- cardboard for making a model first
- bolts, nuts, washers
- plastic sewing machine bobbins
- chalkboard paint
- screw eyes
- conduit clamps
Step 1: Constructing a model
Here is one of the earlier iterations...
Step 2: Making a Paper Pattern
Step 3: Cutting out the Plywood
Step 4: Prime and Paint the Wood
First I gave each piece a thorough coat of primer, then I sprayed them with two coasts of black chalkboard paint.. The chalkboard paint gives a very tough surface and students will be able to mark on the arm with chalk if needed during their investigations. This really helped with the appearance especially since I used plywood.
Step 5: Dill the Holes
Make sure that the holes are slightly smaller than the bolts you are using so they fit in tightly.
Step 6: Adding the Pulleys
For the pulleys, I used plastic sewing machine bobbins. To attach them to the piece I put washers on the top and bottom of the bobbin and threaded the whole thing through a bolt. I fastened the bot with a nut on the other side of the plywood and left it loose enough that the bobbin could turn freely.
Since you are not actually using the bobbins with a sewing machine it doesn't matter which size or model you purchase but some bobbins have seams inside the spindle, I avoided those because I was concerned that the cord might catch on that.
Step 7: Attaching the Pivots between the Pieces
To accomplish this, I put a bolt through the top piece, tightened a nut to the bottom of the first piece, put the end of the bolt through the second piece of wood and then another nut on the bottom. I put washer under the head of the bolt and under each nut to allow the joint to easily rotate.
Step 8: Adding the Screw Eyes
Be careful to choose screw eyes that will not stick out the back of the plywood because the pieces will be sliding past one another and also the tip of the screw eye is pointy and could be a safety hazard if it sticks out the back.
Step 9: Stringing the Model
One goes along the back of the arm and the other the front of the arm. I tied loops on the end of the string to allow a force scale to be easily hooked onto the ends of the cords to monitor how much force is required to different motions of the arm.
Step 10: Mounting the Model to a Ring Stand
I pre-drilled holes in the plywood and used washers, bolts, and nuts to attach the clamp.
To make sure the two clamps are aligned with each other, I put a marker through the two clamps before marking where to drill the holes for the bolts.
I wanted the head of the bolt to be on the front of the model so it was rather difficult to tighten the nut, you could reverse this but I didn't want the students to fiddle with the nut and keep taking off the clamp just with idle hands during their investigations.
Step 11: Make a Few More for Each Group can Have One
Step 12: Lesson Ideas for the Arms
Here are just a few ideas I have, I plan to introduce the models to the students will a structured inquiry activity and then lead them to open inquiry. I plan to combine the arm model with force probes and video cameras so students can record the force required and use video analysis to analyze the motion and energy of the "hand" of the arms.
- Investigate how the length of the arm effects the force required to left an item.
- Determine the work done to lift something to elbow level.
- Determine the power developed when lifting an object.
- Determine the work done when throwing a ball underhand vs. overhand and compare the kinetic energy imparted to the ball in each case.
- Compare the work done by the bicep vs. the tricep for different motions of the arm.
- Determine the mechanical advantage of the arm when the cord is tied to different screw eyes.
- Determine which motions of the arm (lifting, pulling, pushing, curl, throwing overhand, throwing underhand) use which of the muscles of the arm and if they use both when ratio of muscle use is used.
- Figure out the function of each of the joints and muscles.
- Which actions of the arm are examples of first class, second class, and third class levers and what are the advantages of each of these types of levers?