3. Making One Link Control Another

Overview

Students try to build a pegboard mechanism in which one strip controls another. In order to do so, they will not only need two strips, but also need to invent a new way to use a fastener.

Materials

Pegboard bases (one per student), strips (three per student) and fasteners
One Hammer MechAnimation
Science notebooks

Procedure

Demonstrate a Hammer MechAnimation, without showing what’s inside: Ask the class: What makes the Hammer work?
Making one strip control another: Provide each student with a base, three strips and fasteners. Develop the concept of control. Set the design challenge: Make a mechanism using at least two strips. Make one strip control the other.
Whole-class discussion: Students share their constructions. Highlight those in which one strip does actually control another, vs. those where it doesn’t happen. Here are constructions that do not meet the challenge, and a way to make one lever control another. As appropriate, revisit the distinction between structures and mechanisms (see Lesson 2). For those mechanisms where one strip does control another, focus on the pivots. One pivot goes through the base and fixes the mechanism to the base. We call it a “fixed pivot”. The other pivot connects two strips together, but not to the base. When the mechanism moves, this pivot moves over the base. We call it a “floating pivot.” As questions and issues come up, record them on chart paper. Examples of issues might include:
- Some strips can’t move.
- I can’t make one control another.
- I can make one move the other by pushing, but not by pulling.
- The end of the lever does not move in the right direction.
- It doesn’t move far enough.
- I would like my mechanism to have more than one output, like the MechAnimation.
- The strips can move around too much.
  
  These should be kept for future reference, and used as a basis for further exploration and design. It is far more important that children continue to think about these problems than it is for them to have “an answer.” See Troubleshooting for solutions to each of these issues.
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 **Outcome**
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This lesson develops the idea of control: one strip can control another only if it can make the second one move in either direction. Students should discover that in order to make one strip control another, they will need to use a fastener that connects one strip to another, but not to the base.

Science Notebooks

Here are some prompts for the science notebook.

Troubleshooting

Here are some solutions that don’t satisfy the design challenge.

Sometimes students solve the design challenge, but want more.

Force and motion

MechAnimations