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**1. Homework Statement**

We had this elementary experiment in class. An aluminium rod is fixed at one end and at the other end you have a disk. You hand increasingly heavy weights on the disk and measure how much the rod shears (twists). We were asked to make a graph of the shear torque vs. angle of torsion. It came out quadratic. I'm trying to figure out why, physically and mathematically.

**2. Homework Equations**

Shear Torque = F x d; Equation = aΘ² + bΘ + c (specifically: 6888.1Θ² + 87.951Θ - 1.0003) - Quadratic

Oscillatory Torque = -k*Θ (specifically: - 0.3670Θ) - Linear

ω = 3.27051 rad/sec

k = I*ω² = 0.3670 Nm

I = 0.034315 kgm² (Given)

J = (πD^4)/32 = 6.1359*10^-11

T = 1.92/sec

L= 0.57m

**3. The Attempt at a Solution**

I figure out that the rod was trying to resist the forced torque with an oscillatory torque in the opposite sense. I looked on the web and found the equation (see above). My data seem valid since I could calculate the G (Coulomb's modulus or modulus of rigidity or shear modulus) of the material with a percent difference of 10, which I read was acceptable in physics. I think my shear torque equation gives me the *net* torque, i.e. it takes into account the oscillatory torque. Now I'd like to know how to find what ''pure'' torque, i.e. what the torque would be without the oscillatory torque. Do I add up, substract, multiply or divide the two equations? My gut feeling tells me nothing.....Any hint please? Thanks !

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