Question:

Why doesn’t the water fall out of the pail when the pail is upside down?

Answer:

Because of the water’s inertia.

Realize that the water is moving all the time. At the peak of the circular motion, the water has a horizontal velocity. The water’s weight W would have caused it to continue along a parabolic arc. The pail, however, intends to follow a circular path. Relative to the circular path of the bottle, the parabolic path of the water requires the water to pass through the bottom of the pail.

The pail has no choice but to exert a downward normal contact force N on the water, just enough to force the water into following the same circular path as the pail.

So when the pail is upside down, the water experiences not one but two downward forces: weight W and normal contact force N. Together, they provide the centripetal force that makes the water do the circular motion.

In fact, the faster the pail is spun, the harder the pail must push the water to keep the water in circular motion (instead of bursting through its bottom). So as long as the pail is spun fast enough, the water will not fall out.

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