Overview
Topic Area(s) / Grade Level / Cost / Time / Complexity / Objective
· Engineering, Physics.
/ Grade 4-6 / 0.15$/person / About 45min-1hour / Preparation=2Execution=2 / Introduction to basic mechanics.
Hook
Have you ever wondered what kind of mechanism causes the wind shield wiper on the front widow of car to oscillate? Or even better – how come bicycles, trains, or space shuttles actually work? Well, the truth is, lots of them involve many mechanisms.
Necessary Background InformationEngineers, and particularly mechanical engineers study movement and can predict for example how many turns a train’s wheel has to turn in order to travel a certain distance. The cool part is that predicting doesn’t involve any magic, but actually, everything can be explained by mathematics! So think about it. Every machine that you see, a crane, a tractor, a train has been studied carefully and engineers calculated at what speed, distances will each component of a machine move.
So what is actually a mechanism?
Well, mechanisms constitute machines and they’re made of a combination of resistant bodies, geometrical shapes, formed and connected, which move with relative motions with respect to another. You’ve probably encountered simple machines as the lever, wheel and axle, pulley, inclined plane, wedge or the screw.
Every machine has two primary functions: they have to transmit a relative motion and transmit a force. Since you need strength and rigidity to transmit these forces, the choice of material you’ll use is often important. That’s why many machines are made of steel.
Now, some more useful information. A link is defined as a rigid body having two or more pairing elements which connect it to other bodies for the purpose of transmitting force or motion (Ham, C. W., Crank E. J. and Rogers W. L., Mechanics of Machinery, McGraw-Hill, 1958.)
Now, what would a link do? It’s supposed to produce rotating, oscillating, or reciprocating motion from the rotation of a crank or vice versa. And speaking in a more concrete way, they can convert:
1. Continuous rotation into continuous rotation. (Think in which machine that could be useful… hmmm..)
2. Continuous rotation into oscillation or reciprocation (or the reverse). (Like windshield wipers in your car!! A motor turns a crank and the links transform that rotation into a window-washing movement!)
3. Oscillation into oscillation, or reciprocation into reciprocation. (Once again, try to think where you could use such a mechanism…)
A mechanism composed of rigid bodies and joints is called a linkage. Our mechanism in particular, as we shall soon see, will be able to transform a rotation (i.e. a CD spinning) into a horizontal motion (i.e. a bar moving in and out… like a HAND coming straight out at you…).
Materials· 1 useless CD
· Cardboard
· 3 popsicle sticks
· 4 standard beads (with a hole in them)
· Coloured paper (a bit stiff)
· Glue gun and glue sticks
· Drill or scissors to puncture one hole
Safety Considerations· Be careful while using the glue gun…..it is very hot!!
· Cutting the hole with a drill or scissors can be dangerous!
Procedure· PART A: Make a cool base!
1) Cut out a 40 x 20 cm rectangle out of the cardboard.
2) Cut a hole in it approximately about 1.2cm wide, and 12 cm from the bottom, 12 cm from the side. That’s where the CD is going to be attached.
· PART B: Make it spin!
3) Cut a strip of colourful paper. It should be about 3cm wide and 20-25cm long.
4) Now, roll it up in order to make a cylinder. You need to make sure that it fits just right in the hole of the CD as well as the hole in your cardboard! This will be the shaft on which your CD will be spinning!
Note : make sure that your cardboard shaft does not stick out too much from the CD! It should protrude only about 5 mm. Otherwise it will interfere with the movement of the popsicle sticks we’ll soon add!
PART C: Now comes the rest!
5) You’ll need the 3 popsicles now. Place two popsicles end to end, and connect them by gluing a third popsicle stick centered on top.
6) Now, glue the 4 beads and the film container cap as shown in the picture below.
PART D : Finishing touches.
5) You’re almost done your extra-cool mechanism! Now glue the bead (indicated below) which is glued already to the popsicle stick assembly onto the CD.
6) So now, you need to restrain the movement of the “link” (i.e. popsicle stick assembly). You will do that by cutting out some cool shape (it can be as simple as a sheet of paper bent in half). Just remember that it needs to be rigid enough to restrain the movement of your link!
Now’s the trick: you have to place your folded paper in a strategic position, so that as you turn the CD, the link follows, but doesn’t get stuck!!
7) Finally, creativity time! You can attach anything you wish to the end of the popsicle stick assembly and create… a creepy hand coming out of a sleeve, a cute turtle crawling out if its shell… make it yours!
Congratulations! You did it!
…one and a two and a …. …ooh…
…. Is it a bird, a plane… …no! it’s a worm?...
Learning OutcomesThis project helps you understand more clearly how mechanisms work and makes you see how simple or complex in real life they can actually be!
Possible variations of the project:
To modify this project, you can create other linkage mechanisms. Simply use four bars of different lengths (made out of popsicles), then glue one of the sides on some cardboard. Move the other links.
Depending on which side you flue on the cardboard, a different motion will be created. From this different applications are possible.
Check out these sites:
http://www-2.cs.cmu.edu/People/rapidproto/mechanisms/chpt8.html
http://www-2.cs.cmu.edu/People/rapidproto/mechanisms/chpt5.html
http://www-2.cs.cmu.edu/People/rapidproto/mechanisms/chpt1.html