My family and I went to Cheat Lake last week, where we went swimming at the lakeside. My younger brother (who is 5-foot-7) gave my dad (who is 5-foot-11) a piggy back ride while we were in the water. My brother is way too weak to give my dad piggy backs on land, but whenever we were out in the water, giving my dad piggy backs was easy for him.
This is due to buoyancy. The man who discovered this concept was Archimedes. The story is that one day, Archimedes was about to take a bath when he noticed that as he stepped into the water, the water rose. As he put more of himself into the water, the water rose higher. Once he realized this, he jumped out of the water, still naked, and ran through the streets of Greece, yelling, “Eureka!”
Now we don’t know if this actually happened, but Archimedes’ discovery of buoyancy was, nonetheless, important. It was so important that the scientific principle of buoyancy was named after him: Archimedes’ Principle.
Archimedes’ Principle states that an object submerged in water will displace water based on its weight. Water pushes upward on the object. This force is called the normal force. It pushes upward on objects (the opposite direction of gravity, which pushes down on objects). The normal force stops objects from passing through other objects. For example, if there was no normal force, an object on a table would slam through the table, then through the ground and it would keep going down.
The amount of water an object will displace is based on its density. Density is mass divided by volume. Volume is how much cubic space an object takes up and mass is how much matter is in an object. Mass doesn’t change depending on gravity. Weight, which is something some people get confused with mass, is how much gravitational force is being pushed down on a person or an object. Weight changes based on the magnitude of gravity (for example, people’s weight on the moon is one-sixth of their weight on Earth). But mass doesn’t change even if someone is on the moon or on Earth.
A higher-density object (like a bowling ball) will displace more water than a lower-density object (like a beach ball). When the density of an object is greater than the density of water, it will sink. And when the density of an object is less than water’s density, it will float. If you put a bowling ball in water, it will sink and if you put a beach ball in water, it will float. In both cases, the water is pushing up against the ball with a force that is equal to the weight that the ball is displacing. The bowling ball displaces a lot more water than the beach ball since its density is a lot greater.
Additionally, if you’ve ever tried to push a beach ball (or anything else that can float in water) down towards the bottom of water, you might find that there’s a great deal of resistance. I actually once tried to stand on a kick board when I was in a pool, and I almost fell off. This is because of the buoyancy of water.
Because water is pushing up toward objects, objects are technically lighter. Since water is more dense than air, its buoyancy force is stronger than air’s. This is why my little brother can’t give my dad piggy backs on land but is able to do so easily when underwater.
VAAGEESHA DAS is a rising senior at Morgantown High School.
Information comes from:
Anonymous. (n.d.). Why does an object feel lighter in water than in air? How Things Fly. https://howthingsfly.si.edu/ask-an-explainer/why-does-object-feel-lighter-water-air;
Why Is it Easier To Lift Someone in Water than on Land? Wonderopolis. (n.d.). https://www.wonderopolis.org/wonder/why-is-it-easier-to-lift-someone-in-water-than-on-land.
