Onions cause tears because onion vapors react with eye to make sulfuric acid

1. The enzymes in the onions mix with sulfenic acids in the onion as the onion is cut. This reaction produces propanethiol S-oxide. When these propanethiol vapor reacts with your eyes, it forms sulfuric acid (H2SO4), causing your eyes to water in an attempt to wash out this irritant.
2. Buoyancy, the force exerted by a fluid that opposes an object's weight, has the equation:

B= Buoyancy
= The density of fluid 
V_disp= Volume of the displaced liquid
g= gravity