Period of a pendulum

In this activity students use the phyphox app to determine the gravitational field strength using a pendulum.

Equipment

Each student will need:

• A phone with phyphox installed
• A length of string
• A small clear plastic bag which can be sealed (optional)

Procedure

1. Open the phyphox app and click on Pendulum.
2. Click on the top right part of the screen and switch on do a timed run (use the default settings of 3 seconds for start delay and 10 seconds for experimental duration).
3. Securely attach the phone to one end of the string. For example, you could put it in a clear plastic bag and then attach the string.
4. Hold the other end of the string and pull the phone-pendulum to one side. Press play and let the pendulum go.
5. Keeping your hand steady, let the phone swing for more than 13 seconds to enable the app to record for at least 10 seconds. Make a note of the period from the reading.
6. Measure the length of the pendulum l (length of string + distance to centre of phone).
7. Repeat steps 4 to 6 for more pendulum lengths.
8. Plot a straight-line graph to determine the acceleration of free-fall g.

Teaching notes

If students are unfamiliar with the relationship for the period T of a pendulum introduce it:

T = 2π    l g

They can determine g by plotting T² against l (the gradient of the straight-line graph will be equal to 4π²/g).

Learning outcome

Students describe an experiment to determine gravitational field strength.