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This chapter is from the book

The Submarine

A submarine can both submerge and rise to the top of the water. What's the trick?

Version 1: Step By Step


  • 2-quart (2-liter) plastic bottle

  • balloon

  • plastic tubing, approx. 3 feet (1 m) in length

  • silverware (3 or 4 table knives)

  • kitchen sink or pail

Make two holes roughly 1 to 1 1/2 in (3 to 4 cm) in diameter in the upper and lower parts of the bottle, as shown in the figure. Insert one end of the tubing in the opening of the balloon. (It needs to fit snugly; use masking tape if necessary.) Place the knives (ballast) inside the bottle through the holes in the side. Insert the end of the tubing with the balloon into the mouth of the bottle and position the balloon in the middle of the bottle. Your new submarine is ready. Put it on the bottom of the sink or in a pail. Fill the sink or pail with water until it covers your submarine. In order to not waste water, test your submarine when you are washing dishes in the kitchen sink. Blow into the free end of the tubing and see what happens.

Fun Facts

If there were water in the balloon instead of air, the mass of the water would be about a thousand times greater than that of the air contained in the balloon. Density is a measure of the quantity of matter (mass) per unit volume. We can then say that water is a thousand times denser than the air in the balloon. When the balloon fills, the submarine rises so that more water—higher density—will remain at a lower depth in the sink or pail, and the air—lower density—will remain above. In this way, more matter (mass) will remain closer to the Earth's surface. This is a result of the gravitational attraction exerted by the Earth. Otherwise, we would simply float! This happens when we are in a swimming pool, for example, and our lungs are full of air. As soon as we let the air out of our lungs, we start sinking, just as happens with the "submarine" when we let the air out of the balloon.

Version 2: Step By Step


  • 2 party balloons

  • 2 coins

  • 2 plastic bottles, 1 large, 1 small, both with screw-on caps

  • masking tape

  • water

Remove as much air as you can from the balloons. (If necessary, roll them as if they were almost empty tubes of tooth paste.) Insert the coins in the openings of the balloons. (The coins act as valves.) Put the balloons in the bottles, fill the bottles with water, and put the caps on tightly. The small amount of air that remains in the balloons will make them float, as shown. Ask someone to press one of the bottles while you press the other. Discover which of the bottles permits you to make the submarine submerge more easily.

A. Squeeze

Tightly cap an empty plastic bottle, then squeeze it as hard as you can with your hands. You will certainly be able to deform it (at least temporarily). Try to deform a bottle filled with water, as in the experiment with submarines. The small amount of air in the bottle can be compressed, to a certain point. The water cannot, and for this reason it is said to be incompressible.

B. Blow

Make small holes in a plastic soda bottle in various places with a pin or needle. Blow through the mouth of the bottle and feel the air coming out of the holes with your fingers. Fill the bottle with water and cap it well. (If the holes are small enough, hardly any water will come out of them.) Squeeze the bottle. (Be careful! Water will squirt out of all the holes in all directions, like in a shower.) With this experiment, you show that pressure propagates in all directions in a fluid, including air (you can repeat the experiment with the bottle empty).

Fun Facts

The submarine experiments demonstrate that any externally applied pressure is transmitted undiminished to all parts of an enclosed fluid (for example, air or water). Considering that the two plastic bottles in version 2 have different surface areas, when you squeeze them with the same force, you produce a different pressure in the water inside the bottles. This extra pressure, in its turn, forces the balloons to shrink, thus increasing effectively their average density. Eventually, when the applied pressure is large enough, the "submarines" will sink. Now, which bottle will sink more easily?

Version 3 : Step By Step


  • plastic tube from a pen, with one end capped/sealed

  • small vial, such as from a sample of perfume, or a dropper

  • wooden matchstick

  • plastic bottle with screw-on cap

  • water

Fill the tube (submarine) with water until it is almost filled, leaving only a small air bubble at the top. Fill the bottle with water and place the submarine inside so that it is suspended vertically. Close the bottle tightly and then press its sides with both hands, as the picture shows. What about using now the wooden matchstick as a submarine?

Fun Facts

This version is just a variant of the previous model. In the present case, when you squeeze the bottle, the extra pressure produced inside the plastic tube squeezes the air bubble, shrinking it, hence increasing the average density of the submarine. In a real submarine, the effect you produced with your hands is made with suction pumps. They pump water into ballast tanks, effectively increasing the average density of the submarine until it submerges. To surface, they pump the water back out into the ocean, decreasing the average density of the submarine until the submarine rises to the surface. Although ships are made of materials denser than water, they don't sink. This happens because most of the ship's space is filled with air. If the ship's weight plus its load exceeds a certain value, the ship will inevitably sink, just as the submarine does with its ballast tanks full of water. Now, what happens if the bottle is not completely filled with water or if it has a small hole at its top or at its bottom? Will the submarine work the same way?

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