A few years ago I was idly musing some basic physics when this occured to me. I was quite thrilled to independently hit upon a cute clever physics puzzle :) So here goes it:
What's heavier: a kg of lead or a kg of cotton?
Trick question, huh? No, actually. Just a clever one.
Now, some implicit assumptions have to be disclosed. We are talking about the weight of, say a lead ball vs. the weigth of say, a bundle of cotton, on the surface of the earth.
The answer is that the cotton is heavier!
The reason is the dramatically different buoyancy of these two objects in air. In simple terms, air pushes "up" the lighter cotton more than the heavier lead ball. (Because both are fully immersed in the Earth's atmosphere, they each displace an amount of air equal to their respective volumes. The buoyant force is equal to the weight of the displaced air. So, cotton experiences more buoyant force.) When we weigh the cotton bundle to be a kilogram, we need to adjust for the buoyant force, which makes it seem lighter than it actually is. This adjustment applies to lead also, but to much smaller extent. So, the cotton bundle will turn out to be a wee bit more than 1kg in actual terms (when weighed in vaccum) than the lead ball.
Monday, April 25, 2005
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