KS3 · Physics
Calculating speed
Who's faster: someone who runs 100 m, or someone who runs 400 m? You can't tell yet, and working out why is the whole trick of speed.
Physics · Speed
A cyclist rides at a steady 4 m/s. Drag the clock along the bar. Watch the distance: it grows by the same 4 metres every second, and distance ÷ time never changes.
One relationship, three ways round
Tap the quantity you want to find. Here d is distance, s is speed and t is time.
Tap the quantity you want to find. The triangle shows you the formula.
Cover distance, speed or time to reveal its rearranged formula, then plug in numbers to solve.
Watch it done
Problem
A skateboarder rolls 90 m in 15 s. Calculate the skateboarder's speed.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Speed isn't just a feeling of 'fast'. It's a number you can count: how many metres you cover in every single second.
What you need to know
- Speed is a measure of how fast something travels from one place to another.
- speed = distance ÷ time, so you need both the distance and the time.
- m/s means metres per second: the number of metres travelled each second.
- The same relationship rearranges: distance = speed × time, and time = distance ÷ speed.
The big picture
Speed tells you how much distance something covers in each unit of time. Work it out with speed = distance ÷ time, rearrange it to find a distance or a time, and make sure the units match: metres and seconds give a speed in m/s.
Key points
Worked example
Problem
A coach travels 180 km in 3 hours. Calculate its average speed in km/h.
⚠ Watch out
Judging speed from the distance alone, or the time alone. The one who goes furthest isn't always fastest: you need distance ÷ time. And when the answer must be in m/s, change kilometres to metres and minutes or hours to seconds before you divide.
Memory hook
Per means each. m/s is 'metres each second': count the metres, count the seconds, divide.
Check yourself
A scooter covers 50 m in 10 s at a steady speed. What is its speed, and how far would it go in 30 s at that same speed?
Flashcards
(11)What does speed measure?
What is the equation for speed?
What does m/s mean?
In a unit like km/h, what does 'per' mean?
Rearrange speed = distance ÷ time to find the distance, and then the time.
What happens to the distance each second when an object moves at a steady speed?
Two objects travel the same distance. Which one is faster?
What are the three steps of a speed calculation?
Name three units of speed other than m/s.
How do you know the unit of a speed you have calculated?
What must you do before calculating a speed in m/s from km and minutes?
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.
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