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Q & A: action and reaction in buoyant force

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Most recent answer: 01/15/2014
Q:
If Newton's third law states that for every action there is a reaction equal in magnitude and opposite in direction, and Archimedes' principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces. Why doesn't the weight of the body is equal to the upthrust force? Since the weight is an action and the upthrust force is a reaction? Aren't Archimedes' principle and Newton's 3rd law in condradiction then? Thank you!
- Yara (age 17)
Cairo, Egypt
A:

The language people often use about "actions" and "reactions", is very misleading. There's no distinction between "action" and "reaction" in Newton's laws law. A modern statement of the law would be that "Every force from object  A on object B is always accompanied by a force from B on A of the same strength but in the opposite direction."  For the case you mention, the equal, opposite force to the weight (pull of Earth's gravity on the body) is the pull of the object's gravity on the Earth. 

If the object happens to be one that floats, and if it's given some time to settle down and float steadily, not bobbing up and down, then the buoyant force on it will come to have the same strength as its weight. The force that's always equal and opposite to the buoyant force, however, is the downward force on the liquid from the object.

Mike W. 


(published on 01/15/2014)

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