Question

A 50-cm3 block of wood is floating partially submerged in water, and a 50-cm3 block of iron is totally submerged in water. Which block has the greater buoyant force on it?

A) the wood
B) the iron
C) Both have the same buoyant force.
D) The answer cannot be determined without knowing the densities of the blocks.

Answer 1

The buoyant force is the weight of the displaced water. The iron is displacing a full 50 cm^3 of water, but the wood is only displacing part of it. So the buoyant force on the iron cube is greater.

Answer 2

Answer: Option A

Explanation: The buoyant force depends on the amount of displaced fluid (in this case water)

As the wood is less dense than the iron, the wood requires a lot less upwards force to float, and it only displaces a little bit of fluid, in the other case, the iron requires a lot more of force to do it, so it displaces a lot of water and is totally submerged, and the iron block can never float actually, so it sinks down, at the point that the buoyant force applied is negative (the weight of the water upside the block is bigger than the force upwards)

The Buoyant force can be calculated as:

B = weight on-air - weight on water.

The block of wood is floating, so the weight on the water is zero, and the buoyant force is equal to his weight in the air.

For the block of iron, we have:

B = Weight on-air - weight on water, here in both cases we have positive numbers, but not enough information to know them.

Then the only information that we have is that the woodblock displaces less water than the other block, so with this, we only can know that the buoyant force in the woodblock is smaller.