1. The problem statement, all variables and given/known data 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. Relevant 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 !