Why Things Float?
Name: Grace R.
Which items float in water, which do not and why?
Did you ever notice that when something floats in water, part of it is
actually under water? As it sinks (even a little bit) it pushes away the
water until that amount of water weighs the same as the thing that is
floating. If the thing you try to float is too heavy, it cannot push
away enough water to be the same as how much it weighs. If that happens,
the thing will sink.
Ask an adult to help you with an experiment (a test) that can show you
how this works: Float a small plastic boat in water and notice how deep
the boat sinks when it is empty. Then add pennies to the boat and watch
how the boat sinks deeper and deeper the more pennies you add. The
pennies make the boat weigh more and more. If you add enough pennies,
the boat will sink deep enough so that water reaches the top and then
the whole thing sinks.
Blow up a balloon and float it on water. It will not sink very far because
it is not very heavy. If you look really close, where the balloon touches
the water, you can see a little dent in the water under the balloon.
That's the place where the water is pushed out of the way. If you try
this test with a ball that is exactly the same size as the balloon, the
ball will sink deeper before it floats. Because the ball is heavier than
the balloon, it has to push more water out of the way before it can
It is not just how heavy something is that makes it float or sink. Look
how heavy real boats are -- and they still float. Floating or sinking
has to do with the amount of water pushed out of the way. Any boat will
sink if you put enough stuff inside it -- just like your experiment
showed. Small, heavy things like a marble or a rock cannot float because
they cannot push enough water out of the way to be the same as how much
So remember, anything that floats weighs the same as the water pushed
out of the way.
The object is buoyed up (pushed up) by a force that is equal to the weight
of the water that the object occupies that was previously occupied by the
water. If you shape the object is in such a way that it occupies a volume of
water whose weight equals that of the object, the object will float. If it
occupies a volume of water whose weight is less than the weight of the
object, the object will sink.
You can show this to yourself by taking a piece of aluminum foil and making
a water-tight boat out of it. If you carefully put the boat in a dish or pan
of water, you will see it float.
Now take the aluminum foil boat and crumple it up into a ball and put it
back on the water. It sinks! There is the same amount of aluminum foil in
both cases, but in the case of the boat, you shaped it so that it displaced
a lot of water compared to the amount of water that is displaced when you
crumpled the aluminum foil into a ball.
Objects that are heavier than the same amount of water will sink. Things
that are lighter than the same amount of water will float. If you push an
empty bottle under water, you push a lot of water out of the way. That
water would weigh much more than the bottle. The bottle will float. A rock
put under water also pushes a lot of water out of the way. That water would
weigh less than the rock, so the rock can sink.
Dr. Ken Mellendorf
Illinois Central College
Whether an item will float in water has to do with a property called
density. Density relates to the weight of items that are of a specific
Things that have a lower density than water will float in water. This
is because the item weighs less than the water that it displaces.
Because the water is pulled (by gravity) towards the earth with more
force than the item it flows around the item and pushes it out of the
way so that it is closer to the earth. The result is that the item is
further from the earth and thus floating on top of the water.
You can "model" this result using a shallow can full of small beads or
BBs and a polystyrene foam packing peanut. Bury the peanut in the beads so you
can't see it from the top. Now gently 'bounce' the can on the floor
(drop it a small distance, like 1/2 inch) several times so that the
beads move around, just like water molecules do. The peanut should come
to the surface.
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Update: June 2012