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Why do things float?



If you throw a teacher into a swimming pool in a chicken costume, will the teacher float? This is the question that every child ponders while they lay awake at night. Will they sink like a stony stone or float like chicken feathers? Objects float when the buoyant force (the force of a fluid pushing up) exerted on them by a fluid (such as water or air) is greater than or equal to the force of gravity pulling them downward. It’s that simple. It’s a contest between buoyancy and gravity. The teacher is silently hoping that buoyancy wins!



The buoyant force arises from Archimedes' principle, which states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces. So, when you throw the teacher into the water and measure how much of their chicken body is under the water, you can figure out how strong the buoyancy pushing them up is. This is called displacement. Look at a ship below the waterline. The amount of water the ship displaces (pushes back) is equal to the buoyant force pushing it up. When an object is placed in a fluid, it displaces a certain volume of that fluid, and the weight of the displaced fluid creates an upward force.



If the weight of the object is less than the weight of the fluid it displaces, the buoyant force is greater than the gravitational force acting on the object, and it floats. If the weight of the object is greater than the weight of the fluid it displaces, the buoyant force is smaller than the gravitational force, and the object sinks.




A message in a bottle saying, “I stink!”, floats because it’s full of air giving it more buoyancy. If the cork breaks and water floods into the bottle, it’ll sink because now the gravitational force pulling it down is greater than the buoyancy pushing it up. This important message will be lost to the oceans like Atlantis and that marble that I lost in 1994.



That’s the worst possible news for a teachers in a chicken costume because teachers, even teachers in chicken costumes, cannot breathe underwater, and chicken costumes become very heavy, very quickly! In fact, underwater, teachers in chicken costumes do the opposite of breathing. They…


Luckily, teachers in chicken costumes can take OFF the chicken costume. That reduces the gravitational force pulling them down as chicken costumes absorb and hold water, but human skin doesn’t. Teachers also have this amazing ability to breathe (I know..astonishing stuff!). The air they breathe in and hold increases the buoyancy of their body. If you find yourself lost in a giant lake of apple juice, before you do anything else, take a huge breath in and hold it - you’ll float like a baby seal.




The shape and density of an object also play a role in whether it floats or sinks. An object with a lower density than the fluid will float because the buoyant force is greater than the gravitational force. Conversely, an object with a higher density will sink because the gravitational force is greater. For example, a wooden chicken statue floats because wood is less dense than water, while a teacher made of solid metal sinks because it is denser than water.


This principle applies to various objects and fluids, allowing us to understand why some things float while others do not. Sadly, plastic ocean pollution tends to float for this very reason, creating tons of problems for the marine life that swims through it and around it.



Think of 5 things that float and 5 things that don’t - explain why:

They float!

  1. A plastic bottle - it contains an air pocket inside the bottle giving it more buoyancy

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3.


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5.


They don’t float!

1.


2.


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5.

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