AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Describing motion along a line
Practise Velocity. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Velocity is speed in a given direction, so it is a vector. You need to explain the difference between speed and velocity and, on the Higher tier, explain why an object moving in a circle at constant speed has a changing velocity.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define velocityThe velocity of an object is its speed in a given direction.
4
Explain the difference between speed and velocitySpeed is a scalar (magnitude only); velocity is a vector (magnitude and direction).
5
Use + and − signs for velocityAlong a line, e.g. +4 m/s to the right and −4 m/s to the left.
6
Explain when velocity changesVelocity changes if the speed changes, if the direction changes, or if both change.
7
Explain circular motion: constant speed, changing velocityThe direction of motion changes all the time, so the velocity changes even though the speed stays the same.
8
Link changing velocity to acceleration and forceA changing velocity is an acceleration, so there must be a resultant force on the object.
Notes
Speed and velocity
The velocity of an object is its speed in a given direction.
Velocity is a vector quantity; speed is a scalar quantity.
Velocity has the same unit as speed, m/s, but you also give a direction, e.g. 25 m/s north.
Along a straight line, you can show direction with a sign: +3 m/s forwards, −3 m/s backwards.
When does velocity change?
Velocity changes if the speed changes, if the direction changes, or if both change.
A car going round a bend at a steady 15 m/s has a constant speed but a changing velocity.
Two objects with the same speed moving in different directions have different velocities.
A change in velocity is an acceleration, and it needs a resultant force.
Motion in a circle grade 7+
An object moving in a circle at a constant speed is changing direction all the time.
Velocity includes direction, so the object's velocity is changing all the time.
So the object is accelerating, even though its speed stays the same. A resultant force acts on it, towards the centre of the circle.
Examples: a satellite or a moon in a circular orbit, a car driving round a roundabout at a steady speed, a ball whirled round on a string.
Cheatsheet
Velocity = speed in a given direction
Velocity is a vector; speed is a scalar
Unit of velocity: m/s, with a direction
Velocity changes if the speed OR the direction changes
Along a line: + for one direction, − for the opposite direction
Circular motion at constant speed: velocity is changing, so the object is accelerating grade 7+
Changing velocity = acceleration, which needs a resultant force grade 8+
How to answer each type of question
Explain the difference between speed and velocity
2 marks4
Say that speed is a scalar: magnitude only.
Say that velocity is a vector: speed in a given direction.
Example. Explain the difference between speed and velocity.
Show the model answer
Speed is a scalar: it has magnitude only (1). Velocity is a vector: it is speed in a given direction (1).
Velocities with signs
3 marks6
Choose a positive direction.
Give each velocity a sign.
Change in velocity = final velocity − initial velocity.
Example. A ball moving at 5.0 m/s to the right hits a wall and bounces back at 4.0 m/s to the left. Take 'to the right' as positive. (a) Write down the velocity of the ball before it hits the wall. (b) Write down the velocity of the ball after it bounces. (c) Calculate the change in velocity of the ball.
Show the model answer
(a) +5.0 m/s (1) (b) −4.0 m/s (1) (c) −4.0 − (+5.0) = −9.0 m/s, i.e. 9.0 m/s to the left (1)
Compare velocities in a situation
2 marks6
Check whether the speed changes.
Check whether the direction changes.
If either changes, the velocity changes.
Example. A lorry travels at a steady speed of 20 m/s, first along a straight road and then round a bend. Compare the lorry's velocity on the straight road with its velocity on the bend.
Show the model answer
On the straight road the velocity is constant, because the speed and the direction are both constant (1). On the bend the direction changes, so the velocity changes even though the speed stays at 20 m/s (1).
Explain circular motion
3 marks7
State that velocity is a vector (speed in a given direction).
Say that the direction of motion changes continuously.
Conclude that the velocity changes, so the object is accelerating.
Example. The Moon moves around the Earth in an almost circular orbit at a constant speed. Explain why the Moon is accelerating.
Show the model answer
Velocity is a vector: it is speed in a given direction (1). The Moon's direction of motion is changing all the time (1) so its velocity is changing, and a change in velocity is an acceleration (1).
Shortcuts and memory tricks
Velocity = speed + direction.
Steering wheel test: turning the wheel changes your velocity, even if the speedometer reading stays the same.
Circle at constant speed = changing velocity = accelerating. This is a favourite Higher 'explain' question.
Where marks are lost
Saying velocity is 'the same as speed'.
Saying an object moving in a circle at constant speed is not accelerating.
Giving a velocity without a direction or a sign.
After a bounce, subtracting the speeds (5 − 4 = 1 m/s) instead of the velocities (−4 − 5 = −9 m/s).
Exam technique
Whenever a question says 'velocity', think about direction as well as speed.
For circular motion, use the chain: direction changes → velocity changes → accelerating (→ resultant force).
Use the word 'vector' when you explain what velocity is.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What is meant by the velocity of an object?
Its speed in a given direction.
A parcel on a conveyor belt moves 6.0 m to the right in 4.0 s. Calculate its average velocity.
1.5 m/s to the right m/s
A lift moves upwards at a constant 2.5 m/s. Later, the same lift moves downwards at a constant 2.5 m/s. Compare the lift’s speed in the two cases.
The speed is the same in both cases (2.5 m/s).
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) Which of these is a velocity? Tick (✓) one box.[1]
20 m/s
20 m/s due north
20 m
20 m due north
(b) What is meant by the velocity of an object?[1]
(c) Two trains pass each other on parallel tracks. Each train travels at 30 m/s, but in opposite directions. Explain why the trains have the same speed but different velocities.[2]
Show the answer and mark scheme
(a)Answer: 20 m/s due north
(b)Answer: Its speed in a given direction.
its speed in a given direction
(c)Answer: Speed has no direction so both are 30 m/s; velocity includes direction and the directions are opposite.
speed has no direction, so both trains have a speed of 30 m/s
velocity includes direction, and the trains move in opposite directions
Question 2Medium6 marks
A swimmer swims one length of a 50 m pool in 40 s. The swimmer then turns and swims back to the start in 45 s.
(a) Calculate the average velocity of the swimmer during the first length. Give the direction.[2]
(b) Calculate the average speed of the swimmer for the two lengths.[2]
(c) What is the average velocity of the swimmer for the two lengths? Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: 1.25 m/s away from the start m/s
50 ÷ 40 = 1.25 (m/s)
away from the start / towards the far end of the pool
(b)Answer: 1.2 m/s (1.18 m/s) m/s
100 ÷ 85
1.2 (m/s) / 1.18 (m/s)
(c)Answer: 0 m/s
zero
the swimmer finishes where they started, so the displacement is zero
Question 3Hard7 marks
A ball is rolled up a straight ramp. It slows down, momentarily comes to rest, and then rolls back down the ramp along the same line. The velocity up the ramp is taken as positive.
(a) Describe how the ball’s velocity changes as it travels up the ramp, and give the value of its velocity at the highest point.[2]
(b) Explain why the ball’s velocity is negative while it rolls back down the ramp.[1]
(c) The ball leaves the bottom of the ramp at 3.0 m/s up the ramp. It returns to the bottom 2.4 s later, moving at 3.0 m/s down the ramp. Calculate the size of the change in the ball’s velocity.[2]
(d) Calculate the average acceleration of the ball during the 2.4 s.[2]
Show the answer and mark scheme
(a)Answer: Its velocity decreases up the ramp, reaching zero at the highest point.
its velocity decreases (it decelerates) as it travels up the ramp
at the highest point its velocity is zero
(b)Answer: It is moving in the opposite direction to the direction taken as positive.
velocity is a vector, and the ball is now moving in the opposite direction to the (positive) upward direction
(c)Answer: 6.0 m/s (down the ramp) m/s
change in velocity = (−3.0) − (+3.0) = −6.0 (m/s)
size of the change = 6.0 (m/s) (the change is down the ramp)