Why is the net torque not exactly zero.

[snagglepuffin]

Homework Statement

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

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?

 

[rcgldr]

through observation of the setup, we noticed that the meter stick was not hanging perfectly straight, because of the hangers.

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.

 

[snagglepuffin]

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

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?

 

[rcgldr]

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?

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).

 

[Nickie]

I would like to first acknowledge that it is rare for any measurement to be exactly zero. In this experiment, we are dealing with multiple variables and sources of error, which can contribute to a non-zero net torque.

One possible explanation for the non-zero net torque is the presence of friction in the experiment. Friction can cause a force that is perpendicular to the direction of motion, which can result in a non-zero torque. This could be especially relevant in the case of the hanging mass, as it is being pulled downwards by gravity and may experience some friction on the meter stick.

Another factor to consider is the precision of our measurements. While we may have used significant figures, there may still be some inherent uncertainty in our measurements that could contribute to a non-zero net torque. This could be due to the sensitivity of the equipment or human error in recording the measurements.

Additionally, as you mentioned, the positioning of the meter stick may also play a role in the non-zero net torque. The hangers may cause the meter stick to tilt slightly, leading to a torque along the x-axis as you suggested. This could be further exacerbated by any imperfections in the meter stick itself, such as uneven weight distribution or slight bends.

In conclusion, there are multiple factors that could contribute to a non-zero net torque in this experiment. It is important to carefully consider and account for these potential sources of error in order to improve the accuracy and precision of our measurements.