Water Pressure, Why Things Float, Weight of a Cubic Foot of Water - A demonstration of water pressure and a wonderful explanation of why things float.
These experiments are from Robert Krampf - The Happy Scientist
This week's experiment comes from a suggestion made by B. Eschner. It
started with his observations as he washed a plastic bag. It developed into
a demonstration of water pressure, which then turned into a wonderful
explanation of why things float. The more I played with it; the more I liked
it. To try it, you will need:
a deep bowl or sink filled with water
a plastic bag
First, stick your hand into the water. How does it feel? Wet, right? What
else do you feel? Not much, probably. We are used to the sensation of
having our hands in water, so you don't notice anything strange. Next, put
your hand into the plastic bag and then put it back into the water. Be sure
that the water does not flow into the bag. What do you feel this time?
Pressure! You can feel the water pushing the plastic bag against your hand.
The pressure is pretty much the same as before (slightly more due to the size
of the bag), but this time the water presses on the bag, which presses on your
hand. This is a less familiar sensation, so you notice the pressure more.
Put both hands into the bag and spread them apart. This time, you feel more
pressure and you can see the water pushing the sides of the bag inwards. As
you move your hands apart, you may be surprised at how much pressure the
water is exerting.
When you put your hand into the water, your hand pushes some of the water out
of the way. You hand is now taking up space that was occupied by water, and
the water is pushing back. When you put both hands into the bag and spread
them apart, the water pushed back harder. If you measured it carefully, you
would find that the water was pushing with a force equal to the weight of the
amount of water that you push out of the way. When you spread your hands
apart, the bag takes up more space, pushing more water out of the way. You
then have more force pushing inwards and you feel more pressure.
Now we come to the part about why things float. If I put a one foot cube
into the water, it would push one cubic foot out of the way. That means that
the water would be pushing back with a force equal to the weight of one cubic
foot of water. One cubic foot of water weighs 62.4 pounds (for fresh water
at 32 degrees F.) Water is heavy, as you know if you have ever hauled a
bucket of water very far. Lisa is always glad to let me experiment with
that, hauling water for her flowers. Just for reference, one cubic foot of
water is almost 7.5 US gallons. If your cube is heavier than 62.4 pounds,
then it will sink. If your cube is lighter than 62.4, then the force of the
water pushing is stronger than the force of gravity pulling on the cube, and
it floats. We have talked about this in other experiments, but with this one
you get to experience the pressure exerted by the water.
Knowing that one cubic foot of water weighs 62.4 pounds, can you figure out
how many cubic feet of water a boat would have to displace in order to let
you float? To make it easier, we will calculate the weight of the boat as
62.4 pounds, the same as one cubic foot of water. Divide your weight by
62.4, add one cubic foot for the weight of the boat and that will give you
the number of cubic feet of water you would need to displace to just barely
float. Of course, for a boat, you would want to do more than just barely
float, so a well designed boat should displace quite a bit more water.
To take this experiment a bit farther, you might test the density of some
Coca-Cola by adding a scoop of vanilla ice cream to see if you can make it
float. Of course, even if it sinks, you would still have a float, wouldn't
you?
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