Force and motion

MechAnimations

6. Combining Mechanisms

Overview

Students now build on the “same” type and “opposite” type constructions, as they learn to combine two mechanisms into one. They create pegboard mechanisms that feature a common input controlling two outputs. They compare the directions of motion of the two outputs, and learn how a system can be built up from two simpler subsystems.

Materials

Pegboard base, three strips and fasteners for each student
Post-its: red and yellow
Science notebooks
Combining Mechanisms Worksheet

Procedure

1.

In a class meeting: Demonstrate two links joined with a pivot. Ask:

How should I connect these links to a base so one moves the other? Do whatever is suggested as long as it involves only one fixed pivot. Whichever link is attached to the base is the lever, the other the input link. Operate the input link to make the lever move. Draw the resulting mechanism on chart paper. (See a video on attaching the link and drawing it ) This input operates one output. I want this input to operate two outputs. How can I connect a second output link? Tell me how to add the second output to this drawing I have. Follow a student’s instructions, using a red marker to add a second lever, using whatever pivots are suggested. (See a video on drawing a second output). Add the second lever to the mechanism as suggested by the students and shown in the drawing. (See a video on adding a second output and testing the mechanism.) Which direction will the output on the left move when I move the input to the right? What about the output on the right? Operate the 2-output mechanism to see what happens. Talk about the results, especially if they are not what are expected.

2.

Distribute a base, three links and pivots to each student and present this challenge: Make a mechanism with one input that controls two outputs. After students have made a mechanism they draw the mechanism in their science notebooks.

Draw the mechanism with one input and two outputs.
Show the fixed pivots and the floating pivots. Draw an arrow showing the input moving to the right and an arrow over each output showing the direction it moves when the input goes to the right.

3.

Whole-class discussion: Select students to present their mechanisms so that each of the types shown on the Combining Mechanisms Worksheet is represented. (See a video on four mechanisms) For each mechanism, draw the base and links on chart paper. Ask students where to place the pivots and how to draw them. (See a video on drawings of four mechanisms.) Ask how to show the directions of motion. Put the drawings of the four types of mechanisms side by side. Conduct a discussion about why each of the outputs moves as it does. Ask:

How can you be sure that two outputs will move in the same direction?
How can you be sure that two outputs will move in opposite directions?
What would happen if we made the input move in the opposite direction?

4.

Systems and sub-systems. Explain that each mechanism with one input and two outputs is a complex system. This complex system is made from two simple systems and that each simple system has one input and one output. The simple systems are sub-systems of the two output system. Here is a video on these systems and subsystems.

5.

Ask:

What are the simple systems (“same” or “opposite”) that are combined in each of the four drawings?
What has happened to the inputs of each simple system?

Outcome

The simple lever systems “Same” and “Opposite” can be combined to make complex systems where, relative to the direction of the input, the two outputs go “Same – Same”, “Opposite – Opposite”, “Same – Opposite” and “Opposite – Same”.

Students should discover that:

To make two outputs move in the same direction, their fixed pivots must be on the same side of the input link, either both above the input (between the floating pivot and the output) or both below the input link .
To make two outputs move in opposite directions, their fixed pivots must lie on opposite sides of the input link.

Science Notebooks Complete the Combining Mechanisms Worksheet and place in the science notebook.