Brain Teasers
Buoyancy
Science
Science brain teasers require understanding of the physical or biological world and the laws that govern it.Science
Suppose you have a rubber ball 1 foot in diameter, and a latex balloon filled with helium until it is 6 inches in diameter.
Which has a greater buoyancy force?
Which has a greater buoyancy force?
Hint
Look at the formula for buoyancy force. Something people should think is in it is not.Answer
The rubber ball. This seems to contradict what you think, so let me explain.The formula for buoyancy force is:
Buoyancy Force = Fluid Density * Object Volume * Gravity
As you can see, buoyancy force has nothing to do with the density of an object, just the volume, so since the ball takes up more space then the balloon, it has a greater buoyancy force.
But, you may object, then why does the ball sink and the balloon float? Well, again, the density of the object has nothing to do with buoyancy, so even though the balloon has less buoyancy force, it floats because the force from gravity is even less.
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Comments
Firstly, I really enjoyed this teaser because it made me think. The discussion that follows outlines my thought processes and are not meant to degrade the teaser.
I had never heard of "buoyancy force", but reasoned it must be the "equal and opposite" force of displaced liquid. One answer could have been, since I have a ball and a balloon, but no liquid, the buoyance force on each is zero, so they are equal.
But, can air exert a buoyancy force? I don't see why not. Especially given the example of a helium ballon. Still, the ball would have the greater force acting against it.
The statement that buoyancy has nothing to do with the density of the object (in the answer) I question. I would reason that an object is buoyant if its weight is greater than any buoyancy force acting against it, and weight depends on density.
I had never heard of "buoyancy force", but reasoned it must be the "equal and opposite" force of displaced liquid. One answer could have been, since I have a ball and a balloon, but no liquid, the buoyance force on each is zero, so they are equal.
But, can air exert a buoyancy force? I don't see why not. Especially given the example of a helium ballon. Still, the ball would have the greater force acting against it.
The statement that buoyancy has nothing to do with the density of the object (in the answer) I question. I would reason that an object is buoyant if its weight is greater than any buoyancy force acting against it, and weight depends on density.
To help you understand:
1. Yes, air does exert a buoyancy force. It is in itself a fluid for the purpose of the equation, so it represent Fluid Density.
2. I know it doesn't make sense at first, but yes, the density of the OBJECT (not the fluid) doesn't affect the answer. And all objects are buoyant, its just that for some things, the density of the object causes the force of gravity to be stronger.
Hope you understand now. If you want to learn about this more, watch this video from Mark Rober:
https://www.youtube.com/watch?v=My4RA5I0FKs
1. Yes, air does exert a buoyancy force. It is in itself a fluid for the purpose of the equation, so it represent Fluid Density.
2. I know it doesn't make sense at first, but yes, the density of the OBJECT (not the fluid) doesn't affect the answer. And all objects are buoyant, its just that for some things, the density of the object causes the force of gravity to be stronger.
Hope you understand now. If you want to learn about this more, watch this video from Mark Rober:
https://www.youtube.com/watch?v=My4RA5I0FKs
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