Saturday STEM Challenge: Keeping a Fish Underwater
Dreaming of summer trips to the beach? This week we’re exploring different aspects of aquatic systems.
Image via Juliette Lee
For most people, when they go swimming and try to stay underwater, they’ll float to the water’s surface if they stop moving. Fish, though, live underwater, so they stay submerged whether they’re swimming or not (specifically, bony fish—many cartilaginous fish, like sharks and rays, need to keep moving to stay underwater). So how do fish do that?
This answer is a lung-like organ called a swim bladder. By adjusting the amount of air in their swim bladder, a fish can control its buoyancy. No matter how deep or shallow a fish is swimming, it needs it’s swim bladder to maintain it’s neutral buoyancy.
As discussed in Wednesday’s Distance Learning Module, neutral buoyancy occurs when an object's density is equal to the density of the fluid in which it is immersed, resulting in a balance between the upward and downward forces being simultaneously exerted. An object that has neutral buoyancy will neither sink nor rise.
Notice that when the mass sinks, the force of gravity is greater than the buoyancy force; when it floats, the buoyancy force is greater than gravity; and in neutral buoyancy, the forces are equal.
Sometimes, fish can experience issues with their swim bladders, and they can get stuck floating at the surface of the water—which brings us to our STEM challenge!
Suppose we have a pet fish that needs some help achieving neutral buoyancy. We need to give our fish enough weight so that it will sink more into the water of its tank, but not so much that it’s stuck on the bottom.
Your “fish” can be anything that floats—we’re using some cork board cut in the shape of a fish, but you can use a styrofoam packing peanut, a plastic Easter egg, a film canister, or anything else you can find in your home. Then add enough weight—for instance, push tacks into the cork or packing peanut, add coins inside the plastic egg or film canister—so the forces of gravity and buoyancy are in equilibrium. Test if you found the right amount of weight by placing your “fish” is a tall, clear container of water and seeing if it floats, sinks, or stays right in the middle!
