Honors a=F/M Labs

The period 2 lab reports continue to range from amusing to exasperating. At least you all plotted a on the vertical axis and something to do with mass on the horizontal axis. You should have plotted a vs 1/mass. Many of you plotted a vs mass instead of a vs 1/mass. Some of you plotted the 1/masses at equal intervals using , or rather misusing, EXCEL. You must use the scatter plot format to plot data at the proper locations on the horizontal axis. (EXCEL was designed for business purposes, not real calculations so their normal graphs destroy useful data).

Almost no one said what the space between the gates was; how many cm was the delta x in the calculation of a. Information like this belongs in your report

In many instances there were serious errors in calculating a. Some of you forgot to use the correct band length. Some of you rounded so carelessly your graphs came out lumpy instead of linear and your slopes became inaccurate. Someone used cm/s for v but used meters for for delta x so that a came out a factor of 100 high. This extremely high value was then used in the lab report as if nothing was wrong with it. Very few of you divided the kgcm/s2 by 100 to get kg m/s2 which is what you need if you are going to compare the slope to the force in Newtons [ 1N = 1 kg m/s2].

Nearly all of you said confidently that the slope was the force (including those who plotted graphs of a vs mass for which the slope was a/m instead of ma). However only three of you bothered to actually calculate the slope and compare it with the force, which is the only way you could demonstrate that slope which is a/(1/mass) =a x m actually does = F

It is not clear that most of you realize what the force causing the acceleration was in the two runs, so that you would know with what you should compare the slopes once you have calculated them. In fact only two of you indicated that you knew the force causing the acceleration was equal to the mass of the hanging weight x 9.8 N/kg. This comes to .98 N for a 100g hanging weight, .49 N for 50 g hanging weight, and 1.96 for a 200g hanging weight. Fix this. Calculate slopes, compare them with F and show the calculation and discuss the results and the reasons for them in your conclusion. You need to think and then demonstrate that you have thought, learned, and understood.

The last question about why the line of best fit intersects the horizontal axis at some point to the right of the origin elicited a few correct explanations. Think about the relation between a and 1/mass. If F = something greater than zero what does 1/mass have to be for a to equal zero? If a = 0 and 1/mass was not zero (i.e. the mass of the system was not infinite) what does that say about the net force on the system? If the net force is zero when the force of gravity was applying a force on the system what other force is probably present? 10 additional points of credit on the lab if you can identify it and calculate it for at least one run ( not trial). Again show the calculations and discuss the results and the reasons for them in your conclusion. You need to think and then demonstrate that you have thought, learned, and understood.

Your error analysis should not be a laundry list of what might have gone wrong but did not. It should just address the things that actually did or probably did happen. For example, don't say you might have had the timer on the wrong setting when you have only a 5 % error.

Discussions and conclusions are not the place to describe the procedure. Also they are not the place for a review of how you feel about the lab. You can't just say you learned a lot. You must show calculations that justify your conclusions. You must say exactly what you learned, probably expressing it in equations at some point.

I am hopeful that period 4 labs are better since they were warned before handing them in. We shall soon know.