Wednesday, March 12, 2008

Revamping Intro Physics Laboratory - Part 3

OK, I haven't forgotten this yet.

To me, the biggest problem with the current structure of intro physics lab is that we give students a list of things they have to do and measure, and hand-hold them into getting the result. In other words, they don't have to think too much to complete the exercise. They may have to do a bit of thinking and understanding of physics to complete the write-up, but the actual part of performing the experiment requires simply the ability to follow instructions.

I believe that we should have a more open-ended experiment to be given to the students. So I'll give an example. Note that while thing is something that I've thought about for a while, I'm still writing this off the top of my head. So there may be other problems with it that I haven't carefully considered.

Give them a problem to solve such as something like this:

Construct a pendulum clock. To make this clock useful, it would be helpful if the pendulum can swing back and forth once as close to 1 second as possible. Then each complete oscillation will take just one second. That way, this clock and measure time in increments of one second. You may use a stop watch to calibrate your pendulum to verify that it makes a one-second swing. Try to build this as accurately as possible. You must describe in detail in your lab report how you accomplish this task and why you chose to do it this way.


Now, as apparatus, give them a length of string, a set of weights, and a stop watch, plus other necessary items for them to be able to mount the pendulum on something.

Here's what I expect to occur. You'll have some students doing this by trial-and-error. They'll mount a length of string, and then start changing the weights to change the period of oscillation. Of course, there's no guarantee here that there is JUST the right weight for that length of pendulum to produce a 1-second period of oscillation. So students doing it this way may face a problem, but that's OK, because at the end when we discuss on to do such a thing, they'll discover why their technique isn't the best way.

You'll also get a bunch of student who would use a fixed weight, but tries to vary the pendulum's length. Again, they may try this simply by trial-and-error, adjusting it a little bit at a time until the period is close to 1-second interval. This technique is of course, more "refined" than the earlier one, since there's a high possibility of getting the right period.

Of course, what should be done, rather than simply doing a trial and error method, is simply to use a fixed weight, then measure a set of period corresponding to a set of pendulum lengths. Using the table, one can plot period versus lengths, and from there, interpolate (or extrapolate, depending on the range of lengths that were used) the exact length to produce a period of 1 second can be read off. So after the experiment is done and the students write their report, the lab instructor can start a discussion on the best possible technique to get the most accurate result. One can even make it a bit more complicated and ask the students how accurate is their clock as they let it swing for a length of time. This is where if they constructed a clock that swings over a large angle of oscillation, they may discover that it doesn't keep time very well.

What this type of lab forces them to do is think on the relationship between two measured variables. The first group had to figure out how the period changes as they change the weights. The second group is finding out the relationship between the period and the length of the pendulum. There may be a 3rd group that may be changing both the length and the weights simultaneous. If they do, and they're doing this by trial-and-error, god help them! :) But no matter what, the students are forced to think of what to do, and why they're doing it, to accomplish the task. They are not told how to do it. The experiment and the equipment give are familiar enough to them that this isn't something out of the ordinary. In fact, when they were kids, they probably played with something like this. The curiosity with finding how to do things is the purpose of the lab exercise. It is really playing, it is just that now, they have to think on what they are doing, why they are doing it, and how to present it in writing.

Next time, I'll try to present another possible laboratory exercise along this line.

Zz.

3 comments:

Augie Physics said...

"The first group had to figure out how the period changes as they change the weights."

You realize, of course, that it doesn't.

Anonymous said...

I think that this would be a good experiment.

I'm sure that those students who do try to change the weight, and realize the time period doesn't change, will actually *learn* and *understand* how it works, rather than blindly following a given procedure without thinking.

I'm going to speak to the faculty conducting my lab to see if the experiment can be modified this way for future students. I'll let you know if anything happens.

ZapperZ said...

Yes, I know that the period is independent of the weights. But the students don't know that, or at least those who pursued the experiment of changing the weights don't know that. I think that finding out for themselves that it doesn't is a good thing. In fact, if they did that experiment by changing weights and discovered nothing, they should write down their finding as well. They need to record down the fact that when they changed weights, nothing happened.

My idea here is that after this lab when the students actually realize that the period is only affected by the change in the length, you then can as, them something like

"So now that we know T and L are related, can we find out HOW they are related?"

This is where we now require a quantitative aspect of this experiment, rather than a qualitative aspect of knowing that T and L are related. This emphasizes an important aspect of physics where we not only can say that "Everything that goes up must come down", but we must also be able to say "when and where it comes down".

Zz.