This lab has the student comparing the centripetal force on a rotor spring to the gravitational force necessary to stretch it the same distance. It is important the students ensure these forces are stretching the spring the same distances by using the "flag" that is tripped when the string stretches out to a particular length. This is explained in the lab manual, but it is worth emphasizing before lab.
An additional challenge of the lab is to measure the rotation rate of the rotor. To do this the students will attempt to match the rotation frequency of the rotor with an adjustable frequency strobe light. When they match the image of the rotor should appear stationary. Typically this occurs for a strobe frequency of 500-600 Hz. Many students mistakenly record half this frequency, since the image of the rotor when strobed at half the rotation frequency alternates in a manor that makes it almost look like the stationary image the students should be seeing. If this doesn't make sense, try it yourself before lab to see what I am talking about and warn your students of this pitfall. I'm attaching a slide that I've put on transparency that you can use to help point out these two issues.
The rotors in this lab spin rapidly and can be dangerous - keep fingers, hair and other objects away from the spinning rotors. In particular during the stroboscopic measurement the student may be tricked into forgetting the rotor is spinning so rapidly and may let down their guard.