Hands-on Activity: Swinging Pendulum

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

An animation of an oscillating pendulum showing velocity and acceleration.
Students examine swinging pendulums and their conservation of energy
copyright
Copyright © Wikimedia Commons http://upload.wikimedia.org/wikipedia/commons/2/24/Oscillating_pendulum.gif

Summary

This activity demonstrates how potential energy (PE) can be converted to kinetic energy (KE) and back again. Given a pendulum height, students calculate and predict how fast the pendulum will swing by understanding conservation of energy and using the equations for PE and KE. The equations are justified as students experimentally measure the speed of the pendulum and compare theory with reality.

Engineering Connection

Mechanical engineers design a wide range of consumer and industry devices — transportation vehicles, home appliances, computer hardware, factory equipment — that use mechanical motion. The design of equipment for demolition purposes is another example. Like the movement of a pendulum, when an enormous wrecking ball is held at a height, it possesses potential energy, and as it falls, its potential energy is converted to kinetic energy. As the wrecking ball makes contact with the structure to be destroyed, it transfers that energy to take down the structure.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

Suggest an alignment not listed above

Learning Objectives

After this activity, students should be able to:

  • Understand the concepts of potential and kinetic energy.
  • Use the concepts of kinetic energy, potential energy, and conservation of energy to perform an experiment to determine an object's velocity
  • Understand how the concepts of conservation of energy, kinetic energy, and potential energy are used extensively in engineering design

Materials List

Each group needs:

  • 2 stopwatches (Borrow from other teachers or ask students to bring from home)
  • Masking tape (not scotch tape)
  • 10 feet of string or fishing line
  • Heavy object or weight (to tie to string)
  • Calculator
  • Scale (if mass of heavy object or weight is unknown) (One per class; groups can share; borrow from other teachers.)

Introduction/Motivation

Remember that an object's potential energy is due to its position (height) and an object's kinetic energy is due to its motion (velocity). Potential energy can be converted to kinetic energy by allowing the object to fall (for example, a roller coaster going down a big hill or a book falling off a shelf). This energy transformation also holds true for a pendulum, as illustrated in the diagram. As a pendulum swings, its potential energy converts to kinetic and back to potential. Recall that energy may change its form, but there is no net change to the amount of energy. This is called conservation of energy.

A diagram of a swinging pendulum illustrates that the pendulum's potential energy, when at its highest point at the left, is converted into kinetic energy as it drops to its lowest point, and converted back to potential energy as it reaches its highest point to the right.
This diagram shows a swinging pendulum whose potential energy is converted into kinetic energy and back during the course of a swing from left to right.
copyright
Copyright © Chris Yakacki, University of Colorado at Boulder, 2003.

In this activity, students prove that the transformation of energy occurs by calculating the theoretical value of velocity at which a pendulum should swing and comparing it to a measured value.

Three equations will be used in this activity:

PE = m∙g∙h

KE = ½ m∙Vt 2

Vm = distance ÷ time

where m is mass (kg), g is gravity (10 m/s2), h is height (meters), Vt is the calculated velocity (m/s), and Vm is the measure velocity (also m/s). To make the calculations simpler, use the metric system for measurements and calculations. This way, we can approximate gravity as 10 m/s2 and not worry about the English system's wacky units of mass.

Procedure

Before the Activity

  • Gather materials.
  • Designate several areas, depending on the size of your class, for pendulums to swing.
  • Tie the string(s) or line(s) to the ceiling about 2 inches from a wall, leaving enough slack to reach the ground.

With the Students

  1. Divide the class into groups of 4 and hand out the worksheet.
  2. Have each group measure and record the mass of their object or weight.
  3. Have each group pick an arbitrary height at which they will release their pendulums. This should range from 15-40 cm (.15-.4 m) from the floor.
  4. Calculate the potential energy. Each team member should do this, as a way to verify the result.
  5. Calculate the theoretical velocity, Vt , at the bottom of the swing.
  • Remember, KE at the bottom of the swing will equal PE at the top of the swing.
  • If your students do not know algebra yet, derive Vt for them (see the Troubleshooting Tips section).
  1. Have each group move to a designated area and tie their weight to the string/line so that it barely misses the ground while hanging.
  2. Place two pieces of tape on the wall on opposite sides of the hanging pendulum and record the distance between the two pieces.
  • The distance should range from 30-50 cm (.3-.5m). Choose a larger distance for a higher height (i.e., h = 40 cm → distance = 50 cm).
  • The pendulum should rest in the middle of the two pieces of tape.
  1. One or two students pull back the weight until it reaches one of the pieces of tape.
  2. Two students synchronize two stopwatches, each holding one, and start timing when the pendulum is released.
  3. The first student stops his/her stopwatch when the pendulum passes over the opposite piece of tape and the second student stops his/her watch when it returns back to the initial piece of tape.
  4. Record both times and calculate the difference in time.
  5. Repeat the experiment four times so students can exchange roles.
  6. Complete the worksheet.
  7. How close were the values for the theoretical velocity and the measured velocity?

Attachments

Safety Issues

Make sure the students don't use the weighted pendulum to hit one another.

Troubleshooting Tips

If the students have not learned algebra yet, use the worksheet version with Vt already derived.

An approximation is used for calculating measured velocity, Vm . If the tape markers are too far apart, the approximation won't hold true. However, if they are too close together, it may be difficult for students to clock a difference in time. The distance should range from 30-50 cm (.3-.5m). A larger distance should be chosen for a higher height (i.e., h = 40 cm → distance = 50 cm).

Assessment

Pre-Activity Assessment

Question/Answer: Ask the students and discuss as a class:

  • Where will the pendulum have the greatest potential energy? (Answer: When it is pulled back.)
  • Where will it have the greatest kinetic energy? (Answer: At the bottom/middle of the swing.)

Prediction: Ask the students to predict:

  • Will pendulums at higher heights go faster or slower? (Answer: They should go faster.)

Activity Embedded Assessment

Question/Answer: Ask the students and discuss as a class:

  • What happens to the potential energy as the pendulum swings down? (Answer: It turns into kinetic energy.)
  • When the pendulum swings to the other side, what happens to the kinetic energy? (Answer: It turns back into potential energy.)

Post-Activity Assessment

Question/Answer: Ask the students and discuss as a class:

  • If engineers can use potential energy (height) of an object to calculate how fast it will travel when falling, can they do the reverse and calculate how high something will rise if they know its kinetic energy (velocity)? (Answer: Yes, as long as you know either height or velocity, you can calculate the other.)
  • For what might an engineer use this information? (Answer: Other amusement park rides besides roller coasters, or determining how high to build the next hill on a roller coaster, or how to launch something, etc.)

Activity Extensions

So far, students have calculated the mechanical energy when it is either completely potential or kinetic energy. What about when the mechanical energy is composed of both? Have the students create a table and/or graph (depending on their skill level) showing the potential and kinetic energies of their pendulum at heights of 0, ¼h, ½h, ¾h, and h. (Hint: They should already know the values at heights 0 [purely kinetic] and h [purely potential].)

Activity Scaling

  • For lower grades, work through the calculation of average time. Also, explain and derive Vt for them and use the worksheet without algebra.
  • For upper grades, use the worksheet with algebra.

Contributors

Chris Yakacki; Malinda Schaefer Zarske; Denise Carlson

Copyright

© 2004 by Regents of the University of Colorado.

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

Acknowledgements

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

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