HOW COME? Inertia keeps water in its proper place

How come it's so hard to carry a pail full of water without spilling it, but you can swing a pail of water over your own head and not get wet? asks a reader.

It's one of those experiments best conducted outdoors, preferably on a hot, sunny day . . . while wearing a bathing suit. And far, far away from the living-room window. Oh, and better make that a plastic bucket. What makes the frantic spinning work is something we associate with sitting sluglike on the couch: inertia.

What's inertia, and what does it have to do with getting soaking wet? More than 300 years ago, scientist Isaac Newton described it this way: "Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by force impressed thereon." And the effects of inertia can indeed be unpleasant. Say, when you're standing on a bus going 15 mph, and the bus suddenly screeches to a halt, and you're not holding on. The bus may have stopped, but your body hasn't gotten the instant message. You continue moving - right onto the floor.

With the swinging bucket, it's the water's inertia you've got to worry about (or thank). Imagine tying a short rope to a bucket half full of water and then swinging the bucket in a big, vertical circle around your body. You are providing the force Newton was talking about, "compelling" the water to take a circular path.

(The sun's enormous gravity acts the same way on the planets of our solar system, compelling them to swing around in rough circles (orbits) rather than fly off on a tangent into interstellar space.)

The force you're exerting on the bucket is a toward-the-center, or "centripetal," force. Meanwhile, there is another force at work - the gravitational force, which is tugging the pail and its contents down, toward the Earth. And then there is the water's (and the bucket's) own inertia. This makes the object swinging around your head want to fly off on a straight-line tangent. But the bucket is restrained by your grip on the rope or the handle. And the water is stopped by the bottom of the pail.

So why doesn't the water splash you from one side, or fall down on your head at the top of each swing? Generate enough inward-pulling force, bending the bucket's path into a circle with enough energy, and you're safe. Swing the bucket fast (and smoothly), and the water, trying to fly off on its own straight path, presses against the bottom of the pail. As the bucket swings quickly through the top of the circle, the water's motion "up and out" more than balances the force of gravity "down." So none pours out, and you stay dry.

But let go of the bucket at the top of the spin, and it will fly across the yard, spilling water as it goes. Or play "Spin the Bucket" slowly and feebly, and you'll just get drenched.Don't want to risk it? Watch a video of a spinning bucket at www.youtube.com/watch?v=9ljFT_w_qaM.