Labs in General and Torque Lab

All of you should read all of this this because AP will do this lab soon.

With few if any exceptions, the purpose of a lab in our class is to test the truth of a theory or hypothesis or more than one hypotheses. It is NOT to gain experience in balancing, it is NOT to study the effects of moving a balance point , it is not to educate you or give you a feeling of accomplishment. It is to see if something, usually a pretty specific something, is true.

The reason for me requiring you to do a lab, or my purpose for you doing the lab is another matter, and may be something really sinister, such as getting a teenager to think. However this is not the purpose of the lab in itself. It is the purpose of the lab itself that you must put in your report, not any of my or your ulterior motives (which in your case may be simply to avoid getting yelled at by an irate physics teacher.)

Similarly, the conclusion must contain a direct answer as to whether or not the purpose was achieved, i.e was the truth of the hypothesis or hypotheses demonstrated. This should include a restatement of the hypothesis, hypotheses being tested.

For the torque lab, the two hypotheses being tested were

1. Given the definition that torque = Ftangential x distance from point of rotation, then if the clockwise torques = the counterclockwise torques rotational equilibrium is attained,
2. For calculating torque, the weight or force of gravity on an object can be taken as acting on the point which is the center of mass of that object.

Cases 1, 2 and 3 demonstrated the first hypothesis and case 5 demonstrated the first and second.

Question 1 on the torque lab asked about how the sum of the forces = 0. IT said nothing about torque. Many of you made the double mistake of saying the forces were equal on both sides of the pivot. First this was not true, the torques not the forces , were equal. Second the question was not about rotation and torques it was about force. All the weights were downward forces, why did the meter stick not accelerate downward; i.e. what was holding the meter stick up and with how much force?

Question 3 Triple beam; since you never got to use these in chem lab I'll have to explain it to most of you although congratulations to the few who did get this right.

Question 4: your answer should have had a calculation which showed the torque required from mass 3 had to be 4500 gram cm. You could then explain why this was a problem with

M3 = 50 grams and how you might solve it moving M1 or M2 or even the pivot point.

Question 6 should be answered. Write an expression for the ccw torque using 220 g and its distance from the pivot and an expression for the cw torque using 100 g and its distance from the pivot and the mass of the meter stick ( unknown so call it M3) and its distance from the pivot. Set ccw equal to cw and solve for M3. Remember distance is not position, i.e. d not equal x.

Also remember that x means position, not mass, not speed, not force, not torque.