Science | Science Experiment of the Week 6 | 267 - Feeling the Pressure

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.

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This Week's Experiment - #267 Feeling the Pressure

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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