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Imagine a body which is allowed to rotated around its centre of gravity. I place a rotating moment, $M_d$ on a point which is a distance d from the body's centre. $M_d$ is only required to put the body in motion, i.e. overcome the friction and intertia required for the body to start rotating around its centre of gravity. Compare this moment to a moment $M_0$ located at the centre of gravity. Same conditions apply for $M_0$, it only has initiate rotation. Is $M_0 = M_d$? Or is one bigger than the other?

Theoreticaly they should be the same but intuitively when compared to screwing a screw, it is easier to screw the screw if you apply the moment directly on the screw axis of rotation. Could someone please explain if there is a theoretical difference or if it is only my intuition which is wrong.

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  • $\begingroup$ So why do you need a big flat screwdriver for big screws? By your analysis a small flat screwdriver is sufficient... $\endgroup$ – Solar Mike Feb 8 at 16:46
  • $\begingroup$ I think the problem you are describing is more related to points of contact and grip? A larger screwdriver will have contact on the entire screw head compared to a smaller one, thus being able to transfer a larger moment. I wonder if not your answer together with kamran's answer is the one I am looking for. $\endgroup$ – loStraniero Feb 9 at 13:56
  • $\begingroup$ No, the screwdriver will have two points of contact... $\endgroup$ – Solar Mike Feb 9 at 13:59
  • $\begingroup$ Isn't half the surface of one side in contact together with half the surface of the other side? $\endgroup$ – loStraniero Feb 10 at 11:09
  • $\begingroup$ Put engineer’s blue on a screwdriver and test. $\endgroup$ – Solar Mike Feb 10 at 11:19
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A pure torque applied to a rigid body doesn't have a point of application. But if the torque is the result of some forces to that body more likely there will be a force F passing through the Center of the mass of the body and a torque applied to the body.

note how the resultant of the forces have simplified to torque and a force F applied at the CG of the rigid body in the figure.

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rigid body forces

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  • $\begingroup$ Thanks for the answer! I think I understood it when I was reading @SolarMike comment above. If pure momment cannot be applied to a point but as in the case with the screwdriver it is a moment-force-moment transfer situation, where moment is transfered from your arm to the screwdriver. The screwdriver in turn acts on the screw head with forces which cause a rotation (moment) on the CG. $\endgroup$ – loStraniero Feb 9 at 13:59

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