Velocity — AQA GCSE Physics
Test yourself on Velocity with AQA GCSE practice questions.
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Velocity explained
Because a direction is part of it, a velocity changes whenever either the speed or the direction changes.
Read the full explanation
A car driving round a bend at a steady 13 m/s has a constant speed but a changing velocity, and average-speed cameras can only report a speed, since the road between them is not straight. Constant velocity therefore means moving in a straight line at a steady rate, and anything else is accelerating. On a straight road, opposite directions are handled with signs, so a vehicle travelling at 20 m/s that turns round and comes back at 20 m/s has changed its velocity by 40 m/s. Velocity is measured in metres per second, exactly like speed, so the unit alone never tells you which quantity you are holding.
Students should be able to explain the vector–scalar distinction as it applies to displacement, distance, velocity and speed.
Set the four quantities out in pairs. Distance and speed are scalars, with magnitude only and no direction. Displacement and velocity are their vector partners, carrying magnitude and direction. The pairs connect through time, since speed is distance over time and velocity is displacement over time, so the scalar or vector nature carries through the division. A train taking a curved route travels a distance of, say, 12 km along the track but is displaced only 8.5 km in a straight line; over 600 s its average speed is 20 m/s while its average velocity is about 14 m/s in the direction from the first station to the second. Any journey returning to its start has zero displacement and zero average velocity, though neither its distance nor its average speed is zero.
(HT only) Students should be able to explain qualitatively, with examples, that motion in a circle involves constant speed but changing velocity.
An object going round a circle at a steady rate keeps the same speed but is always turning, so its direction, and therefore its velocity, changes at every instant. A changing velocity is an acceleration, and an acceleration needs a resultant force, which here points towards the centre of the circle. Because that force acts at right angles to the motion it does no work, so the speed does not change and only the direction does. For a satellite such as the Hubble Space Telescope the force is the Earth's gravitational pull, which makes it accelerate towards the Earth and keeps it in orbit instead of flying off in a straight line. For a car on a roundabout it is friction between the tyres and the road, and for a conker on a string it is tension.
Your focus
- Explain why a vehicle on a curved road has a changing velocity at constant speed.
- Calculate the change of velocity when a vehicle reverses its direction of travel.
- State the conditions required for an object to move with constant velocity.
Show all 9 objectives
- Classify distance, displacement, speed and velocity as scalar or vector.
- Calculate both the average speed and the average velocity for a curved journey.
- Explain why the average velocity is zero for a journey that returns to its start.
- Explain why an object moving in a circle at constant speed is accelerating.
- Identify the force that keeps a named object moving in a circle.
- Describe why a force at right angles to the motion changes velocity but not speed.
Velocity exam tips
Marking Points
- one mark for velocity as a speed in a given direction
- one mark for velocity as a vector and speed as a scalar
- one mark for a change of direction counting as a change of velocity
- one mark for constant velocity requiring motion in a straight line at a steady speed
- one mark for distance and speed identified as scalars with magnitude only
- one mark for displacement and velocity identified as vectors with magnitude and direction
- one mark for pairing each scalar with its vector partner
- one mark for a worked comparison showing the two differing on a curved route
- one mark for the speed staying constant while the direction of motion changes
- one mark for a changing direction meaning a changing velocity
- one mark for a resultant force directed towards the centre of the circle
- one mark for naming that force in a given example, such as gravity acting on a satellite
Examiner Tips
- 💡If the path is not straight, the velocity is changing; say so explicitly.
- 💡Quote both the magnitude and the direction whenever a velocity is asked for.
- 💡Learn the four as two pairs, with the vector always carrying the direction.
- 💡For a closed loop, write zero displacement and zero average velocity immediately.
- 💡Write the chain: direction changes, so velocity changes, so it accelerates, so a resultant force acts towards the centre.
- 💡Name the actual force in the example - gravity, tension or friction - rather than writing centripetal force alone.
Common Mistakes
- saying velocity and speed are the same because they share a unit
- claiming a car on a bend at a steady rate has constant velocity
- giving a velocity with no direction
- asserting that a changing velocity always means a changing speed
- pairing displacement with speed, or distance with velocity
- claiming displacement and distance are always equal
- saying the average velocity of a round trip equals its average speed
- defining a vector as a bigger quantity rather than as one with direction
- saying the velocity is constant because the speed is constant
- claiming there is no resultant force because the speed does not change
- describing the force as acting along the direction of travel
- stating that gravity changes the speed of an orbiting satellite