Newton's First Law — AQA GCSE Physics
Test yourself on Newton's First Law with AQA GCSE practice questions.
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Newton's First Law explained
A resultant force is what remains when every force on an object is added as a vector.
Read the full explanation
If that total is zero, velocity cannot change: a stationary object stays still, and a moving object keeps the same speed in a straight line. Zero resultant force is not the same as no forces at all. A car cruising on a level road has multiple forces acting on it: weight is balanced by the normal contact force, while the driving force is exactly matched by the sum of drag and friction. A hailstone at terminal velocity has its weight exactly matched by air resistance. In these cases, forces balance out, so the object neither speeds up nor slows down. Because velocity is a vector, this law also means direction cannot change: an object with zero resultant force never turns.
So, when a vehicle travels at a steady speed the resistive forces balance the driving force.
A vehicle is pushed forward by a driving force from the wheels and held back by resistive forces, mainly air resistance plus friction. Air resistance grows as the vehicle goes faster, because the car sweeps through more air every second. When a driver accelerates, the driving force is larger than the resistive forces, so there is a resultant forward force. As the speed rises the resistive forces catch up, the resultant force shrinks and the acceleration falls. At maximum speed the resistive forces have grown until they equal the driving force, the resultant force is zero and the speed stays constant. The driving force has not fallen; the drag has risen to meet it. A steeper hill or a headwind raises the resistive forces at every speed, so the same engine reaches a lower maximum speed.
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, so it carries a size and a direction. Changing either one counts as a change of velocity, and every change of velocity needs a resultant force. A falling hailstone accelerates because its weight is larger than the air resistance on it, so a resultant force acts downwards. A satellite in a circular orbit travels at a constant speed, yet gravity pulls it towards the Earth at right angles to its motion, continually turning it; the direction changes, so the velocity changes and the satellite is accelerating even though its speed never alters. A car following a bending road at a fixed speedometer reading has a changing velocity for the same reason. A resultant force can change the speed, the direction, or both at once, and the direction of the force tells you which.
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.
Applying the first law means reading the motion, then deciding what the forces must be doing. If the description says uniform velocity, steady speed or terminal velocity, the resultant force is zero and you name the balanced pair. If the description says speeding up, slowing down or turning, a resultant force acts and you say which force dominates and in which direction. The Hubble Space Telescope is the classic turning example: the Earth's gravitational force makes it accelerate towards the Earth, which changes its direction of motion but not its speed, so its velocity changes and it stays in orbit. A train rounding a curve at a fixed speed is accelerating for exactly the same reason.
(HT only) The tendency of objects to continue in their state of rest or of uniform motion is called inertia.
Inertia is the name for an object's reluctance to change how it is moving. An object at rest resists being started; an object already moving resists being sped up, slowed down or turned. Inertia is not a force and it does not push anything: it is a property that comes from mass. Because of it, a passenger on a braking bus keeps moving forwards while the bus slows beneath them. On Higher Tier the idea is made quantitative: inertial mass is a measure of how hard it is to change an object's velocity, and it is defined as the ratio of force to acceleration, m = F ÷ a. A trolley needing 12 N to accelerate at 4 m/s² has an inertial mass of 3 kg. Inertia is also why a seat belt is needed: the passenger keeps moving forwards at the original speed while the car stops around them.
Your focus
- Decide from a free body force diagram whether the resultant force is zero, and predict the motion that follows.
- Explain why a hailstone stops accelerating once air resistance has grown to equal its weight.
- Distinguish an object with zero resultant force from one with no forces acting on it at all, identifying all horizontal and vertical forces on a cruising car.
Show all 15 objectives
- Describe how driving force and air resistance compare at the start of an acceleration, part way through, and at maximum speed.
- Explain why a vehicle has a maximum speed even though its driving force stays the same.
- Draw a labelled horizontal force diagram for a van travelling at a steady speed, with arrows of equal length.
- Explain why a satellite moving at a constant speed in a circle is still accelerating.
- Identify which force is larger from a description of an accelerating object such as a falling hailstone.
- Compare the average speed and the average velocity of a car driven along a bending road between two cameras.
- Explain the effect of the Earth's gravitational force on the motion of an orbiting telescope in terms of direction and velocity.
- Work out, from a description of a journey, whether the resultant force on the vehicle is zero or not.
- Explain how a train can accelerate on a curved track without its speedometer reading changing.
- Calculate the inertial mass of a trolley from a measured resultant force and its acceleration, giving the unit.
- Explain why a passenger continues forwards when a bus brakes, without calling inertia a force.
- Compare two objects of different mass by how large a force each needs for the same change of velocity.
Newton's First Law exam tips
Marking Points
- one mark for stating that the resultant force acting on the object is zero
- one mark for saying the object stays stationary, or continues at the same speed in the same direction
- one mark for naming the forces that balance, such as weight and normal contact force, or driving force and drag plus friction
- one mark for stating that the balanced forces are equal in size and opposite in direction
- one mark for the driving force being greater than the resistive forces, giving a resultant forward force
- one mark for air resistance increasing as the speed of the vehicle increases
- one mark for the resultant force decreasing, so the acceleration decreases
- one mark for air resistance becoming equal to the driving force at maximum speed, so the resultant force is zero
- one mark for stating that a resultant force is acting on the object
- one mark for identifying which force is larger, for example weight greater than air resistance
- one mark for stating that velocity includes direction, so a change of direction is a change of velocity
- one mark for linking a force acting at right angles to the motion to a change of direction but not of speed
- one mark for the gravitational force causing the object to accelerate towards the Earth
- one mark for saying this changes the direction of motion but not the speed
- one mark for concluding that the velocity of the object therefore changes
- one mark for saying the gravitational force maintains the circular orbit, awarded only if no other marks are scored
- one mark for defining inertia as the tendency of an object to stay at rest or to keep moving at the same velocity
- one mark for stating that a larger mass gives a greater inertia
- one mark for defining inertial mass as the ratio of force to acceleration
- one mark for a correct calculation of inertial mass from force ÷ acceleration, with the unit kg
Examiner Tips
- 💡Write the words 'resultant force is zero' rather than 'the forces cancel out'; the mark scheme looks for that phrase.
- 💡Whenever a question says steady speed, constant velocity or terminal velocity, make zero resultant force your first sentence.
- 💡Identify all forces acting on an object, including vertical forces like weight and normal contact force, before describing the motion.
- 💡In a levels-marked 'explain how the forces change' question, track both forces from the start of the acceleration to maximum speed.
- 💡Finish with the sentence 'the resultant force is now zero', which is usually needed for the top level.
- 💡Label arrow lengths on a force diagram to show which force is larger at each stage.
- 💡If the question mentions direction changing, say the word velocity, not speed, in your answer.
- 💡State which force is bigger before you state that there is a resultant force; both usually earn marks.
- 💡For circular motion write 'the direction changes so the velocity changes' as a separate sentence.
- 💡Three-mark motion questions usually want force, then what changes, then the conclusion about velocity; write three separate sentences.
- 💡Quote the direction of the force as well as its name; 'towards the Earth' is often a marking point.
- 💡Check whether the object's speed is fixed before you claim it is accelerating or decelerating.
- 💡Say 'tendency to continue' rather than 'wants to keep going'; examiners reject answers that give objects intentions.
- 💡For inertial mass, quote m = F ÷ a and show the substitution before the answer.
- 💡Check the tier: inertia and inertial mass are Higher Tier only, so expect them late in the paper.
Common Mistakes
- writing that no forces act on an object moving at a constant velocity, instead of saying the forces are balanced
- claiming a resultant force is needed to keep an object moving at a steady speed
- treating a car going round a bend at a steady speed as having zero resultant force, when its direction is changing
- assuming only two forces act on a cruising car, ignoring vertical forces like weight and normal contact force
- saying the driving force decreases as the car speeds up, when it is the air resistance that increases
- claiming air resistance becomes greater than the driving force at top speed, which would make the car slow down
- describing only the forces at maximum speed and ignoring how they changed on the way there
- using the vague word friction for a car moving through air rather than air resistance or drag
- assuming an object moving in a circle at a steady speed has zero resultant force
- taking acceleration to mean speeding up only, so ignoring turning and slowing down
- using the words speed and velocity interchangeably in an explain question
- saying gravity pulls a satellite forwards along its orbit rather than towards the Earth
- saying a satellite is weightless because there is no gravity in space
- stating the telescope speeds up because a force acts, when that force is perpendicular to its motion
- writing 'it accelerates' with no statement of what is changing, speed or direction
- assuming distance travelled and displacement are the same along a curved route
- describing inertia as a force that throws passengers forwards when a vehicle brakes
- confusing inertia with momentum, which also depends on how fast the object is going
- claiming an object in space has no inertia because it has no weight
- writing the inertial mass ratio upside down as acceleration divided by force