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Swing Low: Investigate the Motion of a Pendulum

Difficulty
Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety No issues

Abstract

"Swing me higher, Mommy, higher!" Kids love to ride the swings at the playground. The back-and-forth motion of a swing demonstrates the physics of a pendulum. In this experiment, you will investigate the factors that affect the speed and duration of a pendulum's swing.

Objective

Investigate the motion of a simple pendulum and determine how the motion of the pendulum is related to its length.

Credits

La Né Powers

Cite This Page

MLA Style

Science Buddies Staff. "Swing Low: Investigate the Motion of a Pendulum" Science Buddies. Science Buddies, 5 Aug. 2014. Web. 2 Sep. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p016.shtml>

APA Style

Science Buddies Staff. (2014, August 5). Swing Low: Investigate the Motion of a Pendulum. Retrieved September 2, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p016.shtml

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Last edit date: 2014-08-05

Introduction

A pendulum is an object, hung from a fixed point, that swings freely back and forth under the action of gravity. A playground swing is an example of a pendulum. The swing is supported by chains that are attached to fixed points at the top of the swing set. When the swing is raised and released, it will move freely back and forth. The swing is moving due to the force of gravity on the swing. The swing continues moving back and forth until friction (between the air and the swing, and between the chains and the attachment points) slows it down and eventually stops it.

We see pendulums in other areas of our lives as well, such as in long-case clocks, commonly known as grandfather clocks. But pendulums can do more than entertain and help us tell time. Among other applications, they can show that the Earth is rotating! This was done in the mid-1800s C.E. using perhaps the most famous pendulum, Foucault's pendulum. However, pendulums were being used for centuries before this. One of the first known pendulum uses was around 100 C.E., when a Chinese scientist, Zhang Heng, used it to detect distant earthquakes in a device called a seismometer. Today, pendulums have many applications, including measuring local gravity and helping guide ships and aircrafts.

In this science fair project, you will investigate how the period of a pendulum is related to the pendulum's length. A pendulum's period is the time it takes the pendulum to swing back to its original position. In the example of a kid being pushed in the swings at a playground, this is the time it takes the kid to be pushed and then return back for another push. The period of a pendulum is mathematically related to the pendulum's length.

Terms and Concepts

  • Pendulum
  • Gravity
  • Momentum
  • Friction
  • Foucault's pendulum
  • Seismometer
  • Pendulum period

Questions

  • What is a pendulum and what causes it to swing?
  • What are some common uses for pendulums?
  • How do you think the period of a pendulum is related to its length? Will a longer pendulum have a longer period than a shorter one?

Bibliography

Introduction to General Physics Concepts:

  • Hewitt, Paul G, 2002. "Conceptual Physics," Prentice Hall, IL.

Simple Physics Concepts for Kids:

  • Keller, R.W., 2005. "Real Science for Kids: Physics, Level 1," Albuquerque, NM: Gravitas Publications, Inc.

Materials and Equipment

  • Identical chairs (2)
  • String or yarn
  • Metal washers of identical size (10)
  • Meter stick
  • Scissors
  • Stopwatch accurate to 0.1 s
  • An assistant
  • Lab notebook

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Experimental Procedure

  1. Create two pendulums with different lengths.
    1. Place the two chairs back to back. Then space the chairs a little less than 1 meter apart.
    2. Lay the meter stick on the backs of the two chairs, centered on the back of each.
    3. Cut one piece of string to a length of 70 cm. Cut another piece of string to a length of 35 cm.
    4. Tie the two lengths of string to the meter stick, toward the middle of the stick. Space the strings about 20–30 cm apart.
    5. Attach 5 washers to the end of each string.
  2. Time how long each pendulum swings.
    1. Hold the washers tied to the 70 cm long string in one hand and the washers tied to the 35 cm long string in the other hand.
    2. Pull the strings tight and hold the strings at the same angle from the meter stick.
    3. Have an assistant ready with a stopwatch.
    4. At the same time, drop the longer pendulum and have the assistant start the stopwatch. Have the assistant stop the stopwatch when the pendulum returns back to its original position. How long does it take the longer pendulum to swing back to its original position? This is called the period of the pendulum. Note: If the pendulum hit anything as it swung, such as the wall, readjust your setup and try timing the pendulum again.
    5. Make a data table in your lab notebook like Table 1 below. Write down the period of your pendulum in your data table under "Period (s)."
    6. Time the period of the shorter pendulum by repeating steps 2a–2d above, but in step 2d drop the shorter pendulum instead of the longer pendulum. How long does it take the shorter pendulum to swing back to its original position? Write down your results in your table.
    7. For each pendulum, repeat steps 2a–2d above, but instead of then timing the period of each pendulum, let the pendulum swing until it stops moving. What is the total time that each pendulum swings? Write down your results in your table under "Total Time (s)."
  3. For any experiment, it is important to do multiple trials to assure that your results are consistent. Repeat step 2 for at least five separate trials for each pendulum and record your results in your table.
    1. Calculate the average period for each pendulum and write this down in your table under "Average Period (s)."
    2. Calculate the average total time for each pendulum and write this down in your table under "Average Total Time (s)."
Sample Data Table
Pendulum Length (cm) Trial (#) Period (s) Total Time (s) Average Period (s) Average Total Time (s)
35 1        
2    
3    
4    
5    
70 1        
2    
3    
4    
5    
Table 1. In your lab notebook, write down your results in a data table like this one.
  1. Analyze your results. Did one pendulum have a longer period than the other? If so, why do you think this is?
    1. Are the two periods different in the way that you expected them to be? Are you surprised by your results? Why?
    2. Are the periods and total times for each pendulum consistent between your five trials, or do they vary a lot?

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Variations

  • Instead of changing the length of string, try changing the number of weights attached to the string. Does mass affect the speed of the swing or how long the pendulum swings?
  • Try changing the initial angle of the string when you drop it. Does this affect the speed and duration of the swing?
  • Try changing the size of the washers. Does this affect the speed and duration of the swing?
  • For a more advanced challenge, you can calculate the expected periods of your pendulums using a mathematical equation. To do this, check out The California Academy of Sciences resource About Foucault Pendulums. For each pendulum, how does the average period you recorded compare to the expected calculated period? If they are different, why do you think this is?

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