# Homework Help: Why is the net torque not exactly zero.

1. Nov 8, 2012

### snagglepuffin

1. The problem statement, all variables and given/known data
The sum of the net torques is zero even f the numerical result is not. Rather than simply explaining that the net torque is close to zero, justify that your net torque is small, within the context of this experiment.

2. Attached is a pdf of the lab that we had this week. My question is regarding Question 2 on Page 5.

Here is what we got for our measurements.
DATA TABLE 1
Mass (kg) Force (N) Sign of torque Moment Arm (m) Torque (N-m)
Meter stick .1974 -1.93 - .49 -.9457
Hanger 1 .020 -.196 - .66 -.1294
Hanger 2 .0207 -.203 - .98 -.1989
Hanging Mass .0999 -.797 - .66 -.6461
Force sensor no mass 1.98 + .98 1.92
Sum of forces : -1.328N
Sum of torques : 0.02 N-m

3. The attempt at a solution
My lab partner and I were trying to figure out why our measured net torque was not zero, but .02 N-m. We thought that it might have been the use of significant figures, but that did not work. Also, through observation of the setup, we noticed that the meter stick was not hanging perfectly straight, because of the hangers. We thought that there was a torque along the x-axis. Assume that the x-axis runs the length of the meter stick and the y-axis is vertical.

Any other ideas out there why our calculated net torque is not exactly zero?

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2. Nov 8, 2012

### rcgldr

Was there a reason you couldn't adjust the force scale to get the meter stick to be horizontal?

One experiement should have been to only use hangar 2 at the 100 cm end of the meter stick, which would simplify the test and perhaps be used to "calibrate" the test. I'm wondering about the actual net effect of a support or hangar placed at the ends of a meter stick, where each net force could actually act as if it originates somewhat inside the ends of the meter stick. Can the support / hangars be positioned partially beyond the ends of the meter stick in an attempt to get the actual net force to act at almost exactly at the 0 and 100 cm points, although this may not explain why the calculated torque was .02 N m.

Last edited: Nov 8, 2012
3. Nov 9, 2012

### snagglepuffin

The meter stick was about 1mm off on the horizontal, so overall it was level horizontally.

The hangers themselves would not hold it perfectly vertical. I tried multiple hangers and picked the best ones, but there was still some twisting down the meter stick. If you look at the end of the meter stick and it is perfectly straight, you should only see the (3/8)" x 1" profile. Looking down our meter stick I could see the other end, leading me to believe that there is an unaccounted for torque about the x-axis.

Also, the closest the supports could be to the end of the meter stick was 1cm. Could the fact that there was still mass on the other side of the hanger be where the error is?

4. Nov 9, 2012

### rcgldr

What happens if you recalculate the table for torques about 0 cm, but with the outer hangers at 1 cm and 99 cm? (In this case the hanger at 1 cm generates a tiny amount of torque).