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Rotational motion (don't forget friction)

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In a recent lab done in my physics c class, my group was experimentally determining the moment of inertia of six different objects. We set up a ramp for the objects to roll down at an angle of 3.325 degrees. We rolled the objects down the ramp, recorded the time for each object and then found each objects linear acceleration, radius, angular acceleration, mass, net torque and finally moment of inertia. When we checked our answers with our teacher they were horribly wrong, like an average of 200% error. This was because we neglected ed to include for unction in our calculations for torque. 

This was a major mistake seeing as friction is the only force on the objects that provide a net torque. We went back and fixed our equation for net torque, which previously was the radius of the object multiplied by the x component of the force of gravity, to be the x component of the force of gravity subtracted by linear acceleration and the objects mass, this result was multiplied by the radius to get the net torque. Our lab then produced less massive percent errors, so my advice to you if you are doing a rotational motion lab is don't forget friction! 

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