Archive for March 2012
When we did the egg drop challenge a couple of weeks ago, I asked the students to write about their design and the concepts involved in safely landing the egg in their structure.
For them, they had fun and were rewarded for their hard work with no lab report, just a dialog of what they built, why they built it, and the concepts we’ve been studying. I wanted them to talk about forces, gravity, momentum, impulse, collisions, and any other concept we’ve studied in order to explain the physics behind the effort to save the egg.
I’m thinking the egg got off easy. I had to read phrases like “depending upon how fast you dropped the egg,” and “the impact of momentum, ” and best (worst) or all, “the egg has many things to be concerned about it not to break”
Other than labs, I haven’t given a writing assignment before and I now think it needs to be a regular event. Clearly the students can not talk about the concepts. Although we spend weeks problem solving, discussing, and working in the lab, they can’t put the concept into an intelligent sentence. How did this happen? I feel like I’ve failed.
I like saving money for my school. Nothing against Carolina Biological, we’ve been really pleased with their equipment and service, but sometimes their stuff is just way overpriced. Their Hooke’s Law Device is $35 each. I made a similar set of devices for a couple of dollars using what we already had in the lab and classroom.
My device is made with a ring stand and small rubber bands to hold a ruler in place. I used a pendulum clamp as the top support, but any clamp will do. To connect everything, I used a bit of chain that comes with a shop fluorescent lamp and opened up the links. These are in an S shape and twisted to be offset by 90 degrees. You could just as easily purchased a package of S hooks at Home Depot for a couple of dollars. At the top, I hang one side on the clamp and hook the spring over the other opening. Same on the bottom, the hook provides a place to hang a weight.
We use hanging weight sets, I just didn’t have them hand for the picture, this is one of my 500g medicine bottle weights. The springs come from Harbor Freight. They have a box set of 200 springs for about $5. Lots of springs to play with, many look alike but have very different spring constants.
My “indicators” were fabricated on the 3D printer, but you can just as easily make the same thing with two Popsicle sticks and some glue. Drill the holes or it will split.
That’s really the whole thing. I found that if I spun the whole setup one rotation CCW, the spring would try to rotate clockwise, holding the indicator against the ruler and making it much easier to read.
I’m planning on adding a scatter chart to this lab. The students will enter the points in Excel to create the chart and then plot a trendline. We can then use the first order trendline to determine an unknown weight based on the distance the spring stretched. This setup was inexpensive and effective.
Last year I gave all my physics classes the first toy popper problem (1st popper lab). I learned my lesson and only the honors group gets the inquiry version of the lab. This year I followed on to that lab with a second popper lab when we got to impulse. The non-honors class doesn’t get to play with poppers until they get to this lab.
I made it a lot easier for the non-honors group by explaining the steps to calculating the time and acceleration of the popper from just the height and the mass. The honors group got to confirm their methods with the help of my written procedures and they got to test the accuracy of their predictions. As a result, the lab works both as a stand-alone lab or as a follow-on from the previous lab.
About half of the students take calculus (with me) and I took the time to explain how the area under the impulse curve is really the impulse because it is the force (y-axis) multiplied by the time (x-axis). While they wouldn’t have seen that on their own, I think they all understood that the change in momentum (impulse) = Ft.
So here is the latest incarnation of the lab for you to use and enjoy.