4. Representing Mechanisms

Force and motion

MechAnimations

4. Representing Mechanisms

Overview

Students make new mechanisms. They are challenged to represent their mechanisms by a drawing. The drawing must be clear enough so someone else could use it to make the original mechanism.

Materials

Pegboard bases (one per student), strips (three per student) and fasteners

  • Teacher mechanism and drawings of the mechanism
  • Science notebooks

Procedure

1.

Make more mechanisms. Distribute the materials. Tell students they are to use what they learned in Lesson 3 and make more mechanisms where one link will control another link.

2.

Draw your mechanism. After 10 minutes, students draw the mechanism they have made.

3.

Discuss mechanisms and drawings of mechanisms. a. In a class meeting, brainstorm reasons why it might be important to have a good drawing of your mechanism. Reasons could include:

  • I can remember what I made, so I can make it again.
  • I can show what I made to my Mom.
  • I can show it to my friend so my friend can make it.

b. Demonstrate a teacher-made mechanism and three drawings of the mechanism (see videos) – Discuss the problems of each drawing.

  • In the first drawing, there is no correspondence between where the links and pivots are placed, and where they are in the mechanism. Discuss “How should I change where I drew the input link? the lever? the pivots?”
  • In the second drawing, the input link, the lever and the pivots are placed about where they are in the mechanism, but all pivots look the same. Discuss “Does this drawing tell you how to make this mechanism?” If students see no need to distinguish fixed pivots from floating pivots, make the mechanism using all fixed pivots. Demonstrate the mechanisms and ask what is wrong. Develop the idea of distinguishing fixed pivots (l) and floating pivots (m).
  • The third drawing has the links in the proper positions and the correct symbols for fixed and floating pivots. However, holes on the pegboard are included in the drawing. Discuss “How much of your mechanism do you need to show, in order to explain how to build it?”

4.

Re-draw mechanisms. Students re-draw their mechanisms in their Science notebooks so that anyone could make their mechanism from the drawing.

Outcome

Each student should be able to:

1.

Make a drawing of a mechanism from which another could make a copy of the mechanism.

2.

Explain what a good drawing must have.

  • The links must have the same position on the drawing as they do on the mechanism.
  • The pivots must be in the same location on the drawings of the links as on the mechanism’s links.
  • The fixed pivot and floating pivot must be shown with different symbols.