Buoyancy Science Project 2

A ship will float as long as it weighs less than the water it pushes out of the way, or displaces. In order to load a ship with cargo, that ship also has to have air in it.

Air doesn't weigh as much as water. This lowers the weight of the ship compared to the same volume of water. The more air in a ship, the less it weighs in the water, even though it still displaces the same amount of water as a fuller, heavier ship.

Submarines use the principle of buoyancy. When they have more air than a similar volume of water, they will float. When they have the same amount of air, they will stay where they are in the water. When they have less air than water, they will sink.

Submarines are constructed to be able to vary their weight by adjusting the amount of air in the ballast tanks.

No, the submarine you’re going to make doesn’t have to be yellow. But in order to win points with your instructor, you might look up that reference.

Let’s make one to the following specs:

*At least 2 cm long and not larger than an average shoebox.

*Completely self-operational.

*Use a clear storage box or fish tank at least one foot deep, with no hoses.

*Only gasses can be releases into the tank, no chemicals.

*You cannot change the density of the water in the tank.

*Captain’s Log must include an engineer’s report on building this sub and an explanation of its course mechanics. You’ll have the submarine 1) float on water, 2) sink, (and, like you, in life, many times,) RISE AGAIN!


*One paper cup, with a wax covering

*One pencil, sharpened

*Pen knife

*Several marbles


Take the pen knife and cut the top off the cup so that the submarine is a short little thing.

Take the pencil and poke three to five holes through the bottom of the cup.

Poke the pencil through the middle of the cup, so that it sticks out the bottom.

Put the marbles in one at a time.


The cup submarine will float.

Put in as many marbles as needed to sink the cup submarine. And the cup submarine will sink.

Then notice that as the pencil hits the floor of your tank, it tips the cup, dumping the marbles, the ballast out, and up rises the submarine. The cup submarine will rise again.