Why do things float in water?


You can conduct an at-home buoyancy experiment. Why does the ball of foil filled with pennies sink, while the same size piece of foil formed into a raft, filled with pennies, float? (Ann Cameron Siegal/For The Washington Post)
August 12, 2013

Yay, summer! Yay, water! Yay, playing in water during the summer!

While you’re having fun splashing in a pool, lake or at the shore, think about, “Why do you float, but a rock that could fit in your hand sinks?” Or: “Why does a boat float while its anchor sinks?”

Obviously, you weigh more than the rock and a boat weighs more than its anchor. So what makes the difference?

It’s not about weight, is it?

Making room in the water

Did you ever notice how water in a bathtub rises as you lower yourself in?

You displace water (move it out of the way) to make room for your body.

So, part of the story is that you displace more water than a rock. A boat displaces more water than an anchor.

Then what?

Gravity tries to pull things down into the water, but fluids have a force that pushes upward, against gravity. It’s called buoyancy (BOY-an-cy). This is what helps you or the boat float.

A Greek mathematician named Archimedes (Ark-ih-MEED-eeze) discovered the relationship between water’s displacement and its buoyancy more than 2,000 years ago, while taking a bath.

Seeing is believing

Let’s do a simple experiment to see how buoyancy works. You’ll need some aluminum foil, 10 pennies and a sink or large bowl half-filled with water.

1. Cut the foil into two pieces, each six inches square. Put five pennies in the middle of one piece of foil. Then crumple the foil around the pennies very tightly, squeezing out all the air and making a ball.

2. Take the other piece of foil and bend the edges up tightly, forming a boat or raft shape with a flat bottom. Put five pennies in your boat.

3. Gently place both foil-penny combinations in the water.

What happens? Why?

That’s the rest of the story. Objects react differently in water because of their density.

Your foil ball, crumpled into a tight clump, has more density because its pennies are crowded into a smaller space than those in the foil boat. The foil boat has less density because it is spread out and filled with air.

Things float when they have less density than water, but sink when they have more.

Can I keep my foil ball from sinking?

Yes. Gently put the foil ball on top of your foil boat. What happens? It’s the same thing that happens when an anchor is placed in a boat. Because of buoyancy, boats can carry objects that won’t float on their own.

And, a hand-size rock won’t sink if it’s resting on your tummy while you are floating!

Puzzles to try at home

●Which floats: a marble or a tennis ball? An orange with the peel on or an orange with the peel off?

●Will an egg float better in plain water or saltwater?

●We know rocks sink. What happens if you put the rock in a paper cup and then put the cup in water?

●Make several shapes of foil boats to see which shape holds the most pennies

— Ann Cameron Siegal

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