There is maybe a little physics needed
here...
When an object is floating be it because it
is something less dense than water, say polystyrene, or because it is in a
boat, say a brick and the whole boat is less dense than water, then it
displaces in the water it's own mass. For example a 10 Kg lump of lead on a
boat will force the boat to sink by a volume equivalent to 10Kg of water.
Hence displacing 10Kg of water. Or indeed forcing the water level to raise by
an amount equivalent to 10Kg of water. (Like when you get in the bath.)
When an object sinks in water it
necessarily displaces it's own volume.
So when the brick is in the boat it is
displacing it's own mass equivalence in water. When the brick is thrown over
the side it is displacing it's own volume in water.
So which of these is greater?
Well we know the brick to be more dense
than water because it sinks. So the volume of water equivalent to the mass of
the brick is greater than the volume of the brick. And so less water is
displaced after than before. Hence...
The Water Level Goes Down!!!!
If that is not clear then consider the
following example: you need to know that 1 litre of water has a mass of 1Kg:-
Say the brick weighs 2Kg and is 1
litre in volume.
When the brick is in the boat it is forcing
the water to be displaced by 2Kg or as we know 2ltr.
When the brick is over the side and sat on
the bottom of the lake it is displacing just it's own volume or one litre.
BACK
