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Hitler's Lost Sub

Classroom Activity

Objective
To investigate weight and buoyant force as applied to submarines.

• copy of "Build Your Own Submarine" student handout (HTML)
• 2-liter plastic soda bottle, rinsed, label removed, and cut to specifications
• package of 3-ounce drinking cups, plastic preferred
• large needle
• large paper clips
• kitchen utility scissors
• waterproof markers
• water, room temperature
• dishwashing liquid
• for ballast: paper clips and metal washers

1. Before class, prepare diving tanks for each group by cutting around the shoulder of a soda bottle so that the remaining base is tall and straight-sided. For safety, use kitchen utility scissors. This activity involves working with these "tanks" filled with water. Work in a wet lab, outdoors, or place them in plastic dishpans.

2. Organize students into groups and distribute copies of the "Build Your Own Submarine" student handout. Demonstrate for students the construction of a submarine:

   1. Poke two holes with a large needle on opposite sides of the drinking cup's rim.

   2. Bend a large paper clip into a U-shape and attach to the cup through the holes.

   3. Neatly cut a bean-shape hole in the side of the cup about a quarter of an inch (6.35 mm) from the bottom. The submarine will be easier to control if the edge of the hole nearest the cup bottom is straight and parallel to the cup bottom.

3. Supervise students as they build their submarines and prepare their diving tanks. (If students have difficulty making their submarines neutrally buoyant, add some dishwashing liquid to the diving tank. This will reduce the surface tension, making the size of the escaping bubbles smaller and thus making it easier to achieve neutral buoyancy.)

4. Once students have gotten their submarines to be neutrally buoyant, conduct a class discussion about buoyancy using the scenarios listed in the questions section of the student handout.

5. As an extension, change the density of the water and repeat the experiment. First have students predict what might happen, then take out the submarine, dissolve sugar or salt into the water, and put the submarine back in again.

A submarine rises because the weight of water pushing up on the submarine, known as the buoyant force, is greater than the downward force, the weight of the submarine. If the submarine weighs more than the buoyant force, it sinks. If it weighs less than the buoyant force, it will rise. If the buoyant force and the weight are equal it will drift (either on or beneath the surface). At this point, the submarine is neutrally buoyant. That is, there is no tendency for it to rise or sink so the submarine should remain at whatever level it was placed.

Divers use this same principle when they add or let air out of their buoyancy vests in order to control their position in the water. Bony fishes use a swim bladder to maintain neutral buoyancy. Like the diver's buoyancy vest, this saves energy that would otherwise be spent maintaining vertical position.

A submarine fills its tanks with water to submerge and pumps the water out to rise. After a torpedo launch, water must enter the diving hull to compensate for the loss of weight. Otherwise, the sudden unequal forces will drive the submarine to the surface.

During WWII, German submarines routinely escaped detection by switching off their engines and taking advantage of the currents flowing in and out of the Mediterranean Sea. The submarine captains would adjust the amount of water in the diving hull so that they would float within the current of either the surface or lower layer.

Books

Lafferty, Peter. Eyewitness Science: Force and Motion. New York: Dorling Kindersely, 1992.
Discusses the principles behind several different kinds of force and motion, including buoyancy.

Mulligan, Timothy P. Neither Sharks Nor Wolves: The Men of Nazi Germany's U-Boat Arm, 1939-1945. Annapolis, Maryland: United States Naval Institute Press, 1999.
A character study of the men in Germany's submarine force, based on first-person interviews and a survey of more than 1,000 U-boat officers and enlisted men.

Tall, J. J. Submarines (History Series). New York: Barrons, 1998.
The earliest experiments with submarines date from the 16th century, but the first practical submarines were built from the designs of Irish-American engineer J.P. Holland at the beginning of the 20th century. Diagrammatic illustrations, battle illustrations, and photos show submarine development, with emphasis on the vessels of WWI and WWII, and modern nuclear submarines.

Web Site

NOVA Online—Hitler's Lost Sub
http://www.pbs.org/nova/lostsub/
Chronicles the story of a German U-boat wreck discovery off the coast of New Jersey in 230 feet of water. Charts the years of archival research and the dangerous dives onto the wreck to finally identify the sub. Explores the U-boat phenomenon as it played out through two world wars in articles, activities, resource links, and more. Launch date: Friday, November 10.

The "Build Your Own Submarine" activity aligns with the following National Science Education Standards:

Grades 5-8

	Science Standard B: Physical Science

Motions and forces

• The motion of an object can be described by its position, direction of motion, and speed. That motion can be measured and represented on a graph.

• An object that is not being subjected to a force will continue to move at a constant speed and in a straight line.

Grades 9-12

	Science Standard B: Physical Science

Motions and forces

• Objects change their motion only when a net force is applied. Laws of motion are used to calculate precisely the effects of forces on the motion of objects. The magnitude of the change in motion can be calculated using the relationship F = ma, which is independent of the nature of the force. Whenever one object exerts force on another, a force equal in magnitude and opposite in direction is exerted on the first object.

• Gravitation is a universal force that each mass exerts on any other mass.