Build a Pen Cap Submarine
Adapted from a lesson on http://www.unmuseum.org/exsub.htm
Summary
Students will build a submarine out of a pen cap and a clay ball using the principle of buoyancy.
Learning Objectives
Students will be able to:
Build a “submarine” that has neutral buoyancy
Explain neutral, positive and negative buoyancy
Explain how pressure affects buoyancy
Materials
Pen cap (without a hole in the top)
Modeling clay
Plastic bottle with an air tight cap and a mouth large enough to get your “submarine” through it
Water
Deep Tupperware dish to test “submarine”
Optional: A kind of “submarine” retrieval tool like long tweezers or a wire hanger bent into a
straight wire with a U hook at the end
Background
Why does an object float? To answer this
question return to the principles of pressure with depth.
Water has a much higher density than air and can therefore
exert more pressure on objects. This is true not only when
an object descends in the water but also when an object is
supported in the water by floating. The pressure of all of
the water below an object pushes up on the object. That
pressure is greater than the downward pressure exerted by
gravity. A boat, while its materials may be more dense
that the water uses it’s shape to distribute that pressure in
such way that the upward pressure exceeds the downward
pressure. Not until the objects overcomes the pressure of
the water (like when the Titanic filled with water) does the
object sink.
Buoyancy force is the upward force exerted on an
object. Archimedes’ principle states that any floating object displaces its own weight of fluid.
Thus any object that floats does so because the force pushing it up is equal to the weight of the
fluid that was displaced. As a sinking ship takes on water, that water is no longer being
displaced, but sucked in. The boat increases its weight, giving gravity a stronger edge.
Eventually, the boat’s weight overcomes the upward pressure and goes down. The Archimedes
principle did not consider surface tension of fluid, but as this only modifies the amount of fluid
that is displaced, the principle of buoyancy holds true.
Objects that float on or toward the surface
of the water are said to be positively buoyant.
Objects that sink are negatively buoyant. And
objects that hover, that is neither rise or fall, are
said to be neutrally buoyant. The experiments done
in this lesson allow students to discover these
various positions. Any submersible vehicle must be
designed to achieve negative buoyancy so that it
can dive, positive buoyancy so that it can ascend,
and neutral buoyancy if it is to move along at a
certain depth to gather information.
Sources for information:
http://en.wikipedia.org/wiki/Buoyancy
(sketch of forces and buoyancy information)
http://www.lakesidepress.com/pulmonary/books/scuba/sectione.htm
(sketch for negative, netural and positive buoyancy)
Submarines go up or down based on their buoyancy. That is, when they weigh less than the
water they displace, they go up. When they weigh more, they go down. If they weigh exactly the
same as the water they are displacing, they float right where they are. Submarines vary their
weight by adjusting the amount of air in the ballast tanks. This activity uses a pen cap and a bit
of modeling clay to build a submarine that goes up or down as the amount of air in its ballast
tank shrinks and enlarges.
What to do
1. Make your submarine by putting a
ball of clay on the stem of the pen cap
(See diagram). The opening to the
hollow portion of the pen cap should be
facing down. The hollow portion is
your sub's ballast tank.
2. Fill your Tupperware container with
water.
3. Put the "submarine" in the water
(clay down so the air is trapped in the hollow portion of the cap) and then add or subtract clay
until the "submarine" floats just below the surface of the water as in the diagram. Also adjust the
position of the clay on the pen cap stem so that the “submarine” remains upright. It now has
neutral buoyancy.
3. Fill the bottle with water and put the "submarine" into it. The sub should float just below the
neck of the bottle (as in the diagram). If it falls to the bottom or floats on the surface adjust the
amount of clay.
4. Make sure the bottle is full and put the bottle cap on tightly.
5. Squeeze the bottle. This will cause the pressure inside to go up and any gas trapped inside the
bottle (like the air inside the pen cap) will shrink. This will change the buoyancy on your
"submarine" from neutral to negative and it will sink to the bottom. When you release the
pressure the air will expand and the sub will rise.
6. Discuss the concept of neutral, positive and negative buoyancy and why squeezing the bottle
changed the buoyancy. (Answers to these discussion points are in the Background section.)