I understand that pressure varies with depth because of the weight of the water and that buoyancy is a result of pressure variation between the top and bottom of an object but what causes the upward force that enables buoyancy?
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The upward force is described as "displacement".
Consider a 16oz glass ½ filled with water.
Dropping in small stones raises the level of the water.
The lift (displacement) requires force, and the force
acting through the distance of the lift requires work.
Work = force × distance = mass × "g" × height
In the case of an object which doesn't sink;
the submerged portion lifts an equal volume of water.
The weight of the lifted water provides the missing force.
Consider a 16oz glass ½ filled with water.
Dropping in small stones raises the level of the water.
The lift (displacement) requires force, and the force
acting through the distance of the lift requires work.
Work = force × distance = mass × "g" × height
In the case of an object which doesn't sink;
the submerged portion lifts an equal volume of water.
The weight of the lifted water provides the missing force.
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Fluid pressure acts perpendicular to the surface of a solid that contacts it, and acts in a compressive manner. This is true on both the top surface and on the bottom surface, and on all the sides. On net, the bottom net pressure force will exceed that on the top, and that is what forms the buoyancy force.
At the molecular level, what is going on is your standard billiard ball physics that we call the kinetic molecular theory. In the case of a gas, billiard balls make a great model, because the gas molecules are much farther apart than their own diameter, and only interact briefly upon a collision. Liquids are a more complicated story, because of intermolecular bonding. Some gas principles still apply, just not all of them.
Think of how your billiard balls would move, without any friction, and with perfect fully restoring collisions. Would there ever be a force on the boundary bumpers, that is any direction other than outward? Remember, all that the boundary bumpers do, is reverse the perpendicular component of the ball velocity, while preserving the parallel component. The bumpers receive a net outward force, as a result of numerous ball collisions occurring continuously.
The pressure is what we call the force per unit area that the fluid exerts on its boundary. Based upon frequency of collisions per unit area, mass of each colliding object, and how fast it is moving, in our billiard ball model.
At the molecular level, what is going on is your standard billiard ball physics that we call the kinetic molecular theory. In the case of a gas, billiard balls make a great model, because the gas molecules are much farther apart than their own diameter, and only interact briefly upon a collision. Liquids are a more complicated story, because of intermolecular bonding. Some gas principles still apply, just not all of them.
Think of how your billiard balls would move, without any friction, and with perfect fully restoring collisions. Would there ever be a force on the boundary bumpers, that is any direction other than outward? Remember, all that the boundary bumpers do, is reverse the perpendicular component of the ball velocity, while preserving the parallel component. The bumpers receive a net outward force, as a result of numerous ball collisions occurring continuously.
The pressure is what we call the force per unit area that the fluid exerts on its boundary. Based upon frequency of collisions per unit area, mass of each colliding object, and how fast it is moving, in our billiard ball model.
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Upward pressure is due to the resultant of net forces acting on submerged object. The side forces cancel out. But since pressure at bottom is higher than that at top you have net upward forces;