Add option for force values

[ariel-phet]

A user wrote to phethelp:

I love using the Balancing act sim in lessons, it is amazing for teaching students moments quickly. However, it is so frustrating constantly having to explain that Moment is FORCE x distance, so they need to be multiplying the masses by 10 each time. Is there any way that an option to show Weights instead of masses could be introduced. I’m guessing I am not the first teacher to request this!

It will save so many misconceptions!

I replied

Believe it or not, you are in fact the first teacher to request this change to the simulation.

I will add this suggestion to our list of users suggestions for this simulation and we will fully evaluate it next time we update the simulation.

My initial reaction would be that since we include "Force arrows" we could certainly consider a option that allows force values to be also displayed. Unfortunately, I think slightly departing from your suggestion we would still should the masses and if we displayed weights would use the accurate scaling factor of 9.81 -- we would likely get lots of pushback from some physics teachers if we used the rounded value of 10 for "g". Since we wanted to keep the values at round numbers, we would likely have such an option as a checkbox, so the user could see the mass values, the force values, or both.

[ariel-phet]

@arouinfar curious on your thoughts here -- since we will have several people volunteering with the project coming up, doing some minor improvements to this sim might be some nice bite size chunks of work, and this seems like it would be a fairly simple change (see my thoughts above)

[arouinfar]

I think the goal of adding a weight readout would be to better facilitate the calculation of the torque. While I don't think there's currently a significant barrier to calculating torque, I can understand why some teachers may want this feature. While displaying the weight would be a straightforward change for a developer, it's less clear how we would do this in the design.

If we want to think of these values as "force values" they should be tied to the force vectors. The most appropriate way to turn on these values would then be a nested checkbox under "Forces from Objects" (which I don't love). Alternatively, we could change "Mass Labels" to "Values" which would display both the mass and force value (my preference).

If we add labels to the force vectors, I think the best place for them would be below the vector. For the most part, the labels won't overlap, but we would need to shift the labels for the largest masses.

Even with an opaque background, the values will be challenging to read when the ruler is on. Given that the goal is to make the torque easier to calculate, it's not ideal for the force to obscure the ruler.

Alternatively, if we think of values as the weight, it's not necessary to tie them to the force vectors. We could choose to display the weight instead of the mass, and have radio options to view the mass, weight, or nothing.

Since the strings are longer, we should probably put the value and unit on separate lines, which I've done for the people. With this arrangement, there aren't any occlusion issues with displaying both the weight and the distance. Visually, I think the weight readout is the cleanest, but it would muddy up the panel UI a bit, and the extra height would push the panels into the grass.

@ariel-phet thoughts?

[ariel-phet]

@arouinfar I think the more I see this, it seems that in terms of design and the age group we are targeting, this feature is just likely to add clutter and confusion. Younger students are going to have no idea what Newtons are, so I think the right place to put labels would be on the force vectors, but the really gets incredibly cluttered.

I also think if we going to go into "weight mode" we would want to use nice round numbers for everything.

It really seems that one day what we most need is a sim that is devoted to torque, where as this sim is really more focused on a younger audience learning about some basic ideas and proportional reasoning. I am going to mark this one deferred and we can potentially revisit if we get further teacher requests.

[amanda-phet]

Discussed 12/7/23 and we'd like to close this issue. We have the documentation here and haven't received more requests. This request is not appropriate for this age group, and a sim on torque would be more appropriate.

[arouinfar]

@RVieyra shared a user request for this feature over Slack:

Balancing Act: We have one user who is requesting to have the option to visualize brick mass OR weight. I'm curious as to if this was ever considered, or could be a customization option at some point in the future. Message below:

Dear Phet,

I regularly use the 'Balancing act' simulation to introduce the concepts of torque and balance in my classes. My students love it but they are often misled in the calculations of torque.

Torque is in fact the product of a force and a moment arm. It is possible to visualise force vectors in the app, but not force magnitude. Mass is visible instead, so students tend to multiply mass times the moment arm. The result is that they end up believing that torque is equal to mass times moment arm, and its unit is kg·m.

My proposal is to introduce a setting in the simulation where the user chooses whether to visualise either mass values or force values. Users used to masses can use the app as they have done so far, those wanting forces could have forces instead.

To help students, it could be assumed that gravitational acceleration is 10 N/kg, so that a person would show a 60 kg label when using masses, and a 600 N label when using forces, instead of 588 N.
Please let me know what you think about my proposal.

[arouinfar]

I'll tag this for design meeting since we've received a new user request on this topic.

My proposal is to introduce a setting in the simulation where the user chooses whether to visualise either mass values or force values

This would work well as a Preference to switch between Mass and Weight labels. In 'Weight' mode, the checkbox becomes 'Weight Labels' and we would display the weights instead of the masses. To accommodate the longer string, we should break the label onto two lines, as we currently do for the brick masses.

To help students, it could be assumed that gravitational acceleration is 10 N/kg, so that a person would show a 60 kg label when using masses, and a 600 N label when using forces, instead of 588 N.

@ariel-phet also affirmed that using round numbers for weight is desirable:

I also think if we going to go into "weight mode" we would want to use nice round numbers for everything.

The simulation currently uses g = 9.8 m/s² in MassForceVector.js. We could choose to instead use g = 10 m/s², and I think that would be an appropriate approximation for the target audience of this simulation. However, we would likely get pushback for this modification.

I see a few possible options if we want to move forward with this request.

1. Add Mass vs. Weight Preference and leave g at 9.8 m/s². This better supports learning goals tied to torque, but the weight values will be messy making it harder to achieve proportional reasoning goals.
2. Add Mass vs. Weight Preference and round g to 10 m/s². This is an age-appropriate approximation that can better support the facilitation of teaching torque while maintaining nice numbers. Some users may not like this approximation, including PhET-iO clients.
3. Add Preferences for both Mass/Weight and g=9.8/10. This maintains accuracy and allows for more flexibility, but makes the sim more complicated.

For the target audience of Balancing Act, I think (2) is most appropriate. If the team does not want to make changes to g, I recommend closing this issue and deferring until we create a sim about torque.