How can students derive a torque equation using scales and mobiles?

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    Measuring Torque
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Discussion Overview

The discussion revolves around methods for teaching the concept of torque through hands-on activities, specifically using mobiles and scales. Participants explore how students might derive a torque equation by engaging with physical models and experiments, considering both theoretical and practical aspects of torque in a learning environment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant suggests using scales to measure force by placing weights along straws or skewers, questioning the feasibility of this method since the scale supports the mass.
  • Another participant proposes using a see-saw balance to illustrate the usefulness of torque in determining mass or distance, while also questioning the motivation for teaching torque in this context.
  • There is a suggestion to weigh objects after balancing them to calculate force based on changes in position, although this raises questions about what exactly should be weighed.
  • A participant emphasizes the goal of having students discover the relationship between mass and lever arm length (mass1 x lever arm length1 = mass2 x lever arm length2) through their own exploration rather than direct instruction.
  • Another participant shares a successful exercise where students build a balance to weigh small objects, highlighting the need for a controlled setup to avoid confusion and ensure effective learning about torque.
  • Concerns are raised about the sensitivity of mobiles due to applied forces near the axis, suggesting that this could complicate the learning experience.

Areas of Agreement / Disagreement

Participants express various ideas and methods for teaching torque, but there is no consensus on the best approach. Multiple competing views remain regarding the effectiveness of different experimental setups and the clarity of the concepts being taught.

Contextual Notes

Participants note potential limitations in their proposed methods, such as the sensitivity of mobiles and the need for controlled conditions to avoid confusion in data interpretation. There is also mention of the importance of guiding students to discover relationships rather than providing them outright.

Who May Find This Useful

This discussion may be useful for educators looking for innovative ways to teach torque and balance concepts in physics, particularly through hands-on activities and student-led discovery.

Head_Unit
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Hi Folks:

I'm pondering teaching torque and having students build mobiles as part of that. I started wondering if I could have them derive a torque equation.

That started me wondering-if they were placing weights along different lengths of straws or skewers or something, could they weigh that on a scale to get the force? It seemed like it should work, but them I'm thinking maybe not as the scale will support the mass since the mass rests on it.

?

And if that won't work, what would? How could you derive this? (One plan would be to have students make the mobiles first and THEN analyze them, however I'm looking for alternate ideas).
 
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Use a see-saw balance and show how the concept of torque is useful for working out the mass or distance for the balance.

In all your ideas you are comparing linear forces ... you could rig a pulley system t reverse the direction of the force so it pushes down on the scales for eg. But what does that show?

Even with the mobiles, it's all levers - how are you motivating the need for torque?
 
why not weigh afterwards?
 
Gurdian said:
why not weigh afterwards?

Weigh what afterwards?
 
berkeman said:
Weigh what afterwards?

I meant calculate the force afterwards based on the change in position.
 
Simon Bridge said:
...But what does that show?...

Yeah, I'm not sure exactly what I want. Well, I know WHAT I want, just not how. Instead of just giving it to them, I want the students to discover that to balance the forces, mass1 x lever arm length1 =mass2 x lever arm length2 instead of giving it to them.

I suppose they could build the mobiles, then weigh and calculate. Somehow account for the mass of the supporting rods (bamboo barbecue skewers unless someone has an even cheaper idea). Mmmm, maybe a pre-activity just balancing masses on the rod, like a see-saw?

Thanks for the input, all ideas welcome!
 
I want the students to discover that to balance the forces, mass1 x lever arm length1 =mass2 x lever arm length2 instead of giving it to them.
OK - then you want them to discover the lever laws.

I use this approach myself - it can be very effective.
You'll have seen that the trick is usually to choose the situation so the relationship falls out.
If there is too much freedom, then the data will end up just confusing. An additional problem with mobiles is that there is often a couple applied near the axis by the knot in the thread used to suspend each arm... i.e. they can be a tad too sensitive.

An exercise I've used effectively is to get the students to build a balance to weight something small, like a paperclip or a pin, accurately. They have to use some known masses (1-5g say - all heavier than the object to be weighed) and popsicle sticks (or a ruler or whatever).

They'll quickly set up a see-saw, and discover they need to find out how the weight on one end is balanced by the position of the weight on the other end. Those who don't can be nudged.

Torque can be introduced then as a concept that makes that sort of relationship simple ... especially as you devise more complicated experiments.
 

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