1Two things that are always true
Gravity pulls down. Circles pull inward.
You only need these two ideas. Watch each one wiggle:
Gravity never quits
Gravity pulls everything straight down — always. It does not turn off when the bucket flips upside down. At the top of the loop, it's still tugging the water toward the ground.
Circles need an inward yank
To curve in a circle, something must yank you toward the center the whole time. Let go and you shoot off in a straight line. The faster you spin, the harder that inward yank has to be.
2Two ways to swing
The lazy loop vs the speedy loop
Swing slow and gentle
To keep curving slowly, the water only needs a tiny inward pull — smaller than gravity. But gravity is still pulling down full strength, so there's more downward pull than the circle needs. Hold that thought — what do you think that extra pull does to the water at the top?
Swing fast and hard
Now keeping the water curving needs a big inward pull — at least as big as gravity. Swing just barely fast enough and gravity alone is exactly that pull; swing faster and the bucket pushes inward too. We'll see in a second what that does to the water.
3Your turn — be the arm
Swing the bucket and watch the two pulls
Drag the slider to swing the bucket faster or slower. Watch the two force bars at the top of the loop: amber is how hard the water needs to be yanked inward to keep curving, slate is gravity pulling down. When amber reaches slate, gravity is exactly the inward pull the circle needs. (Don't worry about the water yet — that comes next.)
At the top of the loop
4Now try to make it spill
Find the speed where it flips
There's a speed where the water goes from "stays in" to "pours out." Before you go hunting for it — make a call.
Guess before you find out
To keep the water from spilling at the very top of the loop, do you need to swing the bucket faster or slower?