How does a boat or ship carrying hundreds of pounds worth
of stuff float while that same stuff would sink to the bottom of the ocean if dumped
overboard? How come when you're in a pool and you stretch your body out flat you
float. But, if you wrap your arms around your legs and curl up into a ball you sink?
Well, it all has to do with how much water is pushing against you and a little scientific
principle called buoyancy or floatation. When you stretch out flat more
water pushes against you since your body is laid out flatter. When you curl up into
a ball, less water is pushing against you. Want to test this for yourself? Try this
experiment:
- Take a piece of clay and split it into 2 identically sized pieces. Take one of the
pieces and roll it into a ball. Take the other piece and fashion it into a flat
boat shaped object (if needed, get mom or dad to help - that's what they're there
for).
- Now place both pieces into a sink full of water. Which one floats and which one
sinks? Both? Neither?
So you see, if the total area of the object that makes contact with the water is
large enough, the object floats. The object must make room for its own volume by
pushing aside, or displacing, an equivalent (or equal) volume of liquid.
The object is exerting a downward force on the water and the water is therefore
exerting a upward force on the object. Of course the floating object's weight comes
into play also. The solid body floats when it has displaced just enough water to
equal its own original weight.
This principle is called buoyancy. Buoyancy is the loss in weight an object
seems to undergo when placed in a liquid, as compared to its weight in air. Archimedes'
principle states that an object fully or partly immersed in a liquid is
buoyed upward by a force equal to the weight of the liquid displaced by that object.
From this principle, he concluded that a floating object displaces an amount of
liquid equal to its own weight. (Note: if you don't understand these last two paragraphs,
don't worry. They're not on the test).