Cannonball!!! It’s getting hot here and there is nothing my kids like better than jumping in a pool to cool off. The average outdoor pool temperature is about 84 degrees Fahrenheit–so why does it feel so cool in the pool? The short answer is that water is a better conductor than air so it pulls heat off your body more efficiently.
In this 90-degree heat the water feels so refreshing. Before long my kids have found their way to the diving board. “Watch me, Mom! I’m going to make the biggest splash!”
My six-year-old gets a running start, jumps, tucks up his knees, and boom, hits the water in a cannonball. Sure enough, the force of his jump sends water splashing out in all directions. His weight and ball-like form are two pieces of the math puzzle that impact the size of his splash.
An older kid thinks he can get a bigger splash with the greater surface area of the belly flop. I am cringing in sympathy pain even before he hits the water with a loud, but not very splash-inducing, smack.
Turns out the spherical shape of the cannonball is your best bet for making a splash at the pool!
Speaking of water displacement (splashing), I tell the kids how Archimedes calculated the volume of a king’s crown using water displacement. Legend has it that he made the discovery in his bathtub, not the pool, noticing that the water level rose as he stepped into the bath. The volume of the water displaced is equal to the object submerged in water. Archimedes was so excited that he allegedly ran naked into the streets, shouting, “Eureka!” Don’t try that at your local public pool.
You can, however, measure volume, and then mass, using the concept of displacement. Just fill a plastic beaker with some water from the pool and note the volume. Then, place an object inside and note the new level. The difference is the volume of the object.
With the volume of the object, you can now calculate density. This makes for a fun game of sink or float. Get a few objects that you are allowed to bring into the pool, like plastic bath toys, quarters, and diving toys. Ask the kids which objects will sink or swim and see who is right.
Later on, you can find the volume and divide the mass by the volume to find the object’s density. What do you notice about the density of objects that float and the density of objects that sink?
My daughter loses interest in the sink or float game and returns to jumping in again. She likes a different approach than her brother, the pencil dive. She jumps in with her feet together and her hands at her side. “I’m going to make the tiniest splash–hardly any splash at all, Mom! Watch!”
She slices through the water, her tiny surface area making barely any splash, and almost hits the bottom. She pops back up and says, “I can sink and float!” unintentionally revealing a new wrinkle in our earlier density experiments. To prove her point, she spreads out her arms and legs and floats onto her back. I mention that she is spreading her density over a greater surface area. I also tell her that taking a big breath and holding it in also increases buoyancy.
When you head to the pool this summer, don’t forget your suit, your towel, your goggles, and your math!
Image licensed by Ingram Publishing