Force and motion


3. A Mechanism with an Input that Controls It


Children begin Lesson 3 by sharing mechanisms they have made. They develop the ideas of input and output by discussing everyday things. Then they figure out how to use another piece to move their mechanism back and forth. To do so, they will have to invent the floating pivot, which attaches two pieces (links) to each other but not to the base.


  • Classroom set of pegboard Mech-a-Blocks
  • One Butterfly-Net MechAnimation
  • Post-Its™ or markers


  1. Inputs and Outputs:
    In a whole-class meeting, students show mechanisms they have made and what they need to do to make them move. They describe what each one does to make his or her mechanism move. Then students think of other things they push or pull to make them work. Examples might include lights, pencil sharpeners, doors, or retractable ball point pens.

    • What is the part you have to operate on each of these?
      List the names of the parts of the mechanisms that you push or pull or turn, then give a general name for these parts: these are all inputs to the mechanism.
    • When you operate the input of a light or pencil sharpener or TV, what is it you want to happen?
      Develop the name for what you want to happen: the output.

  2. Levers and Handles:
    Demonstrate the Butterfly-net MechAnimation. Ask students:

    • What story is this telling?
    • What do you think is making the net move?
    • Where is the input? How do you know?
    • Where is the output? How do you know?
    • *How does this input control the output?
      Then challenge them: Invent a way to make an input, or a handle, for your own mechanism. As an example, hold a pegboard strip near a mechanism:
    • How could I use this strip as the input to this mechanism?

  3. Class Meeting:
    Students show how they have met the challenge of making a handle (the input) for their mechanisms. They are likely to think of simply pushing the lever with the input, rather than with their fingers directly. Congratulate them on that solution, but then use their mechanism to demonstrate what happens when you try to pull the lever back with the input. Because they are not connected, the input will not be able to pull it back.

    • What’s wrong with this?
    • Why doesn’t it work when I try to make it go the other way?
      Challenge them to think of a way around this problem, and provide a few more minutes for them to experiment. In these videos Travis introduces fixed and floating pivots. Later he introduces adding a handle.

  4. The Floating Pivot:
    At least one student will probably come up with the idea of attaching the input to the lever with a fastener that goes through the two moving parts but not through the base. We will call this type of pivot a floating pivot. If students have not invented the floating pivot yet, make such a mechanism yourself.
    Lead a discussion based on what children have made.

    • _What do you think about _____’s mechanism? Where is the input? where is the output? What controls what?
    • Did anyone make anything similar? What was similar about it? What was different?
    • _Did anyone have the same problem ______ did? What did you do to solve it? What advice do you have for ______ ?_

  5. Science Notebooks:
    Using words and pictures, explain how you made a mechanism with an input, which controls it.

  6. Outcomes:
    Students should be able to:

    • Identify the input and output of a mechanism,
    • Tell if the input is a separate piece that controls the output (or is just another part of the same piece), and
    • make a mechanism with a separate input

  7. Assessment:
    Demonstrate a simple lever, attached to a base with a fixed pivot and ask;

    • Where is the input and output?
    • How would you add an input that could control the lever?