Due Friday: Study the details of and compare both attached files AFTER reading the notice below.
Important to know what's in the two attached spreadsheets before Friday Oct. 18 and how it affects you individually*. A lab performance task has to be completed during the period on October 18. A Part 2 Measurement Performance grade will be assigned by the end of the period. Part 2's objective is to graph acceleration versus varying outside force for a constant total mass of all things being accelerated.
What's in the two spreadsheets gives numerical evidence to support the claim that many technique errors (and I'll list them below) are still being made across the board in the lab space, and since there is a lot of group work going on, these technique errors affect everyone. These errors are fine; it's what school is for. But they need to be corrected before Part 2 is conducted. Part 1, as shown in the attached file named "Discovering F = ma Per 2 2019 Initial Student Measurements", is showing a 122% error. (The percent error is determined from the discrepancy between the slope of the graph and the Newton value that's in the title of the graph.) Part 2 needs to be conducted with more accuracy. Half of the grade of this experiment will be determined by a Part 2 percent error as follows: Percent error of 10% or below will earn an A on half of the lab grade for this assignment. Percent error of 10 to 20% will earn a B on half of the lab grade. Percent error of over 20% will earn a grade of 79.999% on half of the lab grade for this assignment. The preceding percent errors in grade determination will be determined from what happens in Part 2 only. I'll still let the percent error from Part 1 not impact grades on the assignment. Now comes the explanation for why I know a 122% error should not be happening:
When Period 2 ended on Wednesday, 10/16/19, I went to each of the 5 stations to see what I measure as the time to accelerate 50 cm from a state of rest. I used the same mass values that each of you said you used. When I finished going to all 5 stations, these are the setup errors I found:
1) The tracks were insufficiently level at 5 of the 5 stations.
2) The strings were FAR from horizontal at 2 of the 5 stations.
3) Photogate heights were improperly set to correspond to the picket fence dashes at 3 of the 5 stations.
4) A 20 gram mass was hanging from one string. I'm assuming that error wasn't made during measuring, and maybe the group was just testing with a 20 gram mass for the heck of it. It was agreed they'd all be 50 g.
5) One photogate was misbehaving, showing red light for times when the picket fence dashes were not passing the gate. This is not the students' fault but IT IS THEIR JOB to watch the red light when conducting the experiment and switch out the malfunctioning part. Heightened observation. I noticed that strange acting photogate and switched it out before I measured.
6) A track poorly supported by books and slightly unstable at one station. Put the support books at opposite ends.
7) In two cases, the number of bricks left on cars didn't match reported total cart mass. I'll assume these got moved after measuring was done, because I did go around and check that all reported M masses were reasonable in the chart, and they were.
8) Photogates were not mounted perfectly vertical and perpendicular to the track at one station.
I corrected all the setup errors above to the best of my ability and measured by own time T at each of the 5 stations. I did some repetition at each station and in each case I ended up confident in my measured time to the nearest hundredth of a second. My collected data are completely shown and graphed in the attached file named "Discovering F = ma Per 2 2019". I have roughly 3 percent error. On Wednesday 10/16, almost all student stations had high precision but low accuracy.
Before Friday, it's important to compare the two attached spreadsheet files. Each individual needs to arrive at class Friday with heightened awareness about how to measure Part 2 with best technique and avoid the types of errors that are in the list above. I am convinced that this experiment can consistently be done with high accuracy and precision. I've done it myself several times, and I've had classes of students do better than I have done.
It's good that this all happened and that you get to see the discrepancy between the results in the two files attached. And yay, the laws of nature are still intact.
*The individual's responsibility as it relates to everything said the above: If there is a group of 5 people all tolerating a string that is far from level as the experiment is conducted, that means 5 of 5 people are being unobservant. You, as the individual, don't let others do the observing for you. You are each responsible for seeing it individually, no matter how large the group is. Same thing with the malfunctioning sensor showing a red light at instants when the picket fence is not intersecting it. It is your job individually to learn how the equipment works, know what the red light is supposed to do, and see for yourself the obvious situation where the red light is showing malfunction. (Such an occurrence is rare, but it did happen once on October 16.)
