In this experiment you will make a simple Foucault pendulum. When a pendulum is moving freely in air, the air resistance causes the pendulum to slow down and eventually stop. A heavy weight and tall pendulum will reduce the effect of friction. (Foucault's original pendulum consisted of a 62-pound [28-kilogram] iron ball suspended on a 220-foot [67-meter] steel wire.)
You will use a bag of sand as the pendulum's weight, and then note how the sand moves as it trickles from the bag. To observe results from the pendulum it should swing for at least thirty to sixty minutes. After that time, note the apparent change in the direction in which the pendulum is swinging.
To begin this experiment, make an educated guess about the outcome of the experiment based on your knowledge of Earth's rotation. This educated guess, or prediction, is your hypothesis.
A hypothesis should be brief, specific, and measurable. It must be something you can test through further investigation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "The lines of sand falling from the Foucault pendulum will shift slightly over time as Earth is continuously rotating."
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
Moderate to Difficult (because of the difficulty in keeping the swing straight).
Draw the pattern of the sand. Explain the results, including how the Coriolis force influences the direction of the sand lines. For example, a Foucault pendulum set in motion in the northern hemisphere traces out a line that is always shifted toward the right.
How many degrees the pendulum shifts depends on where it is geographically located or its latitude. Latitude identifies the north-to-south position of a point on Earth The equator is 0° latitude; the north and south poles are each 90° latitude. At the equator the pendulum would not shift at all. At either of the poles the pendulum's swing would complete a circle in about twenty-four hours. You can figure out the rate of rotation where you live by finding your latitude and figuring out the following equation through longhand or a calculator. Mathematically, the pendulum's rate of shift is equal to the rate of rotation of Earth multiplied by the sine of the number of degrees of latitude: n = 360 degrees x sine (latitude), where n equals the number of degrees of rotation. The sine of latitude represents the angular distance of a place from the equator.
Here are some problems that may arise during this project, some possible causes, and ways to remedy the problems.
Problem: The bag is moving in a circular, elliptical path.
Possible cause: You may not have pushed the bag in a straight line for the first push or any subsequent pushes. Try practicing a straight-line push with the tape on the bag, and then repeat the experiment.
Problem: There was no shift in the lines of sand.
Possible cause: You may have set the cord slightly off-kilter during one of your pushes, or the pendulum may not have swung long enough. Try practicing a straight-line push with the tape on the bag, and then repeat the experiment, making sure to keep the pendulum swinging for at least sixty minutes.