direction of energy transfer. Examples include water waves, all electromagnetic waves, vibrations on a string, and S-waves (secondary seismic waves).

Longitudinal waves have oscillations parallel to the direction of energy transfer. Examples include sound waves and P-waves (primary seismic waves). Longitudinal waves consist of compressions (regions where particles are close together) and rarefactions (regions where particles are spread apart).

Wave Properties and the Wave Equation

Every wave can be described using the following properties:

• Amplitude -- the maximum displacement from the rest position, measured in metres
• Wavelength -- the distance from one point on a wave to the equivalent point on the next wave (e.g., crest to crest), measured in metres
• Frequency -- the number of complete waves passing a point per second, measured in hertz (Hz)
• Period -- the time for one complete wave to pass a point, measured in seconds

Period and frequency are linked:

period = 1 / frequency

T = 1 / f

The fundamental wave equation relates speed, frequency, and wavelength:

wave speed = frequency x wavelength

v = f lambda

Wave speed is measured in m/s, frequency in Hz, and wavelength in metres. You must memorise this equation and be confident rearranging it.

The required practical for this section involves measuring the speed of waves in a ripple tank and the speed of sound. For the ripple tank, you measure wavelength and frequency to calculate speed. For sound, you can use a two-microphone method or measure the echo time over a known distance.

Reflection and Refraction

When a wave hits a boundary between two materials, it may be reflected, refracted, or absorbed.

Reflection occurs when a wave bounces off a surface. The angle of incidence equals the angle of reflection. Both angles are measured from the normal -- an imaginary line perpendicular to the surface at the point of incidence.

Refraction occurs when a wave passes from one medium to another and changes speed. If the wave enters a denser medium, it slows down and bends towards the normal. If it enters a less dense medium, it speeds up and bends away from the normal. The wavelength changes during refraction but the frequency stays the same.

If light travels from a denser medium (such as glass) into a less dense medium (such as air) and hits the boundary at a large enough angle, it undergoes total internal reflection -- no light passes through the boundary. The critical angle is the angle of incidence at which refraction gives an angle of refraction of exactly 90 degrees; above this angle, total internal reflection occurs.

Sound Waves

Sound waves are longitudinal waves caused by vibrating objects. They require a medium to travel through -- they cannot travel through a vacuum. Sound travels fastest through solids, slower through liquids, and slowest through gases.

The frequency of a sound wave determines its pitch. A higher frequency produces a higher-pitched sound. The human hearing range is approximately 20 Hz to 20,000 Hz. Sounds above 20,000 Hz are called ultrasound.

Ultrasound has practical applications including medical imaging (pregnancy scans), sonar (measuring sea depth), and industrial quality control (detecting flaws in materials). The principle is the same in each case: a pulse of ultrasound is emitted, it reflects off a boundary, and the time delay and speed of sound are used to calculate the distance.

The amplitude of a sound wave determines its volume. A larger amplitude means a louder sound.

The Electromagnetic Spectrum

Electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber. Unlike sound, they do not need a medium and can travel through a vacuum. All electromagnetic waves travel at the same speed in a vacuum -- the speed of light, approximately 3 x 10^8 m/s.

The electromagnetic spectrum, in order of decreasing wavelength (and increasing frequency), is:

1. Radio waves -- longest wavelength, lowest frequency
2. Microwaves
3. Infrared
4. Visible light
5. Ultraviolet
6. X-rays
7. Gamma rays -- shortest wavelength, highest frequency

You do not need to memorise specific wavelength values for each type, but you do need to know the order and that visible light occupies only a very small portion of the full spectrum.

Uses of Electromagnetic Waves

Each type of EM wave has specific applications:

• Radio waves -- television and radio broadcasting, Bluetooth, and WiFi communication
• Microwaves -- microwave ovens (heating food by being absorbed by water molecules), satellite communication, and mobile phone signals
• Infrared -- thermal imaging, remote controls, optical fibre communication, heating
• Visible light -- vision, photography, optical fibre communication
• Ultraviolet -- fluorescent lamps, detecting forged banknotes, disinfecting water
• X-rays -- medical imaging (viewing bones and detecting fractures), airport security scanning
• Gamma rays -- sterilising medical equipment and food, treating cancer (radiotherapy), medical tracing

Dangers of Electromagnetic Waves

The higher the frequency of an EM wave, the more energy it carries, and the more hazardous it tends to be:

• Microwaves -- can cause internal heating of body tissue
• Infrared -- can cause skin burns
• Ultraviolet -- can damage surface cells and eyes, leading to sunburn and increasing the risk of skin cancer. It can also cause premature ageing of the skin
• X-rays and gamma rays -- these are ionising radiations. They can damage DNA in cells, causing mutations that may lead to cancer. Exposure must be minimised, which is why radiographers stand behind lead screens or leave the room during X-ray procedures

Understanding the link between frequency, energy, and hazard level is essential. A common exam question asks you to explain why UV radiation is more dangerous than visible light -- the answer is that UV has a higher frequency and therefore more energy per photon, enough to damage cells and DNA.

Lenses and Visible Light (Higher Tier)

Higher tier students may encounter questions on the behaviour of light through converging (convex) and diverging (concave) lenses. A converging lens brings parallel rays of light to a focus at the principal focus. A diverging lens spreads parallel rays of light apart so they appear to come from a principal focus on the same side as the incoming light.

The magnification of a lens is:

magnification = image height / object height

A magnification greater than 1 means the image is larger than the object; less than 1 means it is smaller. Magnification has no unit.

Black Body Radiation

All objects emit and absorb infrared radiation. A perfect black body absorbs all incident radiation and does not reflect or transmit any. As an object heats up, it emits more infrared radiation and the peak wavelength shifts to shorter wavelengths (towards the visible spectrum). This is why very hot objects glow -- first red, then white.

The temperature of the Earth depends on the balance between radiation absorbed from the Sun and radiation emitted by the Earth. If more energy is absorbed than emitted, the temperature rises. This is the basis of the greenhouse effect and is relevant to understanding climate change.

Exam Tips for Forces and Waves

Show all working in calculations. Forces and Waves are calculation-heavy topics. Write the equation, substitute the values with units, and clearly state the answer with the correct unit. Even if your final answer is wrong, you will pick up method marks for a clear working chain.

Convert units before substituting. Extension must be in metres for Hooke's law, not centimetres. Wavelength must be in metres for the wave equation, not nanometres. Speed must be in m/s if the other quantities are in SI units. Unit conversion errors are one of the most common reasons for lost marks.

Practise graph interpretation. Velocity-time graphs, distance-time graphs, and force-extension graphs all appear regularly. Know what the gradient and area under the graph represent for each type.

Learn the EM spectrum order. A simple mnemonic such as "Rich Men In Vegas Use eXpensive Gadgets" (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma) can help. But more importantly, know the uses and dangers of each type -- these are frequently tested in four-mark and six-mark questions.

Newton's Third Law pairs. Always state that the forces act on different objects, are equal in magnitude, opposite in direction, and of the same type. Leaving out any of these details loses marks.

Momentum questions (Higher). Draw a clear before-and-after diagram. Assign a positive direction and stick with it. If an object moves in the opposite direction, its velocity is negative. This avoids sign errors that can cost you the full calculation.

Six-mark questions. Both Forces and Waves are popular topics for extended response questions. Practise structuring your answer: state the physics principle, apply it to the specific situation, and reach a clear conclusion. Use correct terminology throughout.

Prepare with LearningBro

LearningBro offers targeted courses for both of these topics, built around the AQA specification with practice questions that match real exam style and difficulty.

• AQA GCSE Physics: Forces -- covers scalar and vector quantities, contact and non-contact forces, resultant forces, Newton's laws, work done, Hooke's law, stopping distances, and momentum
• AQA GCSE Physics: Waves -- covers transverse and longitudinal waves, the wave equation, reflection, refraction, sound waves, the electromagnetic spectrum, and the uses and dangers of EM waves

Work through these courses alongside past papers and mark schemes, and you will be well prepared for every aspect of these topics on exam day.

Good luck with your revision.