What changes a pendulum's swing?
Time ten swings of a pendulum. Change the length of the string, the mass of the bob and how far you pull it back, one at a time, and find out which one really changes the swing.
Core concept
In a fair test we change one variable at a time; for a pendulum only the length of the string changes the time of a swing, not the mass or a small swing
Topic: Scientific study
A pendulum is a weight: The pull of gravity on something, measured in newtons. It is less on the Moon. All key words (the bob) hanging on a string, swinging to and fro. Galileo noticed that a swinging lamp always took the same time for each swing, and pendulums were later used in clocks. What decides how long one swing takes? Three things could matter: the length of the string, the mass: How much matter something has, measured in grams or kilograms. It is the same everywhere. All key words of the bob, and how far you pull it back before letting go. These are the variable: Something that can change in an experiment. Change one, measure another, and keep the rest the same. All key words. To find out which one matters, a scientist changes only one at a time and keeps the others the same: a fair test: A test where you change only one thing, so you know what caused the result. All key words. To measure more accurately, we time ten swings and divide by ten.
What you will learn
- Identify the variables in a pendulum experiment.
- Plan a fair test by changing one variable at a time.
- Find that only the length changes the time of a swing.
- Explain why timing ten swings is more accurate than timing one.
Before you start
- Use a stopwatch and divide by 10.
Materials
- String, bobs of 50 g, 100 g and 200 g (or nuts and bolts)
Equipment
- A stand and clamp, a protractor, a metre rule, a stopwatch
Procedure
- Time ten swings of a 60 cm pendulum with a 100 g bob pulled back 10°, and record.
- Change only the mass to 50 g and to 200 g, and record both.
- Change only the angle to 20° and to 30°, and record both.
- Change only the length, recording at least four lengths.
How this simulation is simplified
- g is 9.8 N/kg; the string does not stretch and the air does not slow the bob.
- Swings are small (30° or less), so their size does not change the time.
- A swing is one full to-and-fro.