When you swing a bucket of water up over your head, what does the water do?
Look… a bucket full of water, on a rope. Now you swing it up, up, up, over your head! Way up there, the bucket flips all the way upside down. And the water? It's right above your head! So here's the puzzle… when you swing it round and round really fast, will the water fall out and splash you… or will it stay inside the bucket? Tap what you think… and then watch what the water does!
After you watchWhen you swing a bucket of water up over your head, what does the water do?
The short answer
When you swing a bucket of water round and round fast over your head, the water stays inside the bucket — even when the bucket is upside down at the very top. It does not splash on you, as long as you keep swinging fast.
Try this next
- What if the bucket had only a tiny splash of water in it? Guess first — does a barely-full bucket need a faster or slower swing to stay dry? Then swing a nearly-empty cup and a full one and watch which one splashes first when you slow down.
- What if you swing it flat, like a merry-go-round, instead of over your head? Predict whether the water still tries to spill when the bucket never goes upside down, then swing it round flat and low and watch what the water does.
The whole story
How it works
Swinging the bucket round in a fast circle keeps pressing the bucket's bottom against the water the whole way around. At the top, the bucket is rushing along so quickly that it scoops under the water before the water has any time to fall. So the water keeps getting carried around the loop instead of dropping out. Slow down too much and the bucket no longer rushes under the water in time, and then it spills.
What people get wrong
Most little kids are sure that an upside-down bucket always dumps its water out — because a still bucket does! The surprise is that a fast-swinging bucket does not. Speed is the secret: keep it whooshing round and the water rides along and stays in.
The catch
Swinging fast keeps the water safely inside, but it takes a strong, steady arm and you can't stop or slow down at the top. Swinging slowly is easy and gentle, but if you go too slow the water drops out and you get a cold splash on your head. So fast-and-round is dry; slow-and-lazy gets you wet.
Questions kids ask
Why doesn't the water fall out when the bucket is upside down?
Because you are swinging it so fast that the bucket rushes around the loop before the water has time to fall. The bottom of the bucket keeps scooping under the water, so the water gets carried around instead of dropping out.
What happens if you swing it too slowly?
Then the bucket no longer rushes under the water in time at the top, and the water pours out — usually right onto your head. Swinging fast is what keeps you dry.
Do you need strong muscles to keep the water in?
You need a steady arm and you have to keep swinging round and round. It is the fast swinging, not big muscles, that holds the water inside the bucket.
Talk about it
- Before we swing it — what do you think the water will do when the bucket is upside down at the top?
- Why do you think a still bucket spills but a fast-swinging one doesn't?
- What do you predict happens if we swing slower and slower — and exactly when?
For grown-ups
Anything moving in a circle needs a constant inward (centripetal) pull toward the center. At the top of a vertical loop, gravity points straight down toward the center, so gravity itself can do that inward job. As long as the bucket is swinging fast enough that the inward pull the circle needs is at least as big as gravity, gravity gets entirely used up bending the water around the loop and has none left over to pull it out — so the water stays in. Swing too slowly and gravity is more than the circle needs, the extra drags the water off its path, and it spills. There is no outward force gluing the water in.
Keep going
What else makes you wonder?
- What if you stopped swinging right at the very top — what would the water do then?
- A roller coaster goes upside down in a loop and you don't fall out either — is that the same swinging trick?
- What else can you swing round and round really fast without it flying out?