Lighthouse Initiative for Texas Classrooms

What Variables Affect the Motion of a Pendulum

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What exactly is a pendulum? (pèn´je-lem) A pendulum is a mass suspended from a fixed point so that it can swing in an arc. The length of a pendulum is the distance from the point of suspension to the center of gravity of the mass. The period is the time it takes to complete one swing. A bob is the mass hanging at the end of a string.


The students will discover which variable affects the period of a pendulum. Students will identify dependent variable, independent variable, and constant for each section of the lab.


  • string
  • washers
  • pencil
  • ruler
  • stop watch


Which variable, mass, length of string or angle of release, affects the period of a pendulum?


We believe....


1. Measure 30 cm of string using the ruler in your science notebook.

2. Tie one washer onto the length of the string. Tape the other end of the string to your pencil and balance pendulum off the edge of your desk.

3. Pull washer back to 45 0 and release washer. Measure time to complete 10 periods. Record data.

Angle of Release Time for 10 periods (sec.) Time for 1 period (sec.) Average 1 period (sec.)

Now repeat the procedure for the other variables you are testing (mass and length of string). Record data in a 2 new data charts on your lab sheet. Be sure to do three trials for each variable.

Graph only the data that shows which variable controls the period of the pendulum.


1. List the following for all 3 sections of your experiment:

Dependent variable

Independent variable


2. What is a "control" in an experiment? Why is this important?

3. You did several trials for each variable. Why was this an important part of the lab?

4. What is the conclusion of your lab group? Was your hypothesis correct?