Floating in Water

What is the density of a human being?

You have probably heard that humans are more than 90% water. If this is true, then we must have a density close to that of water, which is 1 gram per cubic centimeter (usually referred to as simply a 'density of 1'). Since your question mentioned floating in water, you must know that anything with a lower density than water will float. A simple trip to a neighborhood pool shows that people can float! Therefore the density of a human must be near 1.

But wait a minute! Sometimes people can actually stay underwater, which means their density is higher than water! Which is it?

Well, the average density of a human is almost exactly 1. But density itself is the mass of an object divided by its volume. If you increase the volume of an object, but keep the mass the same, the object's density decreases. Now we know that air is lighter than water. When you take a deep breath and hold it, you are increasing your volume, but your mass is only going up a little bit (imagine it this way: you are adding air, which is very light, to your mass, so your average mass is going down). This has the effect of decreasing your density, making it easier to float. If you let the air out, your density increases again and you sink a bit. Also, fat has a low density, so very skinny or very muscular people will have a higher density on average than people with more body fat. They will tend to sink easier.

You can measure your own density. Fill a bathtub high enough so that when you get in you can completely submerse yourself. Mark the level of the water on the tub, perhaps with some tape (no doubt your parents won't like you scratching or marking the tub tile, even in the name of science). Then get in, submersing yourself as much as possible, and mark the water level again (note that it went up!). A friend would help a lot here. The difference in water level is your volume. You can measure that by taking a measuring cup and scooping away the water until the water level reaches the first mark you made. Keep track of the amount! Now, get on a scale and weigh yourself. Divide the weight in grams (one pound American is 454 grams) by the volume you found from the tub (one quart=4 cups=2 pints=908 cubic centimeters). That's your density! You should get a number that is close to 1 gram per cubic centimeter.