Maybe I shouldn't have started this blog now, not with everything that's been going on.

In my mechanics lab class last Wednesday, it was the first time in three weeks that I held separate morning and afternoon classes again.

That day our experiment was Atwood’s Machine. From the get go there was a hitch already, because the title of the experiment in their manual was different: Newton’s Second Law of Motion. So as soon as I got to the room there were already complaints from some of the students who were not able to read up on the theory and procedure.

I did not take this too seriously because I knew this was just the last ditch effort of people who did not open their manuals until minutes before the class started. A minimal amount of scrutiny would have told them (and by process of elimination) which experiment dealt with Atwood’s machine.

There were only four questions in the quiz: what are the equations for theoretical and experimental acceleration (3 points each), what is the equivalent of dyne in familiar units and how to the two masses change for each set of trials (2 points each).

So far I’ve shown them that in to get high scores in the quiz they not only have to know the materials, the measured quantities, the equations, the number of tables and graphs and the units (as well as the description) but pertinent points in the procedure as well.

The performance of the experiment itself had a few more stumbling blocks.

First of all, I haven’t used the new equipment before. This time last year we were using the force table with two pulleys to simulate the ascent and descent of two weights on a common string, which was why I commissioned a new set up, with a pulley large enough for the weight pans not to hit each other on their way up/down.

What I didn’t know (since I didn’t test it) and what David found out last term was that the pans still did collide, although I suspect this is due to the students releasing the lower pan from a slightly diagonal (and not completely vertical) position instead of just letting the lower pan hang and just letting the higher (and heavier) pan drop. After all there was one group who was able to perform the experiment without adjustments.

For the rest though, the technician and one of the upperclassmen who failed in David’s class last term, without my prompting, did what David apparently tried last term, which was to hold another smaller pulley at the same height as the first one, but far enough from the first pan not to collide.

I told those who did this though that they had to perform all the trials from the start because the two pans with the added horizontal distance on the string would now have a new height difference, which was needed in computing for one of the accelerations.

In fact, one of the groups in the afternoon class used two of the smaller pulleys instead of one of the larger ones, and since the smaller pulleys have less friction that the larger ones, they probably have a smaller percentage error.

And that was another obstacle in the experiment, just like our set up last year, because of the friction on the pulleys, as the masses on the two pans get closer to each other in value, there were times the pulley would stop moving altogether. So though they started with 600 and 400 grams and were supposed to shift 20 grams until the two weights were 20 grams apart (but with the same weight still heavier) when that happened I told them to increase the heavier weight by 20 instead until they got 5 sets of trials total.

Also in the afternoon class, I printed out and cut up the questions for their quiz, just like David used to do. This is because there were a lot of one answer questions, and only one enumeration question. It was ironic that for once there were students prepared with their one-fourth sheets when I passed the papers around.

That’s Wednesday, and that’s all we have time for this week. I’ll continue on Monday. We’re done here.