April 7, 2013

Drop anchor and lower the lake

 I HAVE A BRAIN TEASER for you today. It comes courtesy of the British Broadcasting Corporation, but it has nothing to do with broadcasting. It concerns boating:

Imagine a boat floating in a lake, and a heavy object such as an anchor (but without a chain) is dropped off the side. What happens to the water level of the lake?

Does it: a) Rise; b) Fall; c) Stay the same?

Answer b) is correct — the water level falls!

The explanation
When it's in the boat, the anchor displaces a volume of water with the same weight as the anchor itself.

When it's submerged in the water, the anchor displaces an amount of water equivalent to its own volume.

Because the anchor — made of steel, say — is heavier than the equivalent volume of water, it displaces more water when it's in the boat. Submerged, it only displaces its own volume, and all else being equal, the water level falls.

Test it at home
You can easily test this yourself in the kitchen sink, using a sandwich box as the boat, and something like a paperweight as the anchor.

Put an inch or two of water into the sink (enough to easily cover the paperweight later). Then put the paperweight into the sandwich box, and float the box on the water.

Once the water has settled, mark the water level in the sink with a felt-tip pen. Now take the weight out of the box, and submerge it in the water.

When it's settled again, you'll see that the water level has indeed fallen (the bigger the paperweight, or the less water there is, the more the water level will fall).

Today’s Thought
Science is wonderfully equipped to answer the question “How?” but it gets terribly confused when you ask the question “Why?”
— Erwin Chargaff, professor of Biological Chemistry, Columbia University

Contrary to popular belief, sex is not hereditary. If your parents never had it, you won’t have it either.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)   

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