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Any sort of angular acceleration produced in a body is due to its torque. Now, consider the fan, it's weight passes through the centre of mass, so torque is 0. Now, is it the motor that is attached to the fan, that puts some force that does not act through the centre?

If yes, then what is that force? Or my conception is wrong about the forces passing through centre produces 0 torque?

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  • $\begingroup$ The electric motor generates a torque, which is transmitted directly via a shaft. The shaft can be fixed to the blades in a number of ways (eg: friction or set screws). If you draw a free body diagram of the fan blades, you will notice that the forces that fix the blades to the shaft do not act through the center of rotation. Thus, torque. $\endgroup$
    – Tyler Olsen
    Aug 27, 2015 at 18:16
  • $\begingroup$ Can you transform it in detail into an answer? $\endgroup$
    – Aneek
    Aug 27, 2015 at 18:17
  • $\begingroup$ Draw a free body diagram of the parts and balance forces. It would take a long time to put this into a nice format on the computer, but 2 seconds to do it on paper. $\endgroup$
    – Tyler Olsen
    Aug 27, 2015 at 18:20
  • $\begingroup$ @TylerOlsen I see. Thanks sir. But one thing. Does the force then acts tangentially? $\endgroup$
    – Aneek
    Aug 27, 2015 at 18:21
  • $\begingroup$ Yes, the force acts tangentially. Imagine that friction holds the fan blades to the shaft. The motor imparts a torque $T$ to the shaft. In order to hold the blades to the shaft, there must be a tangential force present equal to $F = T/R_{shaft}$. $\endgroup$
    – Tyler Olsen
    Aug 27, 2015 at 18:40

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You are correct that any forces acting on the axis cannot transmit a torque relative to that axis. But the shaft of a fan is a macroscopic object and consists of material that is measurably away from the spin axis. This deviation is sufficient for it to supply a useful torque.

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