A pendulum is a simple science project that shows how gravity can make motion repeat in a steady pattern. By hanging a weight from a string and timing its swings, students can investigate what changes the time for one full back-and-forth motion. This time is called the period, and it is useful because pendulums have been used in clocks, seismometers, and physics experiments.
A careful pendulum investigation helps students practice measuring, controlling variables, and using data to support a conclusion.
In a classroom experiment, students can change one variable at a time, such as string length, weight mass, or starting angle. For small starting angles, the length of the string has the strongest effect on the period, while the mass of the bob usually has little effect. The motion is a simple example of simple harmonic motion because gravity pulls the bob back toward the center after it is released.
Using three trials for each setup makes the results more reliable because small timing errors can be averaged out.
Key Facts
- Period is the time for one complete back-and-forth swing.
- For small angles, pendulum period is approximately T = 2π√(L/g).
- L is the length from the pivot point to the center of the bob.
- g is the acceleration due to gravity, about 9.8 m/s² on Earth.
- Changing the mass of the bob usually does not change the period much if air resistance is small.
- Average period can be found with average period = total time for many swings ÷ number of swings.
Vocabulary
- Pendulum
- A pendulum is a weight hanging from a fixed point that can swing back and forth.
- Period
- The period is the time it takes for a pendulum to make one complete swing cycle.
- Amplitude
- Amplitude is the size of the swing, often measured by the starting angle from the center position.
- Variable
- A variable is something in an experiment that can change, such as string length, mass, or angle.
- Simple Harmonic Motion
- Simple harmonic motion is repeated motion where a restoring force pulls an object back toward its middle position.
Common Mistakes to Avoid
- Changing more than one variable at a time: this makes it impossible to tell which change caused the difference in period.
- Timing only one swing: one small reaction-time error can strongly affect the result, so time 10 or more swings and divide by the number of swings.
- Measuring the string length only to the top of the weight: pendulum length should be measured from the pivot point to the center of the bob.
- Pushing the bob when releasing it: a push adds extra energy and makes the test unfair, so the bob should be pulled back and released gently.
Practice Questions
- 1 A pendulum takes 18 seconds to complete 12 swings. What is its average period?
- 2 Using T = 2π√(L/g), estimate the period of a pendulum with length 0.25 m. Use g = 9.8 m/s² and π = 3.14.
- 3 A student tests pendulums with 50 g, 100 g, and 200 g bobs but keeps the length and starting angle the same. The periods are nearly equal. Explain why this result makes sense.