Question

What causes the upward buoyant force on an object placed in a fluid?

Answer 1

The weight of the displaced fluid.
As part of the fluid is displaced by an object, they try to flow back under the object, creating an upwards force.

Answer 2

Surface Tension

Answer 3

I got this from "Charles" The cause is the difference in densities. Simply looking at the equation, we know that the weight and the density are important in the force that is supplied to the object. More precisely, heavier things are pulled harder by gravity. Why is it that we aren't pulled into the earth, why is it that we don't break through the ground toward the center of the earth? It is because the earth below us is being pulled harder and the force that it supplies to us (the normal force) is completely balanced by the graviational force we supply to it. Therefore, as Newton said, since the sum of the forces is zero, we remain in equilibrium. Chemically, this can have to do with chemical reactions between the fluids, but usually it is easier to describe in a physical way, such as that with gravity.

Answer 4

and this from "TomG" The object displaces more of the fluid than it weighs.

All the displaced fluid "pushes" back on the object equal to the weight of the object... so it reaches equilibrium and "floats".

If the object cannot displace more than it weighs... it sinks. If it doesn't weight enough to break surface tension, then it's held on TOP of the fluid by that cohesive force.
Source(s):
30 years engineering. Thank them and thank you.