I have a lab I teach that has evolved over a long period of time. It's described on p. 14 of this pdf: http://www.lightandmatter.com/lab_205.pdf . Briefly, it involves setting up four force sensors around the circumference of a circular table. We tie strings to their hooks, and they all pull on a point at the center of the table. Students add the four force vectors and verify that their sum is zero. I'm not very happy with how this lab is working, and I'm wondering if anyone else has taught or done a similar lab and might be able to give suggestions for improvement. One problem is that it seems to take students an inordinate amount of time (often as much as 1-2 hours) to get the strings tied, tension on the strings, and an appropriate amount of force on each sensor. My current lab manual describes the easiest and most efficient method I've found, which involves tying some knots and using screws to apply tension. Even so, some lab groups really seem befuddled by the whole process. To some extent, this may actually entail good learning, e.g., some students seem to expect that all four forces will be equal in magnitude, regardless of the angles; or they may not understand that if the forces are all too small, one can scale them all up by the same factor. Some of the delays, however, seem not to be physics related, but just due to issues such as their unfamiliarity with knots or the limited travel in the screws. Another problem is that the Vernier force probes we're using seem to have unstable calibrations. The readings on the probes simply drift over time. This is a real pain. It means that the students can't just calibrate the sensors and then build the setup. They have to build the setup, take data, and then retrospectively determine and apply a calibration. The calibration often drifts by a huge amount, like 1 Newton. I have them do a quick graphical addition on the uncalibrated data (which I think has some educational value of its own) to make sure the results are reasonable before they take final data, cut the strings, and do the calibrations. But the calibrations are usually so whacked that even if they're doing everything right, the uncalibrated graphical addition comes out terrible. If it was me doing the experiment, I'd get everything set up and tensioned properly, back off the screws by, say, five full turns, tare the sensors, and then put the tension back on. However, I'm afraid that if I asked my students to do this, they would spend another hour getting the tension back on and rebalancing all the forces. A final problem is that these Vernier force probes just seem to be junk in general. We have about 30 of them, but about 5-10 of them die each year and need to be replaced, at $100 a pop. Also, many of them flake out during lab, when it's too late to substitute another sensor. E.g., we get sensors whose readings vary erratically by half a newton or a newton. Are there better sensors on the market? We used to do this lab with weights hung from pulleys rather than electronic sensors. The problem with that version was that it was a pain to balance the forces, and we had to do complicated stuff to eliminate the effect of friction in the pulley. In theory the electronic version should be superior, but in practice it seems just as painful. I'd be grateful for any suggestions for improvement.