IGCSE Physics / Movement and position
Average speed, distance and time revision notes
Edexcel International GCSE Physics (4PH1), chapter 1: Movement and position, specification point 1.4, 1.1. Free lesson.
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Average speed links the distance travelled with the time taken. Use metres and seconds in the equation, then rearrange it to find a distance or time when needed.
What average speed means
Average speed is the distance moved divided by the time taken. It is the steady speed that would cover the same distance in the same time, even if the real speed changes during the journey. The equation is provided on the equation list. Average speed = distance moved ÷ time taken Average speed is measured in metres per second (m/s), distance in metres (m) and time in seconds (s).
Convert units before using the equation
The values must be in metres and seconds before you substitute them. Convert kilometres to metres by multiplying by 1000, and minutes to seconds by multiplying by 60. For hours, multiply by 3600 to get seconds. To convert km/h into m/s, multiply by 1000 to change kilometres to metres, then divide by 3600 to change hours to seconds. For example, 90 km/h = 90 000 m/h, then 90 000 ÷ 3600 = 25 m/s. This is also 90 ÷ 3.6. The number in m/s is smaller than the number in km/h.
Calculate average speed
Convert the distance and time first, then write the equation and substitute the values. Include the unit with your answer. Dividing 3.6 by 12 gives kilometres per minute, not metres per second, so it does not answer a question asking for m/s.
Rearrange to find time or distance
When the unknown is in the equation, substitute the known values before rearranging. This shows that you have used the correct equation and can earn a substitution mark. To find distance, multiply speed by time. To find time, divide distance by speed.
Velocity and the key method
Velocity is speed in a stated direction. For average-speed questions, keep the order clear: convert first, substitute, then rearrange if required. Average speed = distance moved ÷ time taken. Convert kilometres to metres and minutes or hours to seconds before substituting. Substitute first, then rearrange.