Rotation and RPM
How fast is a spinning wheel really moving? Why the edge always wins the race.
⏱ 5 min
- Everyday comparisonStep 1 of 5
The merry-go-round
Three kids sit on a merry-go-round: one near the middle, one halfway out and one on the very edge.
They all go around once in the same time. But the kid on the edge travels a much bigger circle in that time — so she is moving much faster.
- Try itStep 2 of 5
Spin it
Use the slider to set how many turns per minute the merry-go-round makes. This is called — revolutions per minute.
Each kid shows their real speed. Notice how the kid on the edge always wins.
Challenge: Double the RPM. What happens to every kid’s speed? Now compare the middle kid with the edge kid — how many times faster is the edge?
- ExplainedStep 3 of 5
Speed at the edge = tip speed
The speed of the outer edge is called the . It grows with two things: how fast it spins (RPM) and how far the edge is from the centre (the ).
Double the RPM → double the tip speed. Double the size → double the tip speed.
- ExplainedStep 4 of 5
An impeller is a very fast merry-go-round
A typical pump motor in Europe spins at about 2900 RPM — almost 50 turns every second!
An impeller only 20 cm across, spinning at 2900 RPM, has a tip speed of about 30 m/s. That is around 110 km/h — motorway speed, on a wheel smaller than a dinner plate.
- The impeller connectionStep 5 of 5
Why size and speed matter
The faster the tip of an impeller moves, the more energy it can give to the water. That is why bigger impellers and faster motors make pumps that push harder.
Remember: tip speed = the pump’s muscle.
📌 Remember this
RPM is how many turns per minute. On a spinning wheel, the edge moves fastest — its speed (tip speed) grows with both RPM and size.