I stole another page from Kelly and set this lab practical up as a quiz. The students were told that I would give them both angles and the starting location of a cart on the steeper ramp; their job was to find a formula giving the location of the cart on the shallower ramp such that the carts hit the end stop at the same time.
This is great as a quiz for three main reasons
- It combines multiple models into an interesting problem
- If students get it wrong, they find out physically and can retry the problem if they have time
- It’s awesome to see physically when they get it right.
The picture below has students in the forefront plugging the given values into their formula to prepare for testing (hence the calculators), and students in the background looking at the solutions (yellow sheets), either they already tested correctly or they got stuck. I had a couple of students nail this who have been struggling. It is extremely satisfying seeing the carts hit in the right place.
***Note: This situation is fairly easy to get ‘right’ doing the wrong physics (or by just guessing with a ratio of angles). I always looked at their papers first, and if their answer was close to correct with the wrong work, I cheated and gave one cart a bit of a push at the release so they didn’t hit at the same time. I confessed this to the students later.
A more dynamic view of this collage is available here.
Today we were working on a problem (#4 in Practice 3 in here) and I wanted to investigate the situation a bit more closely. The problem is a modified atwood, and I had one set up with a wireless force detector on a cart, so the situation was different than the problem in that it was essentially frictionless. I showed students what the F vs. t graph looked like when I just held the cart (a horizontal graph, as F is constant). Then I asked them what would happen to that graph if I released the cart. The above is what they drew, below is what actually happens.
After taking the above data students discussed why this occurs using qualitative free body diagrams of the hanging mass before and after the release.
Today we started working some more involved problems in unbalanced forces, from Practice 3 in my UBFPM packet (problems stolen from various places). The above was problem 1, where students found we could look at the system as a whole (at the bottom of the whiteboard), or could find and relate the internal 3rd law forces to solve the problem.
This is a student who has struggled to ‘buy in’ to multiple representations, as he has very strong reasoning skills, but today he started a problem with 4 great representations (though he didn’t to by-on notation for the free body diagram, but I’ll take it).
Today students presented a claim and their data supporting it for whether speed, surface area, weight/mass, or surface type affected friction. The results for the two classes are shown below (as multiple groups did each one). I forgot to take a pic of first hour, but they had a dissenter in SA thinking it did not affect friction. The far right list of Yes and No is the ‘right’ answer.
I was disappointed with the results, but I learned a valuable lesson; good data can’t be rushed. I am under the gun at this point to finish up unbalanced forces by winter break, so we did this experiment very quickly, with no time for revision and with very little actual data. It is very difficult to get good data for this, particularly the speed experiment. When I did the demo lab after we had compiled their data, I put a motion detector up as well as the force detector, so students could see whether I was pulling at a constant velocity (which you have to in order for the force probe reading to equal the force of friction). I legitimately tried to pull with a constant velocity on the medium speed trial, and it wasn’t even close. I’m confident that they found that speed increased friction only because they were actually accelerating the object. I think the surface area experiment is simply prone to lots of random uncertainty. I have seen it done well when I let them talk almost a week to complete it, but I don’t have that time anymore. Which is kind of sad.
Today we started investigating friction. I was inspired to have students design their own experiments by John in this post, and I’m looking forward to comparing notes with him and Frank (see the comments of John’s post).
We started this process by going through the Friction Investigations page in my UBFPM packet here. The graph above was produced for #4. I was pretty happy with how this guided students towards the idea that static friction only acts as much as it has to (does anyone have a better way to say that, btw?), and that kinetic friction tends to be constant. After we finished up with listing then culling the possible variables to affect friction, I gave them some brief instructions on how to use the force detectors in this case and set them loose. I asked them to investigate qualitatively (if they were changing the speed, do a slow, medium, and fast trial), mostly because of lack of time.
I must admit I am conflicted here a bit because I am crunched for time to finish Unbalanced Forces before winter break. I know some groups are going to get the ‘wrong’ answer from their lab, and I would like to have all the groups follow up with either extensions or re-doing their lab (as Frank suggests in the comments), but I’m out of time. So unfortunately I think I’ll just demo the experiments (as I’ve done in the past) to show the “right” answer. But I hope I don’t have to and the data will speak for itself. We’ll see tomorrow.
I wanted a way to compare the various parts of the elevator situations (see yesterday’s post), so I added this chart to the end of the worksheet. I added the highlighting for emphasis only, 3 and 6 could be compared in the same way.