I released the cart from the top - 4.8 meters away - and they could time how long the trip took (3 trials). I also let the pendulum swing for a while, and they could measure whatever they liked. I assigned each group a number of cycles which the pendulum must go through from the time when they release the cart until the cart reaches the bottom of the ramp. They must calculate the location at which the cart must be placed in order to make that happen. There's a piece of magnetic track from my son's train set that makes noise when the magnet attached to the bottom of the pendulum swings just above it - this signals success.
My tests before school were all very successful, so I was hopeful before class about their success.
I had each group write up a whiteboard that just needed data plugged in, and we began the data-taking with about 30 minutes left in class.
In the first class, almost all of the runs looked extremely close, but we couldn't get the sensor to trip - downer. :( In the second class, I adjusted the sensor setup, and two out of four were successful. One of the others had a good method, but was just a touch off in execution and/or measurement.