## What causes rotation, a couple or a moment? (conceptual)

Does a moment have any real (physical) significance or is it just a definition/ an aid to understand and calculate a couple. Is it that the rotation is actually caused by a couple!

I know we can resolve a force causing a moment about a point into a couple and an equal force acting at that point, but what i want to know is how does this actually happen in a real world scenario.

Also,
Say if there was a body (an extended bar or rod) in space so that no force would act on its centre of mass, then if a force F was applied on one of its ends what would happen and why?

forgive me if the question sounds stupid, but this concept has ben bugging me for a very long time now!

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 Quote by Sarin Say if there was a body (an extended bar or rod) in space so that no force would act on its centre of mass, then if a force F was applied on one of its ends what would happen and why?
The object would undergo a linear acceleration per $m\vec a=\vec F$ and would undergo a rotational acceleration per $\mathbf I \,\dot{\vec\omega} = \vec r \times \vec F - \vec{\omega}\times (\mathbf I \,\vec{\omega})$ .

 Quote by D H The object would undergo a linear acceleration per $m\vec a=\vec F$ and would undergo a rotational acceleration per $\mathbf I \,\dot{\vec\omega} = \vec r \times \vec F - \vec{\omega}\times (\mathbf I \,\vec{\omega})$ .
In words, it's because you apply a force "on" the COM ie. linear acceleration, but a the same time, you apply a torque about the COM ie. rotational acceleration (unless the force is applied directly on the COM, then it's just linear).

Mentor

## What causes rotation, a couple or a moment? (conceptual)

 Quote by _DJ_british_? (unless the force is applied directly on the COM, then it's just linear).
A force doesn't need to be applied directly on the CoM (center of mass) to get pure translation. It just needs to be directed "in line" with the CoM. For example, in the case of a force applied at the end of a rod, there is no torque on the rod if the force is directed toward or away from the center of the rod (i.e., if the force is collinear with the rod).

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