MadSci Network: Physics

Re: Difference between wood and metal

Area: Physics
Posted By: John Balbach, Grad student Physics, Washington University
Date: Fri Oct 4 14:16:41 1996
Message ID: 843757303.Ph

Imagine a bucket of water.  Pick an amount of water and put an 
imaginary box around it.  The water inside that box is held up by the 
water under the box.  Another way to say that is that the water underneath
the box pushes up on the water above it.  It turns out that if you replace
the water in the box with anything else (air, wood, metal) the water under
the box will push the same as if the box were filled with water.
          This means that the upward push that an object feels when it is
put in water depends on its volume, not its mass.  Now, imagine a block of
wood and a block of metal that are the same size.  The block of metal will
weigh much more than the block of wood.  The water can push up with enough
force to hold up the wood, but not enough force to hold up the metal.
          The relationship between mass (weight) and volume (size) has a 
name; it is called density.  The metal has a higher density than wood, so
a block of metal will weigh more than a same size block of wood.  Also, a
weight of metal will be smaller than an equal weight of wood.  Now that we
know about density, we can say that anything less dense than water will 
float, and anything more dense than water will sink.

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