Physics & Physical Science Demos, Labs, & Projects for High School Teachers

Good Intentions, Bad Lab

Posted on: February 28, 2012


A couple of weeks ago we did a lab straight from the textbook.  (Here is the Lab Instructions, typed up and put it into my words.)  I’d been looking for a good Conservation of Energy Lab.  I wanted to use the Vernier devices, but there wasn’t anything in their book that I liked.  Rather than make something up from scratch, I decided to work directly from the Holt Physics textbook.

The lab had two parts; the first was to calculate the spring force constant using a Hooke’s Law device.  I didn’t have that device, so I created my own by first designing a simple indicator on Solidworks (3D CAD Software) and then printing eight of them in the 3D printer.  That worked out great.  (I will try to remember to post my Hooke’s device design, you can build them for about $0.25 each.)  Unfortunately, it was the only part of the lab that worked out at all.

We were able to use a ring stand, ruler, and indicator to successfully calculate the spring force constant.  The second part of the lab was supposed to demonstrate conservation of mechanical energy by bouncing the weight and measuring the high and low point.  Quite honestly, it just didn’t make sense.  At first it did, but the more I thought about it, the less sense it made.

To begin with, it was nearly impossible to measure the bottom and top of the bounce with any accuracy.  It was pure guesswork and the kids were really struggling.

This is a great bunch of kids, I warned them ahead of time that this was the first time using this lab, there might be some hiccups.  They were understanding and really tried to make this work, but they were totally frustrated.  I told them I would grade the lab on their effort, spring constant results, qualitative analysis, and attempt at explaining the results.

Clearly, I need a much better lab for next year.  I was originally thinking of calculating the spring force constant, then determining the weight of an unknown object based on distance the spring stretches.  That makes sense for the chapter on oscillations, but not for Conservation of Mechanical Energy.  I don’t have an air track, but I’m really good at McGuyvering solutions, as you all well know.

Please throw your awesome labs my way, I need help.

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6 Responses to "Good Intentions, Bad Lab"

We did marble roller coasters this year and it was great. You can make them with pipe insulation. I had them find the amount of energy lost to friction and then use that to calculate the highest hill the marble could go over and the tallest loop it would complete. The students then had one shot to test out their calculations….which made grading super simple. 🙂

I had a similar experience today, actually, and I think I will be blogging about it later. I do, however, have a couple of good ones with vernier stuff for energy. Email me at crutherf@shakopee.k12.mn.us and I’ll send them your way.

If you’ve got motion sensors, you should be able to measure the high and low point for the spring with reasonable accuracy. Or you could film it an analyze it using video analysis software like tracker video analysis or logger pro.

I have both of those tools. This is a definite possibility. I think I’m going to gather a few more of these ideas and do some tests with the kids to see which gets the info into their brain most directly.

I do two energy labs with Vernier photogates. First, the kids know about work and potential energy, so we do a pendulum energy lab where they use a photogate to measure the speed of the pendulum at the bottom of its swing. They know how much PE it had, therefore how much KE it has at the bottom. They graph KE vs speed and analyze it to figure out KE=1/2 mv^2.

The second lab we do uses Pasco carts (without the tracks) and Vernier photogates. The cart is attached to a spring, the other end of which is attached to a clamp at the edge of the table. Students pull the cart back and let it go, measuring the speed at the equilibrium position. Since they can calculate KE, they know the elastic energy when the cart was let go. Then they graph elastic energy vs stretch distance and come up with EE=1/2 kx^2.

You can find instructions for both these labs at my website: sites.google.com/a/sduhsd.net/lccphysics. Look in the class schedule for Packet 5.

Kevin Fairchild
La Costa Canyon High School

I was going to also suggest something like videopoint. I doubt it is the most fun way to do a conservation of energy lab, it’s just what I am used to and it gets the job done.

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