Build a Pen Cap Submarine - Center for Dark Energy Biosphere ...
Build a Pen Cap Submarine - Center for Dark Energy Biosphere ...
Build a Pen Cap Submarine - Center for Dark Energy Biosphere ...
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<strong>Build</strong> a <strong>Pen</strong> <strong>Cap</strong> <strong>Submarine</strong><br />
Adapted from a lesson on http://www.unmuseum.org/exsub.htm<br />
Summary<br />
Students will build a submarine out of a pen cap and a clay ball using the principle of buoyancy.<br />
Learning Objectives<br />
Students will be able to:<br />
<strong>Build</strong> a “submarine” that has neutral buoyancy<br />
Explain neutral, positive and negative buoyancy<br />
Explain how pressure affects buoyancy<br />
Materials<br />
<strong>Pen</strong> cap (without a hole in the top)<br />
Modeling clay<br />
Plastic bottle with an air tight cap and a mouth large enough to get your “submarine” through it<br />
Water<br />
Deep Tupperware dish to test “submarine”<br />
Optional: A kind of “submarine” retrieval tool like long tweezers or a wire hanger bent into a<br />
straight wire with a U hook at the end<br />
Background<br />
Why does an object float? To answer this<br />
question return to the principles of pressure with depth.<br />
Water has a much higher density than air and can there<strong>for</strong>e<br />
exert more pressure on objects. This is true not only when<br />
an object descends in the water but also when an object is<br />
supported in the water by floating. The pressure of all of<br />
the water below an object pushes up on the object. That<br />
pressure is greater than the downward pressure exerted by<br />
gravity. A boat, while its materials may be more dense<br />
that the water uses it’s shape to distribute that pressure in<br />
such way that the upward pressure exceeds the downward<br />
pressure. Not until the objects overcomes the pressure of<br />
the water (like when the Titanic filled with water) does the<br />
object sink.<br />
Buoyancy <strong>for</strong>ce is the upward <strong>for</strong>ce exerted on an<br />
object. Archimedes’ principle states that any floating object displaces its own weight of fluid.<br />
Thus any object that floats does so because the <strong>for</strong>ce pushing it up is equal to the weight of the<br />
fluid that was displaced. As a sinking ship takes on water, that water is no longer being<br />
displaced, but sucked in. The boat increases its weight, giving gravity a stronger edge.<br />
Eventually, the boat’s weight overcomes the upward pressure and goes down. The Archimedes<br />
principle did not consider surface tension of fluid, but as this only modifies the amount of fluid<br />
that is displaced, the principle of buoyancy holds true.
Objects that float on or toward the surface<br />
of the water are said to be positively buoyant.<br />
Objects that sink are negatively buoyant. And<br />
objects that hover, that is neither rise or fall, are<br />
said to be neutrally buoyant. The experiments done<br />
in this lesson allow students to discover these<br />
various positions. Any submersible vehicle must be<br />
designed to achieve negative buoyancy so that it<br />
can dive, positive buoyancy so that it can ascend,<br />
and neutral buoyancy if it is to move along at a<br />
certain depth to gather in<strong>for</strong>mation.<br />
Sources <strong>for</strong> in<strong>for</strong>mation:<br />
http://en.wikipedia.org/wiki/Buoyancy (sketch of <strong>for</strong>ces and buoyancy in<strong>for</strong>mation)<br />
http://www.lakesidepress.com/pulmonary/books/scuba/sectione.htm (sketch <strong>for</strong> negative, neutral<br />
and positive buoyancy)<br />
<strong>Submarine</strong>s go up or down based on their buoyancy. That is, when they weigh less than the<br />
water they displace, they go up. When they weigh more, they go down. If they weigh exactly the<br />
same as the water they are displacing, they float right where they are. <strong>Submarine</strong>s vary their<br />
weight by adjusting the amount of air in the ballast tanks. This activity uses a pen cap and a bit<br />
of modeling clay to build a submarine that goes up or down as the amount of air in its ballast<br />
tank shrinks and enlarges.<br />
What to do<br />
1. Make your submarine by putting a<br />
ball of clay on the stem of the pen cap<br />
(See diagram). The opening to the<br />
hollow portion of the pen cap should be<br />
facing down. The hollow portion is<br />
your sub's ballast tank.<br />
2. Fill your Tupperware container with<br />
water.<br />
3. Put the "submarine" in the water<br />
(clay down so the air is trapped in the hollow portion of the cap) and then add or subtract clay<br />
until the "submarine" floats just below the surface of the water as in the diagram. Also adjust the<br />
position of the clay on the pen cap stem so that the “submarine” remains upright. It now has<br />
neutral buoyancy.<br />
3. Fill the bottle with water and put the "submarine" into it. The sub should float just below the<br />
neck of the bottle (as in the diagram). If it falls to the bottom or floats on the surface adjust the<br />
amount of clay.
4. Make sure the bottle is full and put the bottle cap on tightly.<br />
5. Squeeze the bottle. This will cause the pressure inside to go up and any gas trapped inside the<br />
bottle (like the air inside the pen cap) will shrink. This will change the buoyancy on your<br />
"submarine" from neutral to negative and it will sink to the bottom. When you release the<br />
pressure the air will expand and the sub will rise.<br />
6. Discuss the concept of neutral, positive and negative buoyancy and why squeezing the bottle<br />
changed the buoyancy. (Answers to these discussion points are in the Background section.)<br />
Extensions<br />
A good buoyancy site <strong>for</strong> high school and college lessons on buoyancy: http://hyperphysics.phyastr.gsu.edu/hbase/pbuoy.html<br />
Another good lesson on buoyancy <strong>for</strong> high school students.<br />
http://phet.colorado.edu/en/contributions/view/3408