GCSE · Physics · Edexcel · Spec 1PH0

Velocity is speed in a stated direction

A car circles a roundabout at a steady 10 m/s, and the speedometer never moves. Is anything about its motion changing? Physics says yes. Let's find out what.

Motion · sort it

Did the speed change? Did the velocity change?

Velocity is speed in a stated direction. So for each motion, ask two quick questions: did the speed change, and did the direction change? Then tick every set it belongs in — one, both, or neither.

A Its speed changes
B Its velocity changes

A car speeds up from 5 m/s to 15 m/s along a straight road
A car goes round a roundabout at a steady 10 m/s
A tennis ball hits a wall at 8 m/s and comes straight back at 8 m/s (compare just before and just after)
A train cruises along a straight track at a steady 30 m/s
A cyclist brakes from 8 m/s to a stop on a straight path
A bus slows from 12 m/s to 6 m/s as it turns a corner

Motion · a car on a roundabout

Same speed, four different velocities

View: Speedometer view. Showing 3 layers: Roundabout, Car positions, Speed readings

View

Explore

Switch views, then tap a point or the arrows.

Read the speed at P, Q, R and S. It's 10 m/s every time.

Motion · what do you think?

What does velocity actually mean?

Four learners each describe velocity. Which one is closest to what you think right now?

Velocity is just the science word for speed. They mean the same thing.If something moves at a constant speed, its velocity must be constant too.A negative velocity means the object is slowing down.Velocity is the direction something moves in, like north or left.Velocity is a speed together with a direction, like 5 m/s left, or −5 m/s.

How sure are you?

I'm sureI think soI'm guessing

Velocity is just the science word for speed. They mean the same thing.

This is a common misconception

They share a number and a unit, but velocity carries one extra piece of information: the direction. '12 m/s' is a speed. '12 m/s north' is a velocity. Two cars at 12 m/s, one going north and one going south, have the same speed but different velocities.

If something moves at a constant speed, its velocity must be constant too.

This is a common misconception

Only if it also keeps going in the same direction. The car on the roundabout never changed speed, yet its velocity changed all the way round, because it kept turning. Constant speed plus a turn means a changing velocity.

A negative velocity means the object is slowing down.

This is a common misconception

The minus sign is a direction, not a slowdown. Once you choose one direction as positive, say to the right, a velocity of −1.5 m/s just means 1.5 m/s to the left. A swimmer heading left at a steady 1.5 m/s has a velocity of −1.5 m/s the whole length of the pool, and isn't slowing at all.

Velocity is the direction something moves in, like north or left.

This is a common misconception

The direction is only half of it. 'North' on its own doesn't tell you how fast. A velocity needs both parts: a speed and a direction, like 12 m/s north. Picture the arrow: it needs a length as well as a way to point.

Velocity is a speed together with a direction, like 5 m/s left, or −5 m/s.

This matches the evidence

Yes. And the direction can be given in whatever way suits the situation: a compass direction (north, south-west), words like left, right, up or down, or a + or − sign once you've chosen which way counts as positive. All three are just ways of stating the direction.

A velocity always has two parts, a speed and a direction. A minus sign is one way of stating the direction; it says nothing about slowing down.

Motion · put it in writing

Say it like a physicist

A runner runs once round a circular track at a constant speed of 5 m/s. (a) State the difference between speed and velocity. (b) Explain why the runner's velocity is not constant. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

(a) Velocity has a direction; speed does not (velocity is speed in a stated direction). (1 mark)
(b) The runner's direction keeps changing as they go round the track. (1 mark)
(b) So the velocity changes, even though the speed stays at 5 m/s. (1 mark)

Model answer

(a) Speed only tells you how fast something is moving. Velocity tells you how fast and in which direction. (b) On a circular track the runner is always turning, so the direction of motion keeps changing. A change of direction is a change of velocity, even though the speed stays at 5 m/s.

In part (b), naming the changing direction earns one mark and linking it to the velocity earns the other. 'Velocity changes' on its own doesn't say why.

Exam line: Name the direction, and say what changed and what didn't: that's where the marks are.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Keep the speed, change the direction, and the velocity changes.

What you need to know

Velocity is speed in a stated direction.
A direction can be given as a compass direction, as left/right or up/down, or as a + or − sign once you've chosen which way is positive.
A velocity changes if the speed changes, if the direction changes, or if both change.
An object moving at a steady speed along a curved path has a changing velocity.

The big picture

Velocity is speed in a stated direction. That means a velocity changes if the speed changes, if the direction changes, or if both change. So an object going round a bend at a steady speed has a changing velocity, but an object can never change its speed without changing its velocity.

Key points

1Speed tells you how fast; velocity tells you how fast and which way.

2Same speed, different direction means a different velocity: 15 m/s north is not the same as 15 m/s south.

3A negative velocity means moving in the direction opposite to the one you chose as positive. It does not mean slowing down.

4The speed can never change without the velocity changing too.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

A lift moves up at a steady 2 m/s, waits at a floor, then moves down at a steady 2 m/s. Taking upwards as positive, give the lift's velocity in each of the three stages. Is its velocity the same on the way up and the way down?

⚠ Watch out

Saying that an object moving at a constant speed must have a constant velocity. That's only true if it keeps moving in the same direction. Any turn, even at a steady speed, changes the velocity.

🧠

Memory hook

Velocity = speed + direction. Turn the wheel and the velocity changes, even if the speedometer doesn't budge.

✓

Check yourself

A cyclist rides round a bend at a steady 6 m/s. Is the cyclist's speed changing? Is the cyclist's velocity changing? Give a reason for each answer.

Flashcards

(7)

What is velocity?

Speed in a stated direction.

What does a velocity tell you that a speed doesn't?

The direction of motion.

Give three ways of stating the direction of a velocity.

A compass direction (e.g. north); words such as left, right, up or down; or a + or − sign once one direction has been chosen as positive.

Taking right as positive, what does a velocity of −4 m/s mean?

Moving to the left at 4 m/s. The minus sign gives the direction; it doesn't mean slowing down.

Can an object's velocity change while its speed stays constant?

Yes, if its direction changes, for example going round a roundabout at a steady speed.

Can an object's speed change while its velocity stays the same?

No. Velocity includes the speed, so any change of speed is also a change of velocity.

If a velocity is drawn as an arrow, what do its length and its direction show?

The length shows the speed; the way it points shows the direction of motion.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Did the speed change? Did the velocity change?

What does velocity actually mean?

Say it like a physicist

More Edexcel GCSE Physics topics