What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms (kg). It does not change with location. Weight is the force of gravity acting on that mass, measured in newtons (N). Weight depends on gravitational field strength (g), so on Earth weight = mass × 9.8 N/kg, but on the Moon it's about one-sixth of that. Remember: mass is a scalar, weight is a vector (it has direction, towards the centre of the planet).

How do you calculate resultant force?

Resultant force is the single force that represents the combined effect of all forces acting on an object. To calculate it, add forces in the same direction and subtract forces in opposite directions. For example, if a car has a driving force of 5000 N forward and friction of 3000 N backward, the resultant force is 2000 N forward. If forces are at an angle, you need to use vector addition, often by drawing a scale diagram or using trigonometry.

What is terminal velocity and how does it occur?

Terminal velocity is the constant speed a falling object reaches when the force of air resistance equals the weight of the object. Initially, the object accelerates due to gravity. As speed increases, air resistance increases. Eventually, air resistance balances weight, so the resultant force is zero and the object falls at a constant speed. For a skydiver, terminal velocity is about 55 m/s before the parachute opens, which increases air resistance and reduces terminal velocity to around 5 m/s.

How do you apply Newton's Third Law to everyday situations?

Newton's Third Law states that for every action, there is an equal and opposite reaction. For example, when you walk, your foot pushes backward on the ground (action), and the ground pushes forward on your foot (reaction), propelling you forward. Another example is a rocket: it pushes exhaust gases downward, and the gases push the rocket upward. Remember, the forces act on different objects, so they don't cancel out.

What is Hooke's Law and how do you use it?

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. The equation is F = kx, where F is force in newtons, k is the spring constant in N/m, and x is extension in metres. For example, if a spring with k = 100 N/m is stretched by 0.05 m, the force is 5 N. You can also use this to find the spring constant from a graph of force against extension – the gradient equals k.

How do you draw a free-body diagram?

A free-body diagram shows all the forces acting on a single object, represented by arrows. Start by drawing a box or dot to represent the object. Then draw arrows from the centre: weight (W) downwards, normal reaction (R) upwards, friction (F) opposite to motion, and applied forces in the direction they act. Make sure the length of each arrow is proportional to the magnitude of the force. Label each arrow with the force name and value if known. For example, a book on a table has weight down and reaction up – if it's stationary, these arrows are equal in length.

Forces

AQA

GCSE

This topic defines the fundamental nature of forces as interactions between objects, categorizing them into contact and non-contact types. It establishes that forces are vector quantities, requiring both magnitude and direction for full description, and introduces the concept of interaction pairs.

Objectives

Exam Tips

Pitfalls

Key Terms

125

Mark Points

Subtopics in this area

Contact and non-contact forces

Moments, levers and gears (physics only)

Newton's First Law

Factors affecting braking distance 1

The distance–time relationship

Acceleration

Distance and displacement

Pressure

Resultant forces

Pressure in a fluid 2 (HT only)

Factors affecting braking distance 2

Stopping distance

Newton's Third Law

Speed

Velocity

Work done and energy transfer

Reaction time

Scalar and vector quantities

Newton's Second Law

Conservation of momentum

Forces and elasticity

Gravity

Momentum is a property of moving objects

Changes in momentum (physics only)

Atmospheric pressure

Topic Overview

Forces are a fundamental concept in physics, describing the interactions that cause objects to change their motion, shape, or state. In the AQA GCSE Physics specification, this topic covers contact and non-contact forces, Newton's laws of motion, and the effects of forces on objects. You'll learn how to calculate resultant forces, draw free-body diagrams, and apply equations such as F = ma and W = mg. Understanding forces is essential for explaining everyday phenomena, from why a ball falls to the ground to how a rocket launches into space.

This topic builds on key ideas from KS3, such as balanced and unbalanced forces, and introduces more advanced concepts like terminal velocity, Hooke's law, and moments. Forces are central to many other GCSE topics, including energy transfers, pressure, and magnetism. Mastering this area will not only help you in exams but also give you a deeper appreciation of the physical world around you. Expect to see questions that test your ability to interpret graphs, calculate forces, and explain real-world scenarios using scientific principles.

In the AQA exams, forces typically appear in Paper 2 (Physics) and can be worth up to 15-20% of the total marks. Questions range from multiple-choice to extended writing, often requiring you to apply your knowledge to unfamiliar contexts. To succeed, you need to be confident with equations, units, and the difference between scalar and vector quantities. Regular practice with past papers and worked examples will build your problem-solving skills and exam technique.

Key Concepts

Core ideas you must understand for this topic

→Newton's First Law: An object remains at rest or moves at constant velocity unless acted on by a resultant force. This explains why seatbelts are needed in cars – without a force, your body would continue moving forward during a crash.
→Newton's Second Law: The resultant force on an object equals its mass times its acceleration (F = ma). This equation is used to calculate the force needed to accelerate a car or the acceleration of a falling object.
→Newton's Third Law: When two objects interact, they exert equal and opposite forces on each other. For example, a rocket pushes exhaust gases downwards, and the gases push the rocket upwards.
→Weight and Mass: Weight is the force due to gravity (W = mg), measured in newtons, while mass is the amount of matter in an object, measured in kilograms. On Earth, g ≈ 9.8 N/kg.
→Hooke's Law: The extension of a spring is directly proportional to the force applied, up to the limit of proportionality (F = kx). This is used in force meters and suspension systems.

What You Need to Demonstrate

Key skills and knowledge for this topic

Definition of a force as a push or pull acting on an object due to interaction with another object
Distinction between contact forces (objects physically touching) and non-contact forces (objects physically separated)
Identification of friction, air resistance, tension, and normal contact force as contact forces
Identification of gravitational force, electrostatic force, and magnetic force as non-contact forces
Recognition of force as a vector quantity
Description of interaction pairs between objects
Definition of a moment as the turning effect of a force
Calculation of moments using M = Fd

Marking Points

Key points examiners look for in your answers

Definition of a force as a push or pull acting on an object due to interaction with another object
Distinction between contact forces (objects physically touching) and non-contact forces (objects physically separated)
Identification of friction, air resistance, tension, and normal contact force as contact forces
Identification of gravitational force, electrostatic force, and magnetic force as non-contact forces
Recognition of force as a vector quantity
Description of interaction pairs between objects
Definition of a moment as the turning effect of a force
Calculation of moments using M = Fd
Understanding that d is the perpendicular distance from the pivot to the line of action of the force
Application of the principle of moments for balanced objects (clockwise moment = anticlockwise moment)
Explanation of how levers and gears transmit rotational effects
Resultant force is zero means no change in motion
Stationary objects remain stationary if resultant force is zero
Moving objects continue at the same speed and direction if resultant force is zero
Velocity only changes if a resultant force acts on the object
Resistive forces balance driving forces when a vehicle travels at a steady speed
Identification of adverse road conditions such as wet or icy surfaces.
Identification of poor vehicle condition specifically relating to brakes or tyres.
Explanation of how these factors increase the distance required to stop.
Understanding the safety implications of increased braking distances.
The gradient of a distance–time graph represents the speed of the object.
A horizontal line on a distance–time graph indicates the object is stationary.
A straight, non-horizontal line indicates constant speed.
For an accelerating object, the speed at a specific time is determined by the gradient of a tangent to the curve at that point.
Acceleration = change in velocity / time taken (a = Δv / t)
Units for acceleration are metres per second squared (m/s²)
Deceleration is a negative acceleration (slowing down)
Gradient of a velocity-time graph equals acceleration
Area under a velocity-time graph equals distance travelled or displacement
Uniform acceleration equation: v² - u² = 2as
Acceleration due to gravity near Earth's surface is approximately 9.8 m/s²
Terminal velocity occurs when the resultant force on a falling object is zero
Distance is a scalar quantity representing how far an object moves.
Displacement is a vector quantity representing the distance an object moves in a straight line from the start point to the finish point, including direction.
Ability to express displacement using both magnitude and direction.
Definition of pressure as force normal to a surface
Correct use of the equation p = F / A
Correct identification of units: pressure in pascals (Pa), force in newtons (N), and area in metres squared (m^2)
Understanding that pressure in fluids acts normal to any surface
Definition of resultant force as a single force with the same effect as all original forces
Calculation of resultant force for forces acting in a straight line
Use of free body diagrams to represent forces on an object (HT only)
Identification of balanced forces when the resultant force is zero (HT only)
Resolution of a single force into two components at right angles (HT only)
Use of vector diagrams for resolution, equilibrium, and resultant force magnitude/direction (HT only)
Calculation of pressure using p = hρg
Explanation of why pressure increases with depth and density
Calculation of pressure differences at different depths
Explanation of upthrust as a resultant force due to pressure differences on the top and bottom surfaces of a submerged object
Description of factors influencing floating and sinking
Work done by friction reduces kinetic energy
Energy is transferred to the thermal energy store of the brakes
Relationship between speed and required braking force
Relationship between braking force and deceleration
Dangers of large decelerations (e.g., overheating, loss of control)
Stopping distance = thinking distance + braking distance
Thinking distance is the distance traveled during the driver's reaction time
Braking distance is the distance traveled under the braking force
Greater speed leads to greater stopping distance
Reaction time is affected by tiredness, drugs, alcohol, and distractions
Braking distance is affected by adverse road/weather conditions and poor vehicle condition (brakes/tyres)
Forces are equal in magnitude
Forces are opposite in direction
Forces act on two different objects
Forces are of the same type
Recall the equation: distance travelled = speed × time (s = vt)
Calculate speed, distance, or time using the appropriate rearranged formula
Identify typical speed values for walking (1.5 m/s), running (3 m/s), and cycling (6 m/s)
Recognise the speed of sound in air is approximately 330 m/s
Calculate average speed for non-uniform motion from total distance and total time
Definition of velocity as speed in a given direction
Identification of velocity as a vector quantity
Distinction between scalar and vector quantities
Qualitative explanation that motion in a circle involves constant speed but changing velocity (Higher Tier only)
Definition of work done as force multiplied by distance moved along the line of action of the force
Correct application of the equation W = Fs
Understanding that 1 joule = 1 newton-metre
Explanation of energy transfer when work is done
Recognition that work done against frictional forces results in a temperature rise
Definition of reaction time as the time taken to respond to a stimulus
Typical range of human reaction times (0.2 s to 0.9 s)
Identification of factors affecting reaction time (tiredness, drugs, alcohol, distractions)
Relationship between reaction time and thinking distance
Methods to measure reaction time (e.g., ruler drop test)
Definition of scalar quantities as having magnitude only
Definition of vector quantities as having both magnitude and direction
Representation of vector quantities using arrows
Understanding that the length of an arrow represents magnitude
Understanding that the direction of an arrow represents the direction of the vector
Acceleration is directly proportional to the resultant force
Acceleration is inversely proportional to the mass
Correct application of the equation F = ma
Units: Force in Newtons (N), mass in kilograms (kg), acceleration in metres per second squared (m/s²)
Inertial mass is a measure of how difficult it is to change the velocity of an object (HT only)
Inertial mass is defined as the ratio of force over acceleration (HT only)
Definition of momentum as mass multiplied by velocity (p = mv)
Statement that in a closed system, total momentum before an event equals total momentum after the event
Application of the conservation of momentum principle to collision scenarios
Correct use of units: kg m/s for momentum, kg for mass, and m/s for velocity
Requirement of more than one force to change the shape of a stationary object
Distinction between elastic and inelastic deformation
Hooke's Law: force is directly proportional to extension provided the limit of proportionality is not exceeded
Calculation of spring constant (k) using F = ke
Calculation of elastic potential energy (Ee) using Ee = 0.5 * k * e^2
Interpretation of force-extension graphs (linear vs non-linear)
Weight is the force acting on an object due to gravity.
Weight is measured in newtons (N).
Mass is measured in kilograms (kg).
Gravitational field strength (g) is measured in newtons per kilogram (N/kg).
Weight and mass are directly proportional (W = mg).
Weight acts at a single point called the centre of mass.
Weight is measured using a calibrated spring-balance (newtonmeter).
Momentum is a vector quantity with magnitude and direction.
The equation p = m × v must be used correctly with standard SI units (kg m/s).
In a closed system, total momentum before an event equals total momentum after the event.
Application of the conservation of momentum to collision scenarios.
Recognition that force equals the rate of change of momentum
Correct application of the equation F = mΔv / Δt
Correct identification of variables including mass, change in velocity, and time interval
Correct use of SI units (kg, m/s, s, N)
Atmosphere is a thin layer of air around the Earth.
Atmospheric pressure is created by air molecules colliding with a surface.
Atmospheric pressure decreases as altitude increases.
Density of the atmosphere decreases with increasing altitude.
Fewer air molecules above a surface at higher altitudes result in lower pressure.

Examiner Tips

Expert advice for maximising your marks

💡Always remember that force is a vector; if a question asks for a force, ensure you consider if direction is required
💡Use the provided examples in the specification (friction, gravity, etc.) to categorize forces correctly in exam questions
💡When describing interactions, explicitly state that forces act on both objects involved
💡Always draw a diagram if one is not provided to identify the pivot and the line of action of the force
💡Ensure the distance used is the perpendicular distance to the line of action of the force
💡Check that all units are in standard SI units (Newtons and metres) before calculating
💡Remember that for a balanced object, the sum of clockwise moments equals the sum of anticlockwise moments
💡Remember that 'steady speed' implies the resultant force is zero
💡Use the term 'resultant force' rather than just 'force' when explaining the law
💡Be prepared to apply the law to vehicles moving at constant velocity
💡Always distinguish clearly between factors affecting thinking distance and those affecting braking distance.
💡When asked about vehicle condition, stick strictly to the specification's focus on brakes and tyres.
💡Use the term 'braking distance' precisely rather than just 'stopping distance' when the question specifically targets the deceleration phase.
💡Always check the axes labels carefully; ensure you distinguish between distance–time and velocity–time graphs.
💡When calculating the gradient of a curve, ensure your tangent is drawn accurately at the precise point requested.
💡Use a large triangle when calculating the gradient of a line to improve accuracy.
💡Always check if the question asks for speed or velocity
💡Ensure you can rearrange the acceleration equation to solve for time or change in velocity
💡When calculating the area under a velocity-time graph, use counting squares for non-linear sections if required
💡Remember that the final velocity squared minus initial velocity squared equals 2as is only for uniform acceleration
💡Always check if a question asks for a vector or scalar quantity.
💡When asked for displacement, ensure you include a direction (e.g., 'north', '30 degrees to the horizontal').
💡Use diagrams to visualize the start and end points to calculate displacement correctly.
💡Always check that area is in square metres before calculating pressure
💡Remember that pressure is a scalar quantity, but the force it exerts is normal to the surface
💡Use the provided equation sheet to ensure the correct rearrangement of the formula
💡Always check the direction of forces before adding or subtracting them to find the resultant
💡For Higher Tier, ensure you can accurately draw and interpret free body diagrams
💡When resolving forces, ensure your vector diagrams are drawn to scale as required by the specification
💡Remember that a resultant force of zero means the object is either stationary or moving at a constant velocity
💡Ensure all units are in SI units before calculating (e.g., cm to m)
💡Remember that the equation p = hρg is provided on the Physics equation sheet for Higher Tier
💡Be prepared to explain the physical reasoning behind the pressure increase, not just perform the calculation
💡Remember that work done by friction is the mechanism for stopping the vehicle
💡Be prepared to explain the energy transfer from kinetic to thermal
💡Understand that higher speeds require greater braking forces to stop in the same distance
💡Link large decelerations to the risk of brake failure or loss of vehicle control
💡Always state that stopping distance is the sum of thinking and braking distance
💡Be prepared to interpret graphs relating speed to stopping distance
💡Remember that reaction time is a time, while thinking distance is a distance
💡When discussing braking distance, mention both road conditions and vehicle maintenance
💡Always identify the two objects involved in the interaction
💡Remember that Newton's Third Law pairs are always the same type of force (e.g., both gravitational or both contact forces)
💡Use the phrase 'equal and opposite' when describing the forces
💡Always check that time is in seconds and distance is in metres before calculating speed in m/s
💡Use the 'triangle' method or algebraic rearrangement carefully to avoid errors
💡Be prepared to interpret data from tables or graphs to find average speed
💡Memorise the typical speed values provided in the specification as they are often tested in multiple-choice or short-answer questions
💡Always check if a question asks for speed or velocity; if velocity, ensure a direction is included in the answer.
💡Remember that any change in direction results in a change in velocity, even if the speed remains constant.
💡Always check that the distance used in the calculation is in the same direction as the force applied
💡Ensure all units are in standard SI units (Joules, Newtons, Metres) before performing calculations
💡Be prepared to explain that work done against friction is dissipated as thermal energy
💡Always distinguish clearly between thinking distance and braking distance in your answers
💡Be prepared to interpret data or graphs showing how reaction time affects total stopping distance
💡When describing methods to measure reaction time, ensure the procedure is logical and repeatable
💡Remember that reaction time is a biological process, whereas braking distance is a mechanical process
💡Always check if a question asks for a vector or scalar quantity, as this determines if you need to state a direction.
💡When drawing vectors, ensure the arrow length is proportional to the magnitude if required by the scale.
💡Always check if the force given is the resultant force; if not, calculate it first
💡Ensure all units are in SI units (kg, m/s², N) before substituting into the equation
💡Use the proportionality symbol ∝ correctly when describing the relationship
💡Remember that inertial mass is a measure of resistance to change in velocity
💡Always check the direction of motion; momentum is a vector quantity, so opposite directions must be represented by opposite signs (e.g., positive and negative)
💡Ensure all units are in SI base units (kg and m/s) before calculating momentum
💡Remember that this topic is Higher Tier (HT) only
💡Always check if the spring constant is given in N/m or N/cm; convert to N/m for standard SI calculations
💡When calculating work done or energy stored, ensure the extension is in metres
💡Use the provided Physics equation sheet for the elastic potential energy formula, but ensure you understand the conditions for its use
💡In graph questions, identify the point where the line stops being straight to determine the limit of proportionality
💡Always check the units in the question; ensure mass is in kg before using the W = mg equation.
💡Remember that weight is a force, so it must be expressed in newtons.
💡Be prepared to rearrange the W = mg equation to calculate mass if given weight and g.
💡Always check the direction of motion; if objects move in opposite directions, one velocity must be treated as negative.
💡Ensure all mass values are converted to kilograms before calculating momentum.
💡Remember that momentum is a vector, so the direction is as important as the magnitude.
💡Always check if the initial and final velocities are in the same or opposite directions when calculating Δv
💡Ensure the equation is rearranged correctly before substituting values
💡Use the provided Physics Equation sheet to verify the formula
💡Show all working steps to gain method marks even if the final answer is incorrect
💡Always relate pressure changes to the number of air molecules above the point of measurement.
💡Use the term 'collisions' when explaining how pressure is exerted by gases.
💡Remember that the atmosphere is very thin relative to the size of the Earth.
💡Always show your working in calculations. Even if you get the final answer wrong, you can earn method marks for using the correct equation and substituting values correctly. Write the equation, then substitute numbers with units, then calculate.
💡When drawing free-body diagrams, make sure the arrows are proportional to the force magnitudes and clearly labelled. Use a ruler for straight lines and include the direction of each force. This helps examiners see your understanding of vector addition.
💡For 'explain' questions, use the PEE structure (Point, Evidence, Explanation). State the physics principle, give evidence from the question or a calculation, then explain how it leads to the observed effect. This ensures you cover all the marking points.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing scalar and vector quantities
Failing to identify that forces always act in pairs
Misclassifying specific forces (e.g., treating air resistance as a non-contact force)
Omitting the directional component when describing vector quantities
Failing to use the perpendicular distance from the pivot
Confusing clockwise and anticlockwise moments
Incorrectly identifying the pivot point in a system
Forgetting to convert units (e.g., cm to m) when calculating moments
Assuming that an object needs a force to keep moving at a constant speed
Confusing velocity with speed when describing the law
Failing to identify that a change in direction requires a resultant force even if speed is constant
Confusing braking distance with thinking distance.
Failing to limit the vehicle condition factors to only brakes and tyres as specified.
Generalising 'poor weather' without linking it to the specific effect on friction or road surface grip.
Confusing the gradient of a distance–time graph (speed) with the gradient of a velocity–time graph (acceleration).
Failing to draw a tangent correctly when calculating the speed of an accelerating object at a specific point in time.
Misinterpreting a horizontal line as meaning the object is moving at a constant speed rather than being stationary.
Confusing speed with velocity
Forgetting to convert units to SI (e.g., minutes to seconds)
Incorrectly calculating the gradient of a curve on a velocity-time graph
Confusing the area under a distance-time graph with the area under a velocity-time graph
Failing to identify that deceleration is a negative value
Confusing distance (scalar) with displacement (vector).
Failing to include the direction when describing displacement.
Assuming distance and displacement are always the same value.
Confusing force with pressure
Failing to convert area units to metres squared (e.g., using cm^2 instead of m^2)
Misinterpreting the direction of the force (must be normal to the surface)
Confusing scalar and vector quantities when describing forces
Failing to account for direction when calculating resultant forces in a straight line
Incorrectly drawing free body diagrams by omitting forces or misrepresenting their direction
Misunderstanding the concept of equilibrium in relation to resultant forces
Confusing the units for density (kg/m³) or pressure (Pa)
Failing to convert height/depth into metres
Misinterpreting the 'h' in the equation as the total height of the container rather than the height of the liquid column above the point
Forgetting that upthrust is a resultant force
Confusing braking distance with thinking distance
Failing to link the reduction of kinetic energy to the work done by friction
Assuming braking force is constant regardless of speed
Misunderstanding the relationship between deceleration and brake temperature
Confusing thinking distance with braking distance
Failing to recognize that stopping distance is a sum of two distinct components
Assuming reaction time is constant for all drivers
Neglecting the impact of vehicle condition on braking distance
Assuming the forces act on the same object and therefore cancel out
Failing to identify that the forces must act on two different objects
Confusing Newton's Third Law pairs with balanced forces acting on a single object
Confusing speed (scalar) with velocity (vector)
Failing to convert units (e.g., minutes to seconds) before performing calculations
Incorrectly rearranging the s = vt equation
Assuming speed is constant in real-world scenarios when it is actually changing
Confusing speed (scalar) with velocity (vector)
Failing to include direction when describing velocity
Assuming constant velocity in circular motion
Confusing work done with energy stored
Failing to ensure the distance is measured along the line of action of the force
Incorrect unit conversions between joules and newton-metres
Misinterpreting the direction of displacement relative to the force
Confusing thinking distance with braking distance
Assuming reaction time is constant for all individuals
Failing to link reaction time directly to the thinking distance component of stopping distance
Incorrectly identifying factors that affect braking distance rather than thinking distance
Confusing scalar and vector quantities
Failing to include direction when describing a vector quantity
Misinterpreting the length of a vector arrow as something other than magnitude
Confusing mass with weight
Failing to use the resultant force in the calculation rather than a single applied force
Incorrect unit conversions (e.g., grams to kilograms)
Misinterpreting the proportionality relationship
Confusing momentum with kinetic energy
Failing to account for direction when calculating momentum (treating it as a scalar rather than a vector)
Forgetting to define the system as 'closed' when applying the conservation principle
Confusing elastic deformation (returns to original shape) with inelastic deformation (permanent change)
Failing to convert units (e.g., cm to m) when calculating extension or spring constant
Assuming Hooke's Law applies beyond the limit of proportionality
Misinterpreting the 'e' in the elastic potential energy equation as the total length rather than the extension
Confusing mass (the amount of matter in an object) with weight (the force of gravity on that object).
Incorrectly using units, such as using grams instead of kilograms in calculations.
Failing to recognise that gravitational field strength (g) will be provided in calculations.
Confusing momentum (a vector) with kinetic energy (a scalar).
Failing to account for the direction of velocity when calculating total momentum in a system.
Incorrectly identifying a system as 'closed' when external forces are acting.
Using incorrect units for mass (e.g., grams instead of kilograms).
Confusing change in momentum (mΔv) with momentum (mv)
Incorrectly calculating the change in velocity (Δv) when initial and final velocities are in opposite directions
Failing to convert units to standard SI units before calculation
Misinterpreting the time interval Δt
Confusing the decrease in density with an increase in pressure.
Failing to link the number of air molecules above a surface to the weight of the air.
Assuming atmospheric pressure is constant at all heights.
Misconception: 'If an object is moving, there must be a resultant force acting on it.' Correction: An object can move at constant velocity with zero resultant force (balanced forces). For example, a car cruising at 50 mph has balanced forces – engine force equals air resistance and friction.
Misconception: 'Weight and mass are the same thing.' Correction: Mass is a measure of how much matter an object contains (kg), while weight is the force of gravity acting on that mass (N). On the Moon, your mass stays the same but your weight is about 1/6 of that on Earth.
Misconception: 'Action-reaction forces cancel each other out.' Correction: Newton's third law pairs act on different objects, so they don't cancel. For example, when you push a wall, the wall pushes you back – these forces act on different objects (you and the wall), so they don't result in zero net force on either.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic understanding of speed, velocity, and acceleration from KS3 or earlier GCSE topics.
•Familiarity with graphs, especially distance-time and velocity-time graphs, as these are used to analyse motion.
•Knowledge of units and prefixes (e.g., N, kg, m/s) and how to convert between them.

Key Terminology

Essential terms to know

Classification of contact and non-contact interactions
Vector representation and free body diagrams
Newtonian interaction pairs and Third Law applications
Field theory as a mechanism for non-contact forces
Rotational equilibrium and the Principle of Moments
Mechanical advantage and force multiplication in levers
Transmission of motion and torque through gear systems
Inertia and its relationship to mass
Resultant force and equilibrium (static and dynamic)
Constant velocity versus acceleration
Vector representation of forces
Work done and energy dissipation
Velocity-squared relationship (d ∝ v²)
Frictional coefficients and road conditions
Newton's Second Law in deceleration
Scalar vs Vector quantities (Distance vs Displacement)
Graphical representation of uniform and non-uniform motion
Gradient analysis as a measure of rate of change (Speed)
Mathematical modelling of constant and instantaneous velocity
Vector nature of acceleration and directional change
Uniform versus non-uniform motion
Graphical analysis of velocity-time relationships
Newton's Second Law (F=ma) and resultant forces
Kinematic equations for constant acceleration (SUVAT)
Scalar vs Vector quantities
Magnitude and direction in kinematics
Path dependence vs change in position
Resultant displacement via vector addition
Relationship between force, area, and pressure in solids
Pressure in liquids as a function of depth and density
Atmospheric pressure and its variation with altitude
Kinetic theory and the mechanism of gas pressure
Vector addition and resolution
Newton’s First and Second Laws of Motion
Free-body diagrams and force representation
Equilibrium and terminal velocity
Mathematical derivation of the pressure-depth relationship (p = hρg)
Archimedes' principle and the physical origin of upthrust
Force equilibrium in floating and sinking objects
Pressure distribution in incompressible fluids
Work-Energy Principle in braking systems
The quadratic relationship between speed and braking distance
Frictional coefficients and road-tire interface conditions
Mechanical integrity of braking components and tire tread depth
Linear relationship between speed and thinking distance
Quadratic relationship between speed and braking distance
Energy dissipation via work done by friction
Human and environmental factors influencing reaction time and traction
Mechanical transfer of energy via work
Conservation of energy and dissipative forces
Power as the temporal rate of energy transfer
Efficiency and energy degradation
Thinking distance and total stopping distance
Experimental measurement and uncertainty in human response
Factors affecting neurological processing speed
Distinction between magnitude and direction
Vector addition and resultant determination
Resolution of vectors into perpendicular components
Scalar and vector kinematic pairs
Proportionality between resultant force and acceleration
Inertial mass as a measure of resistance to acceleration
Vector addition to determine resultant force
Rate of change of momentum as the generalized definition of force
Vector nature of momentum and directional sign conventions
Conservation in closed systems in the absence of external forces
Analysis of elastic and inelastic collisions
Recoil and explosion mechanics
Hooke's Law and the spring constant (k)
Elastic versus inelastic (plastic) deformation
Work done and elastic potential energy storage
Limit of proportionality and the elastic limit
Gravitational field strength and field lines
Newton's Law of Universal Gravitation
Gravitational potential and potential energy
Orbital mechanics and centripetal acceleration
Vector nature of linear momentum
Conservation of momentum in closed systems
Relationship between force and rate of change of momentum
Impulse and its application in safety design
Vector nature of momentum and velocity
Force as the rate of change of momentum
Impulse and the relationship between time and impact force
Conservation of momentum in closed systems
Safety engineering and deceleration mechanisms
Weight of the air column
Altitude-density relationship
Molecular collisions and pressure
Pressure gradients and fluid flow

Likely Command Words

How questions on this topic are typically asked

Describe

Identify

Explain

Calculate

State

Apply

Estimate

Determine

Interpret

Define

Evaluate

Recall

Convert

Represent

Use

Ready to test yourself?

Practice questions tailored to this topic

Forces

Subtopics in this area

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in AQA GCSE Physics

Atomic structure

Energy

Particle model of matter

Particle model of matter

How to Revise Forces — AQA GCSE Physics

Overview & Synopsis

Examiner Tips for Forces

Common Mistakes in Forces

Key Marking Points

Forces

Subtopics in this area

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between mass and weight?

How do you calculate resultant force?

What is terminal velocity and how does it occur?

How do you apply Newton's Third Law to everyday situations?

What is Hooke's Law and how do you use it?

How do you draw a free-body diagram?

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in AQA GCSE Physics

Atomic structure

Energy

Particle model of matter

Particle model of matter