Purpose: Engineering Tests

Last week (or maybe earlier) we fabricated a variety of pinwheels and you explored whether your ideas about what might make them work better held air (so to speak). If you have a new design of a pinwheel that you suddenly feel an urge to explore you are certainly free to do so as long as you also get the tests that we are doing this week completed. You have some sense of which pinwheel spins more easily but the goal this week is to be able to make clearer and more scientific distinctions between the different pinwheels. Ultimately the task of a pinwheel is to move energy and not just spin around and look pretty.

An example of this is a car battery. When a car battery wears out it still provides 12 V which is what it is supposed to do but when you put it under load (asking it to deliver energy) is just can't keep up. Even people can look like they're doing find but if you put them under "stress" you find out whether they can actually deliver. Pinwheels are the same. To put a pinwheel under load we are going to attached it to the axle. Then we will attached a string to the axle. As the pinwheel spins it will wind the string around the axle. If we attach a weight to the end of the string then the pinwheel will have to lift that weight as it spins and the string wraps around the axle. There are two features to think about as the weight is being lifted. Our primary concern is how long does it take the pinwheel to lift the weight through a known distance? If it lifts a bigger weight faster then it must be generating more power than one that is lifting a smaller weight in the same amount of time.

You may need to quickly make a new pinwheel for today's lab. No doubt you are experts now so that will be no problem.

Procedure:

I) String and Hook: The first task is to work out how you are going to attach your string to the axle. This needs to happen so that the axle will wind the string up without jamming the system in any way. At the same time you will attach the pinwheel to the end of the axle using magnets as we did before. When this is complete do some quick testing to be sure that the pinwheel can reliably wind the string up with a 20 g weight attached. You will need to determine where you are holding your pinwheel relative to the "wind machine" (distance and orientation) so that you can reproduce this in future tests. Be sure to record which fan you are using, what speed it is set at, and how far away you are holding the pinwheel. This is your test setting.

II) Testing I: With your initial weight attached to the string time how long it takes for the pinwheel to life the mass through a known distance. This involves some planning so that you know how far the weight is being lifted. Somebody is likely to have a stopwatch app on their phone to measure the time and meter sticks will be available to measure distances. Repeat this measurement at least 3 times at the same test setting and record the time for each test.

III) Testing II: Repeat the same test with a weight which is at least 50% bigger or smaller than your original test and record the time needed to lift the weight. Be sure you are using the same test setting.

 

IV) Analysis I: As always in science the last step is an effort to make sense of the data you have. Consider the results of the part II.

Use the following formuala to determine the power output of your pinwheel for each test in part II.

What is the average power output (in mW) of your pinwheel for your test setting.

 

V) Analysis II: Here are some questions to consider about your power output data and that of your classmates.

Find out what another group's result was for their pinwheel. Explain why their number is different than yours ...did they use a higher fan speed? ...were they closer or further from the fan?.....was their pinwheel bigger or smaller than yours?

How many hours would you have to run your pinwheel to generate 1 kWh of energy? Be sure to present your units as you do this calculation. Given the numbers I've seen in the past you should expect this number to be years -- not a good sign that your pinwheel has commercial potential!

LAB DELIVERABLES (turn in):

Turn in your descriptions, data, and observations in one document (thru Turnitin on Bb) that show your work on each of the steps given above. Drawings of your experimental setup can be inserted as images or drawn using a simple illustration tool if you have access to one. In places where you need to show your data create a table with labels for appropriate columns and rows. Be sure that your conclusions are based on the data shown in lab report.