AP Labs

Two items Centripetal and Torque Labs

1) Centripetal force lab (5 sub items):

A) friction doesn't change the time reading or speed directly. It does affect what is going on in three possible ways. (1) friction between the string and the top of the tube can reduce the tension at the stopper since now the friction is "absorbing" some of the tension produced by the weight of the washers. (2) A tape rubbing on the top of the tube can produce the aforementioned affect in spades. (3) tape rubbing on the bottom edge of the tube will allow tension in the string above the tape to be higher since some of this tension will be "absorbed" by the friction before reaching the washers to hold them up; i.e. tension at washers is lower than at stopper. Effects 1 and 2 will cause Ms x ac to be lower than Mw x g . Effect 3 will cause the reverse.

B) Measuring radii incorrectly, letting washers swing or orbit, and miscounting rotations are also important sources of error, but can be nearly eliminated by using reasonable care.

C) Since we measure L not r, the vertical component of the string is not important directly. Since r = L cos theta horiz and FT horiz is FT cos theta horiz

FT = Ms omega^2 L cos theta horiz/cos theta horiz = Ms omega^2 L. The real reason for a near horizontal orbit is to reduce the importance of friction vis a vis the other forces by minimizing ratio of normal force between string and top of tube to tension.

D) Some of you have mentioned that Fg is zero. This very very wrong. I assume you meant Fnet on the washers is zero so Ft = Fg on the washers, but this error is so important it still calls for a rewrite. Also note Fg on the stopper in never zero so the tension on the stopper MUST have a vertical component! so that Fnet vertical on the stopper is zero.

E) Fnet on this system as whole is not zero. Part of the system is accelerating with respect to the system, but also with respect to the outside environment. Your hand holding the tube is supplying a net force on the system in the direction of the ac. Think about it. The string is pulling on the tube in the horizontal direction. You are providing a force opposing this. You, in effect are providing the centripetal force. (You are also holding the tube up allowing it to provide the vertical forces canceling the Fgs on the washers and the stopper, yielding no net vertical force on the system.)

2) Torque lab is just fill in but make sure you answer the last question even though it is marked Optional. Also with regard to the questions, we do not use triple beam balances in 506. They are convenient but not trustworthy, not durable. However, most of you still know how they work so answer the question if you can. Lab is due Tuesday.