Which is easier to lift? A small stone or an
entire ship? A small stone won't float (unless you cheat and use
pumice) even if it isn't very heavy, yet a huge oil tanker that is too
heavy to pick up floats easily.
Surely the heaviest one should
Take a piece of aluminium foil
and split it into 2 identically sized pieces (with approximately the
Take one of the pieces and roll
it into a tight, very compact, ball. Take the other piece and fold it
into a flat shallow boat shape.
PREDICT: Ask your
students "What will happen when I place these two into this sink (or
tub) of water?"
OBSERVE: Watch what
happens, remembering they have the same weight, when carefully placed
onto the surface of the water.
EXPLAIN: What does this
show us? (seniors might like to consider the area of the ball and boat
in contact with the water)
Which one floats and which one sinks? Can
you explain what you are looking at? It all works on the idea of 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 was a man who lived centuries
ago who figured out that an object fully or partly immersed in a liquid
is buoyed upward by a force equal to the weight of the liquid pushed
aside (displaced) by that object. A floating object pushes aside
(displaces) an amount of liquid equal to its own weight.
The upshot of all this is that the foil
boat pushes aside more water, as a larger area is in contact with the
water compared to the foil ball. Even a ship with a hull made of
concrete will float if the area of the object that makes contact with
the water is large enough.
The ship is exerting a downward
force on the water and the buoyancy of the water is exerting a upward
force on the object. As long as the forces remain equal the ship will
float. Put too much cargo on board and the downward force is greater...
the ship sinks!