AQA GCSE Physics Foundation (8463), Foundation tier · Forces › Forces and motion › Newton's laws of motion
Practise Newton's First Law. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Newton's First Law says that an object's velocity only changes if a resultant force acts on it. You need to use it to explain the motion of objects that are stationary, moving at a steady velocity, or changing speed or direction.
Key facts
Resultant force = 0 → velocity stays the same (at rest, or steady speed in a straight line)
Resultant force ≠ 0 → velocity changes (speed and/or direction)
Steady speed: driving force = resistive forces
Driving force > resistive forces → accelerates
Driving force < resistive forces → decelerates
Notes
The law
If the resultant force acting on an object is zero and the object is stationary, the object remains stationary.
If the resultant force acting on an object is zero and the object is moving, it continues to move at the same speed and in the same direction: at the same velocity.
So the velocity (speed and/or direction) of an object will only change if a resultant force acts on it.
Steady speed and changing speed
diagram
When a vehicle travels at a steady speed in a straight line, the resistive forces (friction and air resistance) balance the driving force.
A moving object does not need a resultant force to keep moving. It needs one to speed up, slow down or change direction.
Driving force greater than the resistive forces: the vehicle speeds up. Driving force smaller than the resistive forces: it slows down.
Balanced forces: constant velocity. Unbalanced forces: the velocity changes.
A car turning a corner at a steady speed is changing direction, so there must be a resultant force on it (sideways friction from the road).
How to answer each type of question
Apply the First Law to find a force
2 marksGrade 4
Spot the words 'constant velocity' or 'steady speed'.
So the resultant force is zero and the forces are balanced.
Set the forces in opposite directions equal.
Example. A car travels at a constant velocity along a straight, level road. The driving force is 2400 N. What is the total resistive force on the car? Give a reason for your answer.
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2400 N (1) The car moves at a constant velocity, so the resultant force is zero (the forces are balanced) (1).
Explain why a moving object does not need a resultant force
2 marksGrade 5
Identify the forces (or say there are none).
Use the First Law: zero resultant force means constant velocity.
Example. A student says: 'A spacecraft travelling through deep space with its engines switched off will slow down and stop.' Explain why the student is wrong.
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There is no resultant force on the spacecraft: there is no air resistance or friction in space (1) so it continues to move at the same speed in the same direction (Newton's First Law) (1).
Don’t lose marks
Saying an object moving at a steady speed must have a forward resultant force on it.
Writing 'the forces are equal' without saying they act in opposite directions, or that the resultant force is zero.
Forgetting that a change of direction at a steady speed also needs a resultant force.
More tips
Memory tricks
First Law in five words: no resultant force, no change (in velocity).
Balanced forces do not mean 'not moving'. They mean 'not accelerating'.
Changing direction counts as changing velocity, so it needs a resultant force too.
Exam technique
Link forces to motion in one sentence: 'the resultant force is zero, so the velocity is constant'.
In questions about changing speed, describe how the forces change, then the resultant force, then the motion.
When a question says 'constant velocity' or 'steady speed', think 'the forces are balanced'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recognise balanced forces on a diagramEqual-sized arrows in opposite directions mean the resultant force is zero.
Grade 4
State Newton's First LawIf the resultant force on an object is zero, it stays at rest or keeps moving at the same velocity.
Grade 4
Find a force from steady motionAt a steady speed in a straight line, the driving force equals the total resistive force.
Grade 5
Explain steady motion using balanced forcesA zero resultant force means there is no change in speed or direction.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A car travels at a steady speed along a straight, level road. The driving force on the car is 2400 N. What is the total resistive force acting on the car?
2400 N
A cyclist rides along a straight, level road. The driving force from the cyclist is 180 N and the total resistive force is 180 N. Describe the motion of the cyclist.
Moving at a constant velocity.
Complete the sentence. An object continues to move at a constant velocity, or remain at rest, unless it is acted on by a ............
Resultant force.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Complete the sentence. If the resultant force on a stationary object is zero, the object will ............ Tick (✓) one box.[1]
start to move
remain stationary
accelerate
move at a constant speed
(b) A car travels at a steady speed along a straight, level road. The driving force on the car is 2400 N. What is the total resistive force acting on the car?[1]
(c) What happens to the velocity of a moving object if the resultant force on the object is zero?[1]
(d) Which statement is correct? Tick (✓) one box.[1]
A resultant force is needed to keep an object moving at a steady speed.
An object can only change direction if a resultant force acts on it.
A moving object always slows down when there is no resultant force.
The resultant force on an object moving at a steady speed is in the direction of motion.
Show the answer and mark scheme
(a)Answer: remain stationary
(b)Answer: 2400 N
2400 (N)
(c)Answer: It stays constant.
it stays the same / constant (the object keeps the same speed and direction)
(d)Answer: An object can only change direction if a resultant force acts on it.
Question 2Medium6 marks
A cyclist rides along a straight, level road. The driving force from the cyclist is 180 N and the total resistive force is 180 N.
(a) Describe the motion of the cyclist.[1]
(b) The cyclist pedals harder, so the driving force increases to 230 N. Calculate the resultant force on the cyclist and describe its effect on the motion.[2]
(c) The driving force stays at 230 N. After a short time the cyclist moves at a new, higher steady speed. Explain why.[3]
Show the answer and mark scheme
(a)Answer: Moving at a constant velocity.
constant / steady speed in a straight line (constant velocity)
(b)Answer: 50 N forwards; the cyclist accelerates N
50 (N) (forwards)
the cyclist accelerates / speeds up
(c)Answer: Air resistance increases with speed, reducing the resultant force until resistive force = 230 N; then the resultant force is zero and the speed is constant.
as the speed increases, the air resistance / resistive force increases
so the resultant force decreases
when the resistive force reaches 230 N the resultant force is zero, so the speed stays constant