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what is the cause of buoyant force?

Hello archit,

Consider a piece of wood, maple, and the water molecules in a pond. The density of maple is 755 kg/m^3. So a piece 10 cm x 10 cm x 1 m has a mass of 7.55 kg. You're thinking about the tiny water molecules in the pond -- they are so small no one has ever seen a water molecule -- how could water stop this 7.55 kg piece of maple, about 73 Newtons, from going to the bottom of this pond? So you throw it into the water. It floats! Why?

Think about the water molecules. They were minding their own business in the pond and this piece of wood dropped in and pushed them aside. Water has a density of 1000 kg/m^3. But that's just the sum of a bunch of individual, disorganized water molecules. Well, turns out they're not so disorganized. Gravity works on the piece of wood and it also works on the water molecules. Trying to pull them closer to the center of the earth. It pulls harder on things with more mass. The wood is also made up of individual molecules and atoms. A framework holds them together in a particular shape, but gravity's pull on it is just the sum of its pull on all its components. Gravity's pull on water molecules also add up.

The water molecules that were displaced when the piece of maple landed in the pond can team up and try to reclaim their rightful place in the pond. The condition when this struggle comes to equilibrium is that the piece of maple does not sink to the bottom. And it floats so that about only about 3/4th of the wood is below the surface. In that equilibrium state, the weight of the water that has not regained its rightful place is equal to the total weight of the piece of maple. Because water is more dense than the maple, a volume of water that is smaller than the entire piece of maple supports the weight of the maple keeping a fraction sticking up out of the water. I said above that "about 3/4th of the wood is below the surface". That is because the density of wood is 755 kg/m^3 and water's density is 1000 kg/m^3.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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