The centrifugal effect
Why spinning things fling water outward — and how that becomes a pump.
⏱ 6 min
- Everyday comparisonStep 1 of 6
The salad spinner
You wash lettuce and spin it in a salad spinner. The water ends up on the outer wall, far away from the leaves.
Nobody pulled the water outward. So what happened?
- Try itStep 2 of 6
Spin the salad
Turn up the speed. At first the water drops ride along with the basket. Spin faster and they break free and fly off.
Look closely at how they leave: not straight out from the centre, but in a straight line along the edge — like a stone from a sling.
Challenge: Find the slowest speed where the drops start flying off. Then go full speed.
- ExplainedStep 3 of 6
Things want to go straight
Every moving thing wants to keep going in a straight line. This is — the reason you slide sideways in your seat when a car turns.
The spinning basket keeps forcing the drops to turn. When the drops can’t hold on any more, they simply continue straight, along the . From the centre, it looks like they were thrown outward.
- ExplainedStep 4 of 6
“Fleeing the centre”
This outward fling is called the effect — Latin for “fleeing the centre”.
The faster you spin, the stronger it gets. Twice the RPM gives four times the outward push. That is why the drops suddenly fly off once you pass a certain speed.
- The impeller connectionStep 5 of 6
Now make it a pump
Put curved blades () on the spinning disc and let water enter in the middle. The vanes grab the water, spin it, and fling it outward — faster and faster as it moves toward the edge.
Congratulations: you just invented the . This one is running in slow motion so you can follow the water.
- The impeller connectionStep 6 of 6
Your first look at a real one
Here is a real-looking impeller in 3D. Drag to turn it around. Notice the hole in the middle (the ) where water comes in, and the curved vanes that fling it out.
In the next module you will learn every part of it by name.
📌 Remember this
Spinning things fling fluid outward (the centrifugal effect), because the fluid wants to keep going straight. An impeller uses this to throw water out fast — that is how most pumps work.