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If newton's third law states that for every action there is a reaction that is equal in magnitude and opposite in direction, and Archimedes' principle states that when a body exerts a force on a liquid, the liquid exerts a buoyant force that is equal to the weight of the displaced liquid.

Why isn't the weight of the body equal to the buoyant force since they're action and reaction, aren't Newton's law and Archimedes' principle contradict each other in that way?

If not, please explain the difference.

The weight is equal to the buoyant force if the object is floating.  You have to understand that the fluid molecules moving around are what provide the force. That means that the important quality is the fluid density and the volume of fluid displaced.  Those quantities multiplied give the weight of the fluid displaced.  If the weight of the object was the buoyant force, then everything would float.

Newton's Laws still apply.  If you suspend a lead weight that displaces 20N worth of water but weights 220N in a container, the buoyant force will be 20N and the weight of the container it contains (if the weight is suspended above the bottom so only the buoyant force contributes) then the weight of the container will also increase by 20N.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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