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.
Today we worked on a capm lab practical. We first found the acceleration of a cart down a ramp. Then we took down the motion detector and I showed them two photogates. The photogates measure the change in time between them (among other possibilities, but that’s all I showed them for now). The students must choose a starting location for the cart on the ramp that is at least 10 cm above the first photogate, then find where the two photogates should go such that the change in time was some particular value, different for each group. As you can see above, this went swimmingly in third hour. The other two, however, were failures on my part.
First hour I tried to go over a problem we worked on yesterday first, and then students didn’t have even close to enough time to do the problem and test it, especially because I only had one photogate setup that I had planned to move around to all the different ramps. In 2nd hour I skipped the problem at the beginning, but I still used different setups for each group and it proved impossible to move it around. For third hour we just found the acceleration of one setup as a class, then I gave them all different times and they only had to adjust the photogate location on the one setup to test. This worked MUCH better.
One thing that surprised me was how well this worked. The groups that did the calculation correctly were closer than a hundredth of a second off. It is very difficult to get the photogates in the right place, as they are 10 cm above the track. I figured there would be a bit more uncertainty; but the first group that did it right is the one that was right on, to the thousandth of a second! We all agreed there was a bit of luck in there.