- #1
PhyStan7
- 18
- 0
Homework Statement
Ok i have the following problem. There is a pulley with radius 7.5 cm and 2 forces acting on it (2 tensions of a string) acting in opposite directions either side. One is 18.2N, the other 7.5N. I know the acceleration is 3.75 meteres per second squared.
Find the moment of inertia.
Homework Equations
Change in torque = Ia
The Attempt at a Solution
Ok so the torque of each individual force is r x F (F being the force and r the radius in meters). As the forces are acting perpendicular, the torque of each force is rF.
So i thought the total torque is τ = r(18.2-7.5) = Ia.
So i thought the moment of inertia would equal would equal...
I = [r(18.2-7.5)]/a
However my answer seems to be a factor of r out, i.e the equation [(r^2)(18.2-7.5)]/a sems to yield the correct answer.
Why is this? I do not undersdtand where the extra r comes from which it is multiplied by.
Thanks