Edexcel IGCSE Physics · Spec 1.2E
Speed: Using the Equation (Worked Examples)
Worked examples calculating speed, distance and time.
Physics revision video
Speed: Using the Equation (Worked Examples)
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Explained
Speed calculations, and where the marks are hidden
Speed is distance travelled divided by time taken, written v equals s over t. It is the first equation in the course and it keeps appearing, so the habits you build here are worth more than the equation itself.
The three forms
Speed equals distance over time. Distance equals speed multiplied by time. Time equals distance divided by speed.
Rather than learning three versions, put the numbers you have into the first one and solve. A car covering 150 m in 6 s gives v equals 150 over 6, which is 25 m/s. If instead you know the speed and the time, write 25 equals s over 6 and solve for s.
Units first
Metres and seconds give metres per second. Kilometres and hours give kilometres per hour. Mixing them gives nothing useful.
To convert from kilometres per hour to metres per second, multiply by 1000 and divide by 3600, which is the same as dividing by 3.6. So 72 km/h is 20 m/s.
Average speed is not instantaneous speed
Average speed uses the total distance and the total time for the whole journey, including any time spent stopped. A journey of 100 km in 2 hours has an average speed of 50 km/h even if the car never actually travelled at 50.
Instantaneous speed is the speed at one moment, which is what a speedometer shows. On a distance time graph, average speed comes from the overall gradient between two points, while instantaneous speed comes from the gradient at a single point.
What the mark scheme accepts and rejects
Edexcel International GCSE Physics mark schemes usually split a calculation into separate marks for the substitution or rearrangement and for the evaluation. That means writing the equation with the numbers in it earns credit even if the arithmetic that follows goes wrong, and writing nothing but a wrong final answer earns none.
The notes column shows how the rest is judged. A power of ten error costs one mark rather than all of them. Where a question demands two significant figures, the mark scheme states that any final value expressed to two significant figures gets that mark, and it lists the acceptable range around the exact answer. Elsewhere it rejects a value that has been rounded incorrectly.
An examiner report on the same paper notes that many candidates completed the physics correctly but failed to give their answer to the required two significant figures, having read the question too quickly.
The summary advice in these reports is to show all working, so that some credit can still be given for answers that are only partly correct.
A routine that protects the marks
Write the equation. Convert the quantities to matching units. Substitute the numbers in before rearranging. Evaluate. Write the unit on the answer. Then reread the question for an instruction about significant figures or standard form.
Six steps, none of them difficult, and between them they cover almost every way this calculation is marked.
Spec 1.2E
What you need to know
- Recall the speed equation
- Rearrange it to find distance or time
- Use it to solve problems
Active recall
Quick check
Answer each question before opening the answer.
What is the equation linking speed, distance and time?
Speed = distance ÷ time (v = s ÷ t).
A runner covers 150 m in 10 s. What is their average speed?
15 m/s (150 ÷ 10).
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