Simple Harmonic Motion
Forces and Motion

Pendulum bags

Practical Activity for 14-16 16-19 IOP RESOURCES

A simple demonstration to introduce the idea of a pendulum using everyday objects.


  • Students‘ bags
  • Timer with large display
  • Simple pendulum made of string with a metal bob at one end (optional)


When selecting students’ bags look for those which have a loop at the top for holding, or which have a long strap. You will need a varied selection of bags to give a range of periods of oscillation when they swing.


  1. Select 3 or 4 students’ bags from the class . Choose one bag and hang it from a finger.
  2. Pull the bag to one side and release it so that it swings from side to side. Repeat, showing that it swings with the same period.
  3. Repeat with another bag to show that it has a different period of oscillation.

Discussion prompts

  • Why does one bag swing at a different rate to another?
  • How could we measure the time period?
  • How could we change the period of a swinging bag?

Teaching notes

This demonstration introduces pendulums by drawing on students’ everyday experience. Explain that the time period (T) is the time for one complete back-and-forth motion and that it is difficult to measure the time for a single swing and so is better to time a number (eg 10T) to find an average. Also discuss whys it is better to count from the point where the bag passes through the midpoint of the swing rather than at the ends (it is instantaneously stationary at the ends so the time difficult to judge).

Students may suggest changing mass, length or amplitude as ways to alter the period of a swinging bag. To illustrate that period doesn’t depend on the mass add books to the bag.

This activity can used as a precursor to a more formal investigation into the factors that affect the period of a pendulum. Make the link to simple pendulums by explaining that in science we try to design experiments so that we remove superfluous complications. The swing of a bag can be modelled using a pendulum made of string with a heavy bob as the moving mass.

Learning outcome

Students measure the period of a pendulum

This experiment was safety-tested in March 2020.

Simple Harmonic Motion
can be analysed using the quantity Natural Frequency
can be described by the relation a=-(w^2)x
is used in analyses relating to Pendulum Mass on a Spring
is exhibited by Oscillating System
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