Why do objects placed in fluids experience a buoyant force?

Answer 1

All fluids exert pressure on the surface of any object in that fluid, but the pressure is not uniform over the surface of the object. Parts of the object that are deeper in the fluid will have more pressure from the fluid pressing against them than parts that are shallower. The amount of the object's surface that is being pressed down by fluid above it will be equal to the amount of surface being pressed up, but the pressure per unit area pressing up will be greater, on average, than the pressure that is pressing down.

A 1 square inch column of salt water 1 foot tall weighs just under 0.45 pounds, so it exerts a pressure of (about) 0.45 PSI. That pressure will be in addition to the pressure of any fluid that is pressing down on the top of the water column, such as the atmosphere. Thus the pressure on the surface of the ocean is about 14.7 PSI and the pressure at a depth of 1 foot is about 14.7 + 0.45, or 15.15 PSI.

If a cubic object 1 foot on each side was placed in the water with its top right at the surface the top of the cube would have 14.7 PSI * 144 square inches, or 2116.8 pounds of pressure pushing down on it. The bottom would have 15.15 PSI * 144 square inches, or 2181.6 pounds of pressure pushing up on it. The pressure on each side would equal the pressure on the opposite side, so there would be no net pressure pushing sideways. The difference in pressure pushing up and pressure pushing down would be 64.8 pounds, so there would be a buoyant force of 64.8 pounds pressing the object upward. Since the actual weight of the water column is a little bit less than 0.45 that actual buoyant force would be 64 pounds, which is the weight of 1 cubic foot of water. Thus the object is buoyed up by a force equal to the weight of the fluid it displaces.