Newton's First Law — AQA GCSE Combined Science
Test yourself on Newton's First Law with AQA GCSE practice questions.
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Newton's First Law explained
Newton’s First Law describes what happens to an object’s motion when the forces on it balance.
Read the full explanation
If the resultant force is zero, the object keeps doing what it was already doing: a stationary object stays at rest, and a moving object continues at the same speed in a straight line. This is why a puck sliding on frictionless ice keeps moving, and why a book resting on a table does not start moving on its own. The law is not about “no forces” — balanced forces can still act — but about the resultant being zero. It explains why seat belts are needed: a passenger keeps moving forward when a car stops suddenly because no resultant force acts on them during the brief interval.
If the resultant force acting on an object is zero and:
This statement introduces the two cases that follow from Newton’s First Law when the resultant force is zero. Case one: the object is stationary, so it stays stationary. Case two: the object is already moving, so it continues at the same speed in the same straight line. The key idea is that zero resultant force means no change in velocity, whether that velocity is zero or non-zero. For example, a cyclist travelling at a steady 6 m/s on a straight flat road has zero resultant force; the driving force balances friction and air resistance. If the cyclist stops pedalling, friction and air resistance become the resultant force, so the motion changes.
the object is stationary, the object remains stationary
Newton's First Law says that if the resultant force on an object is zero, its velocity does not change. This statement covers the stationary case: an object already at rest stays at rest. 'Stationary' means its velocity is zero, so it is not moving and its direction is undefined. If all the forces on it balance, there is no resultant force to start it moving, so it remains stationary. For example, a book resting on a desk has weight acting downwards and a normal contact force acting upwards; these are equal and opposite, so the resultant force is zero and the book stays still. A student should identify the forces, show that they cancel, and conclude that the velocity stays at zero.
the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.
Newton's First Law states that if the resultant force on an object is zero, its velocity does not change. This statement covers a moving object: it keeps the same speed and the same direction, so its velocity is unchanged. Velocity is a vector, so a change in direction is a change in velocity even if speed is constant. For example, a spacecraft drifting far from any planet has negligible resultant force and continues in a straight line at constant speed. A car moving at a steady speed along a straight road also has zero resultant force: the driving force balances friction and air resistance, and weight balances the normal contact force. A student should identify balanced forces and conclude that speed and direction stay the same.
So, when a vehicle travels at a steady speed the resistive forces balance the driving force.
A vehicle moving at a steady speed has zero resultant force, so the driving force exactly balances the total resistive forces. The driving force is actually provided by the static friction between the driven tyres and the road pushing the vehicle forward. Resistive forces include air resistance (drag) and rolling resistance. If the driving force were larger, the vehicle would accelerate; if smaller, it would decelerate. For example, a car cruising at a constant 20 m/s on a level road has a forward driving force of 600 N and a total resistive force of 600 N, giving a resultant of 0 N. This is a direct application of Newton's First Law: a body continues at constant velocity unless acted on by a resultant force.
So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
Velocity is a vector: it has both magnitude (speed) and direction. A resultant force is needed to change either the speed, the direction, or both. If the resultant force is zero, the object continues at constant velocity in a straight line. For example, a car turning a corner at constant speed still changes velocity because its direction changes, so a resultant force acts towards the centre of the turn. Similarly, a cyclist accelerating from rest has a forward resultant force. This statement is the core of Newton's First Law: without a resultant force, velocity cannot change.
Students should be able to apply Newton’s First Law to explain the motion of objects moving with a uniform velocity and objects where the speed and/or direction changes.
Newton’s First Law says that if the resultant force on an object is zero, the object stays at rest or keeps moving at constant velocity; if the resultant force is not zero, the velocity changes. Constant velocity means both speed and direction are unchanged, so a car cruising at 20 m/s due north has zero resultant force. When speed and/or direction changes, the object accelerates, so a resultant force must act. A cyclist turning at steady speed is accelerating because direction changes, so a resultant force acts towards the centre of the turn. To apply the law, identify all forces, find the resultant, then link it to the motion: zero resultant gives uniform velocity; non-zero resultant gives changing speed, direction or both.
(HT only) The tendency of objects to continue in their state of rest or of uniform motion is called inertia.
Inertia is the tendency of an object to keep doing what it is already doing: staying at rest, or continuing to move with uniform velocity. It is not a force and it does not cause motion; it explains why a resultant force is needed to change motion. A stationary book stays on a desk until a resultant force acts; a spacecraft coasting far from gravity continues at constant velocity because no resultant force acts. Inertia depends on mass: a larger mass has greater inertia, so a larger resultant force is needed to produce the same acceleration. This Higher Tier concept links directly to Newton’s First Law and prepares students for F = ma, where mass measures how strongly motion resists change.
Your focus
- State Newton’s First Law in terms of resultant force and motion.
- Describe the motion of stationary and moving objects when the resultant force is zero.
- Apply Newton’s First Law to explain a familiar situation involving balanced forces.
Show all 24 objectives
- Describe the motion of a stationary object when the resultant force is zero.
- Describe the motion of a moving object when the resultant force is zero.
- Explain why a non-zero resultant force is needed to change an object’s motion.
- State that a stationary object remains stationary when the resultant force on it is zero.
- Identify balanced forces acting on a stationary object in a given example.
- Explain that zero resultant force means no change in velocity, so a stationary object stays at rest.
- State that a moving object continues at the same speed and in the same direction when the resultant force is zero.
- Explain that constant speed in a straight line means constant velocity.
- Apply Newton's First Law to a moving object using balanced forces in a named example.
- Describe the forces acting on a vehicle travelling at steady speed.
- Explain why the resultant force is zero when speed is constant.
- Calculate an unknown driving or resistive force given that the vehicle travels at steady speed.
- Define velocity as speed in a given direction.
- Explain that a resultant force is required to change an object's speed or direction.
- Apply Newton's First Law to examples including acceleration, braking and turning.
- Identify when the resultant force on an object is zero from a description of its motion.
- Explain how a non-zero resultant force changes an object’s speed, direction or both.
- Apply Newton’s First Law to unfamiliar examples involving uniform velocity or changing motion.
- Define inertia in terms of an object’s state of rest or uniform motion.
- Explain how mass affects inertia and the resultant force needed to change motion.
- Apply the concept of inertia to everyday situations involving sudden changes in motion.
Newton's First Law exam tips
Marking Points
- State that Newton’s First Law applies when the resultant force on an object is zero.
- Explain that a stationary object remains stationary when the resultant force is zero.
- Explain that a moving object continues at constant speed in a straight line when the resultant force is zero.
- Distinguish between balanced forces and zero forces, noting that balanced forces give a zero resultant.
- Apply the law to a real context, such as a passenger continuing forward when a vehicle brakes sharply.
- Use the term resultant force correctly, meaning the single force that has the same effect as all the forces combined.
- Identify that the statement sets out two cases: a stationary object and a moving object.
- State that a stationary object remains stationary when the resultant force is zero.
- State that a moving object continues at the same speed in a straight line when the resultant force is zero.
- Explain that zero resultant force means no change in velocity, including no change in direction.
- Apply the two cases to a named example, such as a car cruising at steady speed on a straight road.
- Recognise that a non-zero resultant force is needed to start, stop, speed up, slow down or change direction.
- States that a stationary object has zero velocity and is not moving.
- Explains that when the resultant force is zero, there is no change in velocity.
- Applies the idea to a named example, such as a book on a desk or a parked car.
- Identifies the balanced forces acting on the object, for example weight and the normal contact force.
- Concludes that because the resultant force is zero, the object remains stationary.
- States that a moving object with zero resultant force keeps the same speed.
- States that the object keeps the same direction of motion.
- Explains that constant speed in a straight line means constant velocity.
- Applies the idea to a named example, such as a car at steady speed on a straight road.
- Identifies the balanced forces acting on the moving object, for example driving force and resistive forces.
- State that steady speed means the vehicle's velocity is constant, so acceleration is zero.
- Identify the driving force acting forwards (due to friction between driven tyres and road) and the resistive forces acting backwards.
- Explain that the resultant force is zero because the two sets of forces are equal in size and opposite in direction.
- Use the equation resultant force = driving force − resistive force and show that it equals 0 N for steady speed.
- Apply the idea to a numerical example, such as 600 N forward and 600 N backward giving a resultant of 0 N.
- Recognise that resistive forces include air resistance and friction, and that air resistance increases with speed, whereas solid friction generally does not.
- Define velocity as speed in a stated direction, so a change in either speed or direction is a change in velocity.
- State that a resultant force is required to change an object's velocity.
- Explain that if the resultant force is zero, the object moves at constant velocity in a straight line.
- Give an example where speed is constant but direction changes, such as circular motion, and identify the resultant force.
- Give an example where direction is constant but speed changes, such as a car accelerating along a straight road.
- Link the size and direction of the resultant force to the change in velocity produced.
- State that zero resultant force means the object remains at rest or moves with uniform velocity.
- Explain that uniform velocity requires both constant speed and constant direction.
- Identify that a change in speed, direction or both means a non-zero resultant force acts.
- Apply the law to a named example, such as a car at steady speed on a straight road having zero resultant force.
- Recognise that an object turning at constant speed is accelerating because its direction changes.
- Link the direction of the resultant force to the direction of the velocity change.
- Define inertia as the tendency to remain at rest or to continue moving with uniform velocity.
- State that inertia is not a force and does not itself change an object’s motion.
- Explain that a resultant force is required to overcome inertia and change speed or direction.
- Relate greater mass to greater inertia, so more force is needed for the same acceleration.
- Apply the idea to examples such as a passenger continuing forward when a bus brakes.
- Distinguish inertia from momentum and from the forces acting on an object.
Examiner Tips
- 💡Always link your answer to the resultant force being zero, not just to “balanced forces”.
- 💡When describing motion, state both the speed and the direction to show velocity is constant.
- 💡Use a short everyday example, such as a ice skater or a car at steady speed, to make the application clear.
- 💡Write both cases explicitly: stationary stays stationary, moving keeps constant velocity.
- 💡Use the phrase “same speed in a straight line” to cover both magnitude and direction.
- 💡Check whether the question asks for a description or an explanation, and include the resultant force in either case.
- 💡Start by stating the resultant force is zero before concluding the object stays still.
- 💡Name the balanced forces in your example rather than saying 'forces cancel' without detail.
- 💡Use the word 'velocity' as well as 'stationary' to show you understand that velocity is zero.
- 💡Write 'same speed and same direction' together to show velocity is unchanged.
- 💡Use a straight-line example so the direction is clearly constant.
- 💡Link your answer to the resultant force being zero before describing the motion.
- 💡Always state the direction of each force when describing a balanced-force situation.
- 💡Use the phrase 'resultant force is zero' rather than 'no forces' to show precise understanding.
- 💡If a numerical value is given, calculate the resultant force explicitly before concluding that the speed is steady.
- 💡Use the word 'velocity' rather than 'speed' when direction may change, and state the direction where possible.
- 💡When explaining circular motion, mention the resultant force acting towards the centre of the circle.
- 💡Check whether the question asks about speed or velocity; a change in direction alone is enough to change velocity.
- 💡Name the forces and state whether they balance before concluding anything about motion.
- 💡Use the phrase ‘resultant force’ rather than ‘the force’ when explaining changes in motion.
- 💡For turning objects, state explicitly that direction changes, so velocity changes even if speed is constant.
- 💡Use the wording ‘tendency to continue in its state of rest or uniform motion’ when defining inertia.
- 💡Link inertia to mass whenever a question asks why a heavier object is harder to accelerate or stop.
- 💡Avoid saying inertia ‘pushes’ objects; describe the motion and the resultant force instead.
Common Mistakes
- Saying “no forces act” when the resultant is zero; correct this by stating that forces may be balanced, giving a zero resultant.
- Claiming a moving object must have a forward force to keep moving; correct this by explaining that constant velocity needs zero resultant force, not a driving force.
- Confusing constant speed with constant velocity; correct this by noting that a change of direction is a change of velocity and needs a resultant force.
- Thinking a moving object with zero resultant force will slow down and stop; correct this by stating it continues at constant velocity.
- Believing a stationary object with balanced forces will move; correct this by explaining that balanced forces produce no change in motion.
- Treating “steady speed” and “constant velocity” as identical when direction changes; correct this by noting that a change in direction is a change in velocity.
- Saying that no forces act on a stationary object; the correction is that forces do act but they balance, giving a resultant force of zero.
- Confusing 'stationary' with 'constant speed'; the correction is that stationary means velocity is zero, whereas constant speed can be non-zero.
- Claiming that a resultant force is needed to keep an object still; the correction is that a resultant force would change its velocity, so zero resultant force keeps it still.
- Saying a moving object needs a constant resultant force to keep moving; the correction is that a zero resultant force keeps the velocity constant.
- Treating speed and velocity as the same; the correction is that velocity includes direction, so a change of direction changes velocity even at constant speed.
- Forgetting to mention direction and only stating constant speed; the correction is to state both constant speed and constant direction.
- Thinking that a moving vehicle must have a resultant force in the direction of motion; correction: at steady speed the resultant force is zero, and motion continues because of inertia.
- Confusing balanced forces with no forces acting; correction: forces are still present and balanced, not absent.
- Assuming the driving force is always larger than resistive forces whenever the vehicle moves; correction: the driving force is larger only when the vehicle is accelerating.
- Believing that an object moving at constant speed cannot be accelerating; correction: changing direction at constant speed is still a change in velocity and requires a resultant force.
- Thinking that a resultant force is needed to keep an object moving; correction: a resultant force is needed only to change velocity, not to maintain it.
- Treating speed and velocity as identical; correction: velocity includes direction, so a change in direction alone changes velocity.
- Thinking a moving object always has a resultant force acting; correction: at constant velocity the resultant force is zero.
- Believing constant speed alone means no acceleration; correction: a change of direction at constant speed is still acceleration.
- Assuming a forward force is always larger than resistance for a moving object; correction: at uniform velocity the forward and resistive forces balance.
- Calling inertia a force; correction: inertia is a property of matter, measured by mass, not a force.
- Thinking heavy objects always fall faster because of inertia; correction: in free fall without air resistance all masses accelerate equally.
- Confusing inertia with momentum; correction: inertia depends on mass alone, while momentum depends on mass and velocity.