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I’ve been struggling with feeling like I’m falling a bit behind in my curriculum this week. But at the same time Students were reporting feeling confused by the lab where we tried to determine what factors affect the force exerted by a spring—known by most teachers as the Hooke’s law lab.

In an effort to increase our understanding, and not lose too much time, I decided to try to put a timer to use in an interesting way. I’m well away that timed tests are universally loathed by most students. I hate them too. I think there are few instances in life where one’s success rides on being able to solve physics problems under intense time pressure. But at the same time, I notice that nearly every single sport does have a timer, and the pressure of the clock counting down does, in many cases, bring out the best in athletes. So how could I bring this positive pressure to my classes?

Here’s what I decided to do:
I wrote up the following instructions on the board:

1. Pick a spring and label it
2. Measure the spring constant of that spring
3. Share the data you used to measure the spring constant with the other lab groups (use google docs or a whiteboard).
4. Make a graph force vs stretch graph for all six springs on the same set of axes, and be sure to include all the qualities of a good graph (title, axes labels, lines of fit, equation of fit for your spring, and a two sentence caption).

I told students we’d have 25 minutes for this, and put up a time from countdown timer on the screen. I asked them if they thought they could get everything done in time—they weren’t completely certain. But when the clock started, everyone got to work very productively. I played the sports announcer, walking from table to table announcing the time.

With my first group, no students started to post their data until less than 10 minutes were on the clock. And even then, people were posting data in grams, Newtons, cm and meters. It was a bit of a mess. Groups were so caught up in their own work that they didn’t notice this problem until they started to try to plot everyone else’s data with less than 2 minutes on the clock. And so this is where I stopped the clock, yelled “STOP” and called a time out—which is another great feature of making lab more like a sports match. I got to be the coach, and said “ok, what’s the problem we’re facing now?” Everyone saw that we needed common units, and that some groups didn’t understand that force had to be measured in Newtons.

Our mismash of data, revised

“Can we fix it?,” I asked.

“Sure.” they responded, and I started the clock again. Unfortunately, time ran out on regulation, and I went around and asked people where they were. Most had only plotted 1-2 lines. This is when the second greatest invention comes around: 5 minute overtime. I again get to give a little halftime speech—”Where are we now? What do we need to focus on?” And students quickly listed off everything we needed to do. Clock starts, and we get to work.

Again, when the clock is winding down, I’m able to call a 20 second timeout and ask the class where we’re at. Students seem far more focused, and the sports metaphor is helping out—especially as we trying to figure out what we need to do in the closing seconds to pull out a win. Suddenly everyone realizes it would have been a lot easier to share data with google docs rather than have everyone recopy from the whiteboard.

Finally, after triple overtime, everyone was able to complete the graph, and so we went into the playoffs. As the new robotics coach, I was able to use the robotics lab to quickly chop up a scrap 2×4 and put wood screws on either end of a block. I then taped these blocks to a dynamics cart, and taped the cart to the track, as seen in the picture below.

duck tape is the most essential of all lab supplies.

Students are then told to pick another group, and place their springs on the screws and figure out how hard they each need to pull so that the cart will remain at rest when the tape is cut. I like this goal because it focuses on collaboration, and students have multiple ways of doing this—by looking at the graph, or comparing their equations and solving for where the force is equal, or simply by comparing spring constants and reasoning it out. After that, I asked students to whiteboard out the free body diagram for the block and the springs, and explain why the forces were equal (some forces are equal because of N3, and other forces are equal because of N1, the object is as rest, and there are only 2 forces acting on the object

And that’s where we decided to end the game. While I don’t think I’ll use the timer on everything we do, I do think this was a useful way of raising the level of focus and purpose in lab, without dialing up the pressure as well.

1. October 3, 2011 8:03 am

I am way behind you. When did you start school? How much time do you have for class each week?

• October 3, 2011 8:09 am

We started around August 15, and I have 6-55 minute classes per week with students. Also remember that I’ve changed the ordering of my units—we do CVPM, BFPM, CAPM, UBFPM.

2. October 3, 2011 12:38 pm

I’m a bit behind you also, but we have only one 2-hour slot a week, and have met once. I like the idea of having the students measure spring constants, predict how far to stretch springs to make them balance forces, and then test in the real world. Now I have to find some (mismatching) springs. The only springs I have in my junk pile right now are only good for compression, not tension, so I might have to order an assortment of springs—maybe from http://www.sciplus.com/search.cfm/scategory/SHS/term/spring/srch.fp/1

• October 10, 2011 10:09 pm

Thanks for the tip on the springs. I ended up ordering both sets of springs on that website, and unfortunately, they aren’t the best for physics labs—they’re a bit too short, but I’m sure they’ll come in handy for some project or another.

• October 10, 2011 10:17 pm

I actually ordered the box of 200 springs from The Battery Geek, who had them at a slightly lower price. I agree that the springs are a bit small, but I think that they’ll do for measuring spring constants and for examining the effects of putting springs in series and in parallel. I’ve only got 2 students, so I don’t need things to be visible to a whole classroom.

I’ll have the students design a rig for measuring the spring constants using household items this week.