Feeling the Pressure

This 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. I hope you will have as much fun (and learn as much) as I did.

To try it, you will need:

  • a deep bowl, bucket, or sink filled with water
  • a large, plastic bag

Start by sticking your hand into the water. How does it feel? Wet, right? What else to 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 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. That means you have more force pushing inwards.

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, which is 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. My wife 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.

Now back to putting your one foot cube into the water. If your cube is heavier than 62.4 pounds, then it will sink. The 62.4 pounds of pressure from the displaced water is not strong enough to hold it up. 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.

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?