Thank you. This clears up a lot of issues I've been having with buoyant force.
No matter what, once an object is 100% submerged, whether it be just under the surface, or all the way at the bottom, the amount of water it displaces is equal to its volume. Further, the amount of water it displaces is independent of the object's mass, density, shape, or anything else. In other words, a piece of lead and an bean of the same volume will displace the same amount of fluid so long as they are 100% submerged, even if one is sunken further in the fluid than the other.
Sorry to threadjack, but can someone explain how the calculation works when an object is floating and only partially submerged? Say an object is 50% submerged, does it displace a volume of fluid equal to half of its volume? That seems right to me, but I thought it had something to do with weight and/or density?
Cool, you have it right about the fully submerged objects.
You are also right that a 50% submerged body displaces a volume equal to 50% of its volume. If that body is floating, you can deduce a few things about its weight and density. First, floating means that the buoyant force and the weight are same in magnitude. Then you know the volume of the displaced fluid and its density, so you can calculate the buoyant force. From there, you know how much the body weights. From there you can get its mass. And since you know its volume, you can calculate its density.
Here is an example: A 2 m^3 cube is floating on a water surface and is 50% submerged. What is the density of the cube, if density of the water is 1000 kg/m^3?
Volume of cube: 2 m^3
Volume submerged: 50% * 2 m^3 = 1 m^3
Volume of displaced water: 1 m^3
Mass of displaced water: 1 m^3 * 1000 kg/m^3 = 1000 kg
Weight of displaced water: 1000 kg * g = 10000 N
Buoyant force: 10000 N
Weight of cube: 10000 N
Mass of cube: 10000 N /g = 1000 kg
Density of cube: 1000 kg / 2 m^3 = 500 kg/m^3
Is that helping or more confusing? If we're clear so far, I'll mention a few gotchas with which you need to be careful.