# Developing the Balanced Torque Model

Here’s how students in AP physics developed the idea of ‘turning effect’ (eventually named ‘torque’) using a bike tire, brooms (push-o-meters), meter sticks, and spring scales (pull-o-meters).

1.  Observe the following scenarios, record any observations. (more details on these here)

2. In a small group, establish a ‘set of rules’ to describe your observations.  Make your rules simple and applicable to as many of the scenarios as possible.

3. Since ‘turning effect’ depends on the location of the force (radius) and the amount/direction of the force (must be perpendicular to the radius), investigate the relationship between ‘radius’ and ‘perpendicular force’ for a simple object.  Hey, how about a meter stick?… it’s already got the radii markings on it!  Hang a mass on one side, then use a ‘pull-o-meter’ to measure the force at different radii.

4. Graph F vs. r.  (gives inverse relationship)  Linearize by graphing F vs. 1/r.  Write an equation.  Example equation: F= (0.5 Nm) 1/r.  Groups will end up with different slopes.

Compare graphs with other groups.  Realize that slope was due to the ‘turning effect’ caused by the location and size of the hanging mass chosen.  Rearrange equation to yield: F*r = 0.5 Nm . Define the quantity F*r = Torque!

Formalize by including the sine of the angle between F and r.

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### 4 responses to “Developing the Balanced Torque Model”

1. Thanks for this post. I am teaching AP Physics 1 this year and I wanted to have my students build both a balanced torque model and then an unbalanced torque model. Any suggestions on a paradigm lab or deployment for developing an unbalanced torque model?

2. I introduce it qualitatively with a demo and discussion… see here https://vine.co/v/bJr5hJd0zW1
I then define moment of inertia and give them the equation Tnet=I*alpha and discuss how it is similar to N2L for linear motion. If anyone has a good way/setup to quantify the relationship I’m all ears!

• Thanks Sam,

I have looked over the material presented in the Rotational Motion Model in the AMTA downloadable material. They refer to 5 different activities in the curriculum sequence, but then don’t actually provide any details about the activities (unless I am missing them?). In the notes provided in the download, they briefly hint at the activities – it looks like they do a rolling disk/hoop experiment for example – but then they stop at rotational energy and do not present any curriculum on angular momentum. If you find anything I would love to hear about it.

-Steve.