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Education Program
USS Ohio (SSBN-726)
Ohio Lab

Submarine Demonstration V: Acoustics

notes:
skill level: beginner
manipulation score: high
computation score: beginner
computer knowledge: beginner
Simulations: java simulation, Explore Science simulation (require web browser)
Historical link: Turtle, Revolutionary War
Scientific Principles: Archimedes' Law, Pascal's Law

Goal: to demonstrate the principles of buoyancy using floating models

Background: Archimedes long ago studied buoyancy, and derived certain principles we can use to predict whether an object will float or sink when immersed in a fluid.

His law states that whenever the mass of the fluid displaced by an object is greater than the mass of that object, it will float. For example, a large ship might displace many tons of sea water. As long as the "many tons" is more than the mass of the ship, the ship will float. If the ship takes on water, it is displacing less, and it therefore gets closer to sinking.
Submarines use this principle to submerge and surface: instead of filling the entire sub with water (this would kill the crew), tanks on the outside of the sub are filled with sea water to make the sub more dense than the surrounding water, so it sinks.

When the submarine wants to surface, it pushes out the water, replacing it with air, which has a negligible mass compared to sea water.

In this lab, we will a small container (the "diver") filled with air and some water. The bottom of this container is open to the water. If we add pressure to the water by pushing on a rubber sheet covering the container, Pascal's law tells us that this will compress the air in the "diver", making the diver more water-like and less air-like. This makes the diver sink. In reverse, if the pressure on the tank is decreased, the diver should go up.

We will be making this Archimedes diver using an eyedropper and a graduated cylinder (an Archimedes diver model may be used instead)

Scientific Principles: Archimedes' Law, Pascal's Law

Archimedes' Law: Objects may displace fluids. If the mass of the displaced fluid is greater than the mass of the object, the object will float. If it is less, the object will sink. The density of an object is the mass of the object divided by its volume. Objects denser than the fluid they are in will sink, less dense objects will float.

Pascal's Law: Any pressure applied to a fluid is distributed uniformly throughout the fluid. This assumes that the fluids do not compress very much (like water).

Materials: Eyedropper or Archimedes diver model, graduated cylinder, rubber sheet or balloon, rubber band, wood block, metal block, spring scale ruler, water.

Directions:

Part One:
Fill the graduated cylinder with water. Test the eyedropper to see if it floats in the water first. Squeeze the dropper and add more water to make it more dense. See if it floats now. Continue this until the dropper sinks, then squeeze out a small bit of water, so that the dropper just barely floats in the cylinder.

Cover the graduated cylinder with the rubber sheet, and secure it with the rubber band.

Push on the top of the sheet, and notice what the diver does. See if you can control the upwards and downwards motion of the diver.

Part Two:
Measure the length, width and height of the wooden block in centimeters. Centimeters are the units smaller than inches on the other side of the ruler. The mass of the block should be marked on it in grams. Multiply the length times the width. Multiply that number by the height. This is the volume of the block. To find the density of the block, divide the mass in grams by the volume (in units of cm x cm x cm). This will be the density in grams/cubic centimeter (g/cc for short).

Repeat this process with the metal block, and determine the density of the metal block.

Try floating the wooden block in the water (you may remove the rubber sheet from the cylinder). Try floating the metal block.

Repeat this process (find volume, find mass, calculate density, see if the object floats) for other solid objects if you have access to a gram balance or spring scale.

Observations:

Describe your observations here:

Simulations:

You may test out your guesses using the computer simulation for buoyancy. You will be given an object in a liquid, and be able to change their densities. You can also use the simulations to answer some of the questions below:

Questions:

1. We showed above that metals sink, and wood floats: we can understand why ships made of wood might float, but how do ships made of metal float?

2. If the First Submarine named the Turtle was made of wood, how did they make it sink?

3. If the density of the water was 1.0 gram/cc, compare the density of the wood block and the metal block. Do you notice anything? Which was greater? Which was less?

4. Hotter fluids tend to be less dense than cooler ones. Should the bottom of a swimming pool be cooler or warmer than the surface? Would this also happen in a pond? If cooler water holds more oxygen, which the fish breathe, where would you drop your line to catch these fish: the top of the pond or the bottom?

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