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Forces on the Human Molecule

Grade Level: 7 (6-8)
Group Size: Not defined
Time Required: 100 minutes
Summary: Students will conduct several simple lab activities to learn about the five fundamental load types that can act on structures: tension, compression, shear, bending, and torsion. In this activity, students will play the role of molecules in a beam subject to various loading schemes.
Engineering Connection: Engineers must consider many forces when planning and actually building a structure. This lesson introduces students to several of these important forces.

Materials and structures can sometimes fail when subjected to large enough loads. Each different type of load can cause its own mode of failure. Stresses, fatigue, and failure can sometimes be seen with the naked eye on the surface of the member or structure. However, the movement of tiny individual molecules is where the real failure begins.

Vocabulary/Definitions
Elastic: The ability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.
Fracture: A break, split, or crack in an object or a material.
Inelastic: The inability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.
Procedure

Modeling Loads on Structures Using “Human Molecules”

-Each person will represent a molecule of steel inside a steel bar and their arms will represent the internal bonding forces, which hold molecules together (a molecule is the smallest piece of steel that can exist with the chemical and physical properties of steel – billions of molecules link together in lines to make a piece of steel).

1. Form two lines of ten people each, lining up side by side, facing each other (see diagram). These two lines represent a structural element. Each person must use his/her left hand to hold hands with the person whom they are facing in the other line. Each person should then lock his/her right arm around the arm of the person on his/her right. See Figure 1.
Human Molecule Figure 1

te.jpg

2. Four other students will act as an applied load. Position one student at each end of both lines, and have them pull with equal force (if possible). Have the students pay attention to what they are feeling while the molecules are being pushed and pulled. Next, form the same lines again, but have the four people applying the loads push equally on each line end. The job of the molecules is to try to maintain their original formation, like a solid non-elastic object.

Group Discussion:

-What type of load did you model this time?
-What did it feel like to be a molecule inside the material?

te2.jpg

3. Now have the “applied load” students pull one line of molecules to the left, and the other line of molecules to the right (as shown in the Figure 2).

Group Discussion:

-What type of load did you model this time?
-What did it feel like to be a molecule inside the material?

Investigating Questions

Group Discussion Questions are embedded in the above activity procedure.

1. Describe the fundamental loads and the effect each load (of force) has on a structure or structural member (or component).
2. Give real life examples of tension, compression, shear, bending, and torsion.

Assessment

Students may receive credit for participation in group discussions and answering Investigating Questions.

Owner: K-12 Outreach Office, Worcester Polytechnic Institute

Contributors: Funded by, Pratt & Whitney

Copyright:  2005 by Worcester Polytechnic Institute
including copyrighted works of other educational institutions; all rights reserved.

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