From everyday motion to exam questions, learn how to solve speed, distance and time problems with clear explanations and worked examples.
Speed tells you how much distance an object covers in a given amount of time. When a rickshaw travels from one end of the city to the other, or when a cricket ball flies off the bat toward the boundary, speed is the quantity that describes how fast that motion is happening.
In physics, speed is a scalar quantity — it has magnitude (a number and unit) but no direction. When you add direction to speed, it becomes velocity. For most Class 6 to O Level numericals, the two terms are used interchangeably, but knowing the difference earns you marks in theory questions.
Speed is measured in metres per second (m/s) in SI units, but kilometres per hour (km/h) is equally common in everyday problems. You will need to move confidently between both — and this guide shows you exactly how.
The relationship between speed, distance, and time is one of the most important in all of physics. Everything flows from this single equation:
Speed = Distance ÷ Time
Written as symbols: v = d ÷ t, where v is speed, d is distance, and t is time.
From this one formula, you can rearrange to find any of the three quantities if you know the other two:
A useful trick many students learn is the DST triangle. Draw a triangle with D at the top, and S and T at the bottom left and right. Cover the quantity you want to find — what remains shows you the operation. Cover D and you see S × T. Cover S and you see D ÷ T. Cover T and you see D ÷ S. It sounds simple, but it stops you from mixing up the formula under exam pressure.
The single most common reason students lose marks in speed, distance and time questions is a unit mismatch. If speed is in km/h but time is given in minutes, the calculation will give you a wrong answer — even if your method is perfectly correct.
Always check that your units are consistent before substituting into the formula. Here are the two unit systems you will encounter:
These two conversions come up constantly in O Level and Cambridge-style papers, so memorise them both:
Why 3.6? Because 1 km = 1000 m and 1 hour = 3600 seconds. So 1 km/h = 1000 ÷ 3600 m/s = 1 ÷ 3.6 m/s. Once you understand the reason, you will never forget the rule.
Another conversion that trips students up is time given in minutes when speed is in km/h. Always convert minutes to hours by dividing by 60 before substituting.
Each example below follows the same method a good student uses in an exam: state the known values, identify the unknown, write the formula, substitute, and calculate. Follow this pattern every time.
A car travels 36 km in 1.5 hours. What is its speed?
Known: d = 36 km, t = 1.5 h
Unknown: v = ?
Formula: v = d ÷ t
Substituting: v = 36 ÷ 1.5
Answer: v = 24 km/h
A bus travels at 90 km/h for 2.5 hours. How far does it travel?
Known: v = 90 km/h, t = 2.5 h
Unknown: d = ?
Formula: d = v × t
Substituting: d = 90 × 2.5
Answer: d = 225 km
A train covers 240 km at a speed of 80 km/h. How long does the journey take?
Known: d = 240 km, v = 80 km/h
Unknown: t = ?
Formula: t = d ÷ v
Substituting: t = 240 ÷ 80
Answer: t = 3 hours
A cyclist travels 15 km in 20 minutes. What is their speed in km/h?
Step 1 — Convert time: 20 minutes = 20 ÷ 60 = 1/3 hour
Known: d = 15 km, t = 1/3 h
Formula: v = d ÷ t
Substituting: v = 15 ÷ (1/3) = 15 × 3
Answer: v = 45 km/h
This example is a favourite in exams precisely because students forget to convert minutes. Write the conversion as its own step — it takes five seconds and protects your marks.
An athlete runs 48 metres in 6 seconds. What is their speed in m/s and in km/h?
Known: d = 48 m, t = 6 s
Formula: v = d ÷ t = 48 ÷ 6
Speed in m/s: v = 8 m/s
Converting to km/h: 8 × 3.6 = 28.8 km/h
Average speed is not the average of two speeds. This is one of the most common mistakes in O Level physics, and it costs marks every year.
The correct formula is: Average Speed = Total Distance ÷ Total Time
Here is why simply averaging two speeds fails. Imagine a car that travels 120 km at 60 km/h on the way there, and then travels 120 km at 40 km/h on the way back. The total distance is 240 km. But the total time is not 2 + 2 = 4 hours. Let us calculate properly:
Average speed = 240 ÷ 5 = 48 km/h
If you had incorrectly averaged the two speeds, you would have got (60 + 40) ÷ 2 = 50 km/h — which is wrong. Always use total distance divided by total time.
Why average speed is never the average of speeds: The car spends more time travelling at the slower speed (3 hours at 40 km/h) than at the faster speed (2 hours at 60 km/h). The slower speed therefore pulls the average down. The total distance and total time capture this automatically — averaging the speeds does not.
Understanding this formula is not just about passing exams. Engineers use it to design road speed limits and calculate safe stopping distances. Pilots use it to plan fuel consumption. Athletes and coaches use it to analyse performance. Even Google Maps uses speed and distance to estimate your arrival time. The formula you are learning is the same one professionals use every single day.
Students who consistently score full marks on speed, distance and time questions follow a clear routine. Here is exactly what they do:
O Level marking tip: Cambridge marking schemes typically award one mark for the correct formula, one mark for correct substitution, and one mark for the correct answer with units. Writing your working clearly means you can earn two out of three marks even if your final calculation is wrong.