What Factors Determine the Period of a Pendulum?

Personal Lab 3

Background

Swinging pendula with magnificent massive brass bobs are displayed in many science museums and university science departments. ASU has such a pendulum just outside the classroom in the F wing. These impressive displays are usually used to demonstrate that the Earth rotates. Once the brass bob is set swinging back and forth, the pendulum will swing in the same arc. If the pendulum is suspended from a sufficiently high ceiling, and the point where it is fastened to the ceiling is free from frictional forces, then the pendulum will swing back and forth in the same arc, in theory, forever. How does the ASU pendulum demonstrate that the Earth rotates?

Here you will explore the properties of a homemade swinging pendulum that will give you insights into appreciating the profound science underlying these fascinating museum and school displays. You will also understand why a pendulum clock was used for telling time for centuries.

Purpose

1. To investigate the factors that determine the period it takes for a pendulum to swing back and forth.

2. To understand the causes for the periodic motion of a pendulum.

Procedure

1. Find a place to fasten one end of one of the two lengths of string of your pendulum. Tie one of the fishing weights to the other end. One suggestion would be to secure a wire coat hanger by weighting with books to a tall book shelf. Hang the pendulum from the protruding coat hanger.

2. Measure the height of the fishing weight from the floor. Record in a Data Table in your Lab Book. Create a Data Table with five headings: String Length, Fishing Weight Mass, Height of Weight above Ground, Height of Weight when Released, Time for 10 cycles.

3. Pull the pendulum to one side. Measure the distance above the floor of the weight, just before you release it. Take care in your release not to introduce additional motion other than a smooth fall of the weight. (That is, make sure that the pendulum swings back and forth in one plane, not in an elliptical path.)

4. Time the moment you release the fishing weight pendulum. (Try to record measurements of time to the nearest second).

5. Count off 10 back-and-forth cycles (periods) of the pendulum. Record the time for the 10 cycles in your Lab Book, together with the mass of the fishing weight and the string length.

6. Perform several controlled experiments by changing one of your variables at a time (fishing weight, string length, release height).
Remember the objective is to find which factors (variables) affect the period of the pendulum’s swing.

Questions

Turn in a copy of your Data Table, and any graphs that you plot.

1. Examine your Data Table carefully for patterns and trends. Plot graphs if you suspect a trend among any two variables. Graphs should have titles, labeled axes, and a statement which explains the meaning of the graph.

2. Which variables affected the period of the pendulum?

3. Which variables did not affect the period of the pendulum?

4. What are the main sources of error in this experiment?

5. What causes the motion of a pendulum?

6. At what point in the cycle of a pendulum is the fishing weight’s speed maximum? minimum?

7. At what point in the cycle of a pendulum is the fishing weight’s acceleration maximum? minimum?

8. Sketch a force diagram indicating all of the forces acting on the pendulum’s fishing weight when it is at its minimum height above the ground.

9. How would your experiment be different (if at all) if you were to perform it on the Moon? Explain your reasoning.

Personal Lab 4: What Happens to the Spring Scale Readings in a Moving Elevator?

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Last modified 9 Aug 1997
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