Re: Why does an apple float?

The apple floats because the apple is less dense than water. When the apple is resting on water, it displaces an amount of water which weighs the same as the whole apple. Objects more dense than water sink because even when fully submerged they do not displace enough water to equal their weight.

Taking the foregoing just a little deeper, there are two forces acting on the floating apple. The force due to gravity is the most obvious, it is directed downward and is equal to the mass of the apple times the acceleration due to gravity. The second force, referred to as the buoyant force or buoyancy, is directed upward and is a consequence of Archimedes' principle that "... a body wholly or partly immersed in a fluid is buoyed up with a force equal to the weight of the fluid displaced by the body." (p.431, Halliday & Resnick's 1966 edition of Physics)

                         __  |  __     ^ 
                       /    \|/    \   |         |
            ----------/             \--|------   | Force of gravity
            |         |    Apple    |  |     |   | (mass of apple X 
            |          \           /   |     |   V  acceleration from gravity)
            |  water    \    /\   /    |     |     
            |             ---  ---     |     |
            |          Boyant force pushing  |
            |          against the apple     |
            |________________________________|

http://www.wnet.org/wnetschool/software/buoyancy/

Thanks for your question.

sid