Tag Archives: N2L

Adding Friction: N2L Labs That Work

I recently wrote about some Newton’s 2nd Law Labs that have actually worked for students to get solid data. One negative, however, is that they are all single force labs. I set out, as a result, to find another lab that would work for students. Caveat; I haven’t actually performed this lab with students. We are past our Unbalanced Forces unit, and I collected this data myself just to see how well it would work; astonishingly well, it turns out.

All I did was add a friction block being towed by the cart in a modified Atwood setup where the tension is measured directly (see last post for more information on that setup).

mod atwood cart setup

I’ve done a similar lab in the past, using just the friction block. The problem there is that there is a very narrow range of hanging masses that work; once it starts accelerating from a mass, a small addition of mass results in large increases in acceleration and thus difficultly in getting good measurements. This version, however, has enough system mass where hanging masses from 50-100 grams gave very nice results.

screen shot 2019-01-26 at 5.52.26 am

I first pulled the cart at a constant velocity, and measured friction directly as 0.37 \pm 0.03 \ N, where I used the mean and standard deviation of the force vs. time graph for those values. The mass of the system of the cart and block was measured to be 0.554 \ kg. The N2L equation, linearized, is F_t=ma+F_f, so the slope should be the mass of the system and the intercept should equal friction. The results show below that we are well within uncertainties on those values.

screen shot 2019-01-26 at 5.57.45 am

The trend is nice, the accelerations are reasonable and therefore should be relatively easy to collect, and best of all, it’s a simple extension of a lab students have already performed. I’m excited to try this with my students in the future. I may even do the first version with the block on top of the cart so that the only change in the 2nd version is the addition of friction; the system mass stays the same, thus so does the slope, but the intercept changes.

Newton’s 2nd Law Labs that Work

Modified Atwood with Ft measured directlyIn the last few years I have finally found some N2L labs where students in a general level physics class can consistently get decent data. I have compiled those labs here for use and modification. Below are some teacher notes about these labs.

First of all, the course for which these were designed is a general level physics course taken by a high population of seniors (approximately 1/4 of the graduating class), most of whom are not going into science. Those going into science tend to take our AP offerings.

Another thing to notice is that I have students plot acceleration as the horizontal variable and force as the vertical, knowing that this violates the x: independent variable, y: dependent variable guideline. I think doing so has two benefits; it allows for the slope of the line to be easily recognized as the mass of the system, and it shows students that we can manipulate the axes if it is convenient to do so, a precursor to linearization. The guideline is in fact only a guideline.

The fan cart lab is obviously only possible with fan carts. I got lucky this year and was able to pull them together. I have the pasco carts ($250 each!), but vernier makes a cheaper version ($105 each). I can’t speak to the effectiveness of the vernier carts as I have never used them.

I am confident that the force readings for both the cart on a ramp and the fan cart labs could be done with decent spring scales rather than force probes. Modified Atwood, however, requires force probes. I like the direct measure of tension as it takes away the black box of Modified Atwood setups where masses have to be switched from the cart to the hanging mass; I am confident that my students wouldn’t understand the nuance there unless we dived deep into it, and I prefer to make the system pretty obvious (the cart by itself).

I have also done the modified Atwood lab with friction blocks with some success. It is more difficult, however, for students to consistently get good results. There is a pretty narrow range of masses that will actually accelerate the cart, but not too much so that acceleration is difficult to measure.

The last thing to note is that I set aside at least two 45 minute class periods for each lab. Generally the first day is data collection and the second is analysis and discussion (often a board meeting where students compile data on a whiteboard and then we compare their results). I try to have groups with different masses for both the fan cart and modified Atwood labs so that the relationship between slope and mass is more obvious. I like having a day between collection and analysis where students can work on something else, that way if students were gone or their data didn’t work out well they can perform the lab on that in-between day.  Any lab worth doing is worth doing again!

Let me know in the comments or on twitter if you have questions or ideas!

Google Doc of N2L labs that work