MadSci Network: Physics

Re: Why do boats float and silverware sink?

Date: Wed Feb 4 11:45:33 1998
Posted By: John Haberman, Space Scientist, NASA Goddard Space Center, Greenbelt MD
Area of science: Physics
ID: 885784236.Ph


   Objects can float for several reasons.

   If an object or a material has a lower density than the liquid in which 
it is placed it will float.  When we consider the liquid to be water: oil 
and ice cubes float, most types of wood will float, the rock type pumice 
will float, and some types of plastic will float.  The wood, rock, and 
plastic that float all are less dense than the water because they have a 
lot of air trapped within their molecular structure (between the elements 
from which they are made) and this causes them to be less dense than water.

   We also can see that objects built from materials that have densities 
greater than that of water can float.  Ships of all types are commonly 
made from metal, wood, concrete, plastic, fiberglass and such.  When a 
craft is designed so that when "it is immersed in a fluid it is bouyed up 
by a force equal to the weight of the displaced fluid" it will float.  
This is called the "Archimedes' Principle".  Ships are designed so that 
large surface areas are in contact with the water.  This creates upward 
forces that do not allow the ship to sink even though it is made from 
materials that are more dense than water.  Archimedes' Principle also 
explains why helium filled balloons float in the air.

   Water also has a strong surface tension property.  In simpler terms, 
the water molecules on the surface stick together quite strongly and form 
a thin, membrane like, layer protecting what's underneath (think of this 
like the plastic wrap used to protect your sandwich.)  This surface 
tension is why small insects can be observed walking on water.  You can, 
to demonstrate, float a metal sewing needle in a bowl of calm water by 
very gently placing it on the surface.  However if you simply drop the 
needle into the bowl or if you disturb the bowl the needle will sink.  You 
can also cause the needle to sink by placing a drop of soap (dish 
detergent) in the bowl because the soap destroys the water's surface 
tension property.

John Haberman, Physical Chemist

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