Hands-on Activity: Bend That Bar

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

A diagram shows a force (downward arrow) acting on an I-beam, causing it to bend.
Bending of an I-beam.
Copyright © 2007 Avenafatua, Wikimedia Commons http://commons.wikimedia.org/wiki/File:I_beam_bending0.png


Students learn about material properties, and that engineers must consider many different materials properties when designing. This activity focuses on strength-to-weight ratios and how sometimes the strongest material is not always the best material.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

Finding the best engineering solution for a given design challenge requires a delicate balance between many factors such as weight, strength, cost, performance, safety and ethics. For example, the strongest design might be too expensive, or the safest design might be too heavy. An engineer's final design is a compromise between all considered criteria. The strength-to-weight ratio is an important balance engineers must find when designing airplanes because we want airplanes to be very strong, but also as light as possible.

Learning Objectives

After this activity, students should be able to:

  • Explain the significance of a material's strength-to-weight ratio.
  • Compare and contrast how useful a material is for a project based on its strength, weight and other properties.
  • Measure and record mass, height and distance of a bending bar.

More Curriculum Like This

May the Force Be with You: Weight

Students study the properties of common materials and why airplanes use specific materials. This lesson helps students understand the relationship between the mass and the weight of an object.

Close Encounters of the Polymer Kind

Students explore the basic characteristics of polymers through the introduction of two polymer categories: thermoplastics and thermosets. During teacher demos, students observe the unique behaviors of thermoplastics.

Exploring Nondestructive Evaluation Methods

Students learn about nondestructive testing, the use of the finite element method (systems of equations) and real-world impacts, and then conduct mini-activities to apply Maxwell’s equations, generate currents, create magnetic fields and solve a system of equations. They see the value of NDE and FEM...

May the Force Be with You: Thrust

Students study how propellers and jet turbines generate thrust. This lesson focuses on Isaac Newton's third law of motion for every action there is an equal and opposite reaction.

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.

  • Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Understand the concept of a ratio and use ratio language to describe a ratio relationship between two quantities. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently divide multi-digit numbers using the standard algorithm. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Students will develop an understanding of and be able to select and use transportation technologies. (Grades K - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Appropriate measurement tools, units, and systems are used to measure different attributes of objects and time. (Grade 4) Details... View more aligned curriculum... Do you agree with this alignment?
  • Formulate, represent, and use algorithms with multi-digit whole numbers and decimals with flexibility, accuracy, and efficiency (Grade 5) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use the particle model of matter to illustrate characteristics of different substances (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use tools to gather, view, analyze, and report results for scientific investigations about the relationships among mass, weight, volume, and density (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Materials List

Each group needs:

  • Bend That Bar Worksheets, one per person
  • (optional) spring scale (or groups take turns if fewer are available)
  • 1 bar 15"-18" (38-46 cm) in length and 1/8"-1/4" (3-6 mm) in diameter of each of the following: steel, aluminum and wood, and if possible, plastic, brass and copper; the metal rods and wooden dowel can be found at most hardware and hobby stores
  • 1 Sharpie® or marker/pen to write on the bars
  • yard/meter stick
  • (optional) calculator
  • bathroom or top-loading scale to weigh individual bars


Engineers must carefully select the materials they will use to build different parts of airplanes.The body and wings may be made from a metal such as aluminum. The seats and storage bins may be made from some type of plastic. Every material in an airplane must have some level of strength to perform its task. The wings must hold the weight of the entire plane, while the seats need only support a single person. No matter what the purpose, every object in an airplane must be made from the lightest materials possible and yet be strong enough so that it will not break apart during normal use and, of course, flight.

In today's activity, you will see how different materials have very different physical properties and how engineers must chose which material has the best properties for each part of a bigger design project, like creating an airplane.


Note: This activity may be conducted in English or metric units (1 inch = 2.5 cm; 1 cm = .4 inch), but since scales usually measure in grams, it may be easiest to do this activity entirely in metric units.

Before the Activity

  • Cut each bar into the desired lengths if they are not already in the specified lengths.
  • Make sure the scales are available when you need them and have been recently calibrated.
  • Make copies of the Bend That Bar Worksheet, one per student.

With the Students

  1. Review the correct use of the scales so that students get accurate results. It is helpful to demonstrate the experiment using one bar so students understand the process better.
  2. Have students measure the weight of each bar and record on the worksheet. Measure the length of each bar, and make a mark 10 inches (about 25 cm) from one end and 1 inch (2.5 cm) from the same end.
  3. Have one student hold a bar against the top of a table or chair so that 10 inches (25 cm) sticks out from the table or chair. They need to hold it fairly tightly.
  4. Have another student hold the yard/meter stick vertically next to the bar so that the yard/meter stick rests on the ground. This is so students can measure how far the bar bends. Assign one student the job of reading the measurement, kneeling to be at about the same height as the bar to see it more easily. For best results, place the yard/meter stick at the very tip of the bar.
  5. Have students record the initial height of the bar. Make sure to measure the height at the top of the bar each time.
  6. Have the fourth student hang the scale 1 inch from the end of the bar on the mark made previously. If 1/8" (6 mm) bars are being used, expect the weight of the scale to be sufficient to noticeably bend the bar. If the wooden bar does not bend more than 1 inch, have students pull on the scale with 1 to 5 pounds so that the wooden bar bends more than 1-1 ½" (2.5 to 3.8 cm), but does not break
  7. Have students record the new height of the top of the bar.
  8. Have students repeat #6 and #7 with the remaining bars.

A photograph shows a student using a meter stick to measure the deflection of an aluminum rod after applying a force to one end.
Figure 1. A student measures the height of a bar after bending.
Copyright © 2003 Tom Rutkowski, University of Colorado Boulder

  1. Have students complete the remainder of the worksheet questions.
  2. Discuss the worksheet answers as a class. Have students give examples of their values for how far each bar bent. Discuss what they observed about the weight and strength of each bar. Expact that students observe that aluminum and steel have similar strengths. The aluminum bar, however, is much lighter. Discuss how a high strength-to-weight ratio is desirable for building airplane parts. Aluminum has a higher strength-to-weight ratio than steel because it has similar strength but it is much lighter than steel. Strength-to-weight ratios are important to consider when designing airplanes because aircraft must be as light and strong as possible.


Safety Issues

Thin wooden dowels break easily if too much pressure is applied; if breakage occurs, have students discard broken pieces immediately, as ragged ends can be dangerous.

Troubleshooting Tips

Using bars that are 1/8" (3 mm) diameter is recommended. Bars this size are thin enough to bend easily without becoming permanently deformed; however, a wooden dowel this small is usually not perfectly round and will likely break under the weight of the scale. A shorter section of bar may be used instead of a 10-inch (25 cm) section to reduce the likelihood of breaking the wood bars.

In order to compare different materials, it is important that all bars are the same diameter and length. Avoid comparing solid and hollow rods.

You may want to use c-clamps to attach the rods to the tables because with some of the stronger rods, a lot of applied force will be used and and it may be difficult to hold them down.

Spring scales can be purchased for about $6 each from any science education product catalog. If a spring scale is not available, students can just bend the materials until they almost break (clearly, unless supplies are unlimited, it is important to stress to students that they NOT break the bars).


Pre-Activity Assessment

Prediction: Before the activity, hold up 3-6 bars of various materials and ask students to answer the following questions. Record their answers on the classroom board:

  • Which material would you predict is the strongest?
  • Which material do you think weighs the least?

Activity Embedded Assessment

Worksheet: Have students record measurements and compare and contrast materials on the Bend That Bar Worksheet. After they have finished the worksheet, have them compare answers with their peers. Discuss as a class.

Post-Activity Assessment

Ranking/Discussion: Have the class rank the materials in order of their strength-to-weight ratio on the classroom board or using the overhead projector. Additional discussion questions:

  • List one example of a good use for each material. Explain why. (For example: Steel is good for structures since it is strong and does not bend much.)
  • Which material would you use to build an airplane? Why? (worksheet question 9)
  • Could different materials be used to build different parts of an airplane? Why?

Activity Extensions

If you have thin bars you may choose to have students pull each bar until it bends permanently or breaks. (Note: have students be very careful if they break the bars! Ragged ends are very sharp.) The point where it bends permanently is called the yield point. Have students measure the force needed to break each bar and divide this number by the weight for a new strength-to-weight ratio. This is more accurate than the above procedure because a material may be very stiff but break under less total force than a material that is more elastic. However, this is still not exactly the strength-to-weight ratio used by engineers because it is dependant on the length and thickness of the bar. The strength-to-weight ratios used by engineers use equations to make sure the same material gives the same measurement no matter the size of the material being tested.

Have students convert their results between metric and English units.

Activity Scaling

  • For younger students, this activity may be more effective as a demonstration because of the complicated procedure. Have students in the class volunteer for jobs as the teacher demonstrates the activity. Discuss the results as a class.
  • For older students, require them to make bar graphs with the strength-to-weight ratio on the vertical axis and the different materials on the horizontal axis. This gives students a graphical representation of their results.


Tom Rutkowski; Alex Conner; Geoffrey Hill; Malinda Schaefer Zarske; Janet Yowell


© 2004 by Regents of the University of Colorado

Supporting Program

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


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

Last modified: May 25, 2017