Let's say we have several containers, each containing a different liquid. We find that a wooden block will float in all of these liquids. We observe that in one liquid the block is 30% submerged, in another 90% - the amount submerged is different because the liquids have different densities. In one particular liquid the wooden block, which has a weight of 100 N, floats exactly half submerged. What is the buoyant force exerted on the block by this liquid?

1. 100 N (22/109) (20%)
2. 50 N (34/109) (31%)
3. It depends on the density of the block (5/109) (5%)
4. It depends on the density of the liquid (14/109) (13%)
5. It depends on the both the density of the block and the density of the liquid. (34/109) (31%)

The key here is that the object is in equilibrium, so the forces must balance. The only two forces acting are the downward force of gravity (100 N) and the upward buoyant force. So the buoyant force must be equal to 100 N.

This is true for all floating objects: if the only forces acting on the floating object is the force of gravity and the buoyant force then the buoyant force must balance the force of gravity.

Lesson 1: Drawing a free-body diagram can give you a lot of information about the buoyant force.