The speed of a moving object is rarely constant. When people walk, run or…

The idea

Real journeys are not steady. A walker slows at a kerb and speeds up again, and a car accelerates away from lights, brakes for a junction and waits in traffic, so a graph of its motion is a curve rather than a straight line. This is why average speed is the quantity normally quoted: it summarises a varying journey in one number, found by dividing the whole distance by the whole time, and it says nothing about the speed at any particular moment. A car averaging 12 m/s across a town journey may have touched 20 m/s and also stood completely still. When a question asks you to calculate the average speed it is inviting the totals; when it says at this instant it wants a tangent or a single reading.

What earns the marks

• Recognising that the speed varies through a journey.
• Average speed as total distance divided by total time.
• Stating that the average does not give the speed at any particular moment.
• Linking a changing speed to a curved distance-time graph.

Common mistakes and how to put them right

• Assuming the speed at the start of a journey applies throughout.

• Calculating an average from the highest and lowest speeds rather than from totals.

• Leaving stationary time out of the total time.

• Describing the distance-time graph of real motion as a straight line.

Exam tips

• Include time spent stopped in the total time when averaging; it is the usual trap.
• Read the command carefully: average speed needs totals, instantaneous speed needs a tangent.