AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Forces, accelerations and Newton's Laws of motion
Practise Newton's First Law. 12 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, and (Higher) explain what inertia means.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recognise balanced forces on a diagramEqual-sized arrows in opposite directions mean the resultant force is zero.
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.
4
Find a force from steady motionAt a steady speed in a straight line, the driving force equals the total resistive force.
5
Explain steady motion using balanced forcesA zero resultant force means there is no change in speed or direction.
6
Explain changing motion using resultant forceVelocity (speed and/or direction) only changes if a resultant force acts.
7
Define and use the idea of inertiaInertia is the tendency of objects to continue in their state of rest or of uniform motion.
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
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.
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).
Inertia grade 7+
The tendency of objects to continue in their state of rest or of uniform motion is called inertia.
Example: when a bus brakes suddenly, standing passengers lurch forwards, because their bodies tend to keep moving at the original velocity.
The greater an object's mass, the harder it is to change its velocity (see inertial mass in Newton's Second Law).
Cheatsheet
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
Inertia = the tendency to stay at rest or in uniform motion grade 7+
How to answer each type of question
Apply the First Law to find a force
2 marks4
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.
Show the model answer
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 marks5
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.
Show the model answer
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).
Explain changes in motion using forces
3 marks6
Compare the driving force with the resistive forces.
Say how the resistive forces change as the speed changes.
Say what happens when the forces balance again.
Example. A cyclist is moving at a steady speed. She then pedals harder, so the driving force increases. Explain how the cyclist's speed changes and why it eventually becomes constant again.
Show the model answer
The driving force is greater than the resistive forces, so there is a resultant force forwards and the cyclist accelerates (1). As her speed increases, the air resistance increases (1). When the resistive forces equal the driving force again, the resultant force is zero and she moves at a new, higher, constant speed (1).
Explain using inertia
2 marks7
Say that the object tends to stay at rest (or keep moving): inertia.
Explain why the force acting is not enough to change its velocity much.
Example. A magician pulls a smooth tablecloth very quickly from under a plate. The plate stays in almost the same place. Use the idea of inertia to explain why.
Show the model answer
The plate tends to stay at rest because of its inertia (1). The friction force from the smooth cloth is small and acts for only a very short time, so it cannot change the plate's velocity much (1).
Shortcuts and 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.
Where marks are lost
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.
Calling inertia a force. It is a tendency (a property of the object), not a force.
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'.
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.
What is meant by inertia?
The tendency of an object to stay at rest or to keep moving with the same 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
Question 3Hard6 marks
(a) What is meant by inertia?[1]
(b) A passenger is standing, without holding on, in a bus that is moving at a steady speed. The bus brakes suddenly and the passenger falls forwards. Use Newton’s First Law to explain why.[3]
(c) Explain why a fully loaded supermarket trolley is harder to stop than an empty trolley moving at the same velocity.[2]
Show the answer and mark scheme
(a)Answer: The tendency of an object to stay at rest or to keep moving with the same velocity.
the tendency of an object to continue in its state of rest or of uniform motion (constant velocity)
(b)Answer: The bus decelerates because of a resultant force on it, but the passenger’s upper body has no resultant force on it, so it keeps moving forwards at the same velocity.
a (backward) resultant force acts on the bus, so the bus decelerates
(almost) no resultant force acts on the passenger’s upper body, so it continues to move forwards at the same velocity (inertia)
so the passenger moves forwards relative to the bus
(c)Answer: It has more mass (more inertia), so a bigger force is needed to give the same deceleration.
the loaded trolley has a greater mass, so a greater inertia / inertial mass
so a larger (resultant) force is needed to give the same deceleration / to stop it in the same time