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    The title is intentionally simplistic so as to helpfully convey a dilemma I've come across during some electromagnetism reading (specifically when reading over the torque that results on a rectangular current loop in a uniform B field). I'm used to computing and thinking that I physically understood torque...

    Whilst I'm happy with the nut and bolt calculation RE displacement vector crossed with force applied, I realised that I do not know what the physical justification is for the existence of torques.

    For example taking this thought experiment:

    There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?

    Returning to the source of my doubt, the current loop, why should the loop rotate about the central axis of the loop - what determines that this will be the rotation axis?
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    (Original post by Sm0key)
    There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?
    Both.

    Think about why objects orbiting each other 'wobble' when referenced to a third fixed point not associated with either body. Now apply a similar thought process to the rocket attached to the rod.
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    (Original post by uberteknik)
    Both.

    Think about why objects orbiting each other 'wobble' when referenced to a third fixed point not associated with either body. Now apply a similar thought process to the rocket attached to the rod.
    Are you referring to the barycentric orbit for 2 bodies (like here: https://en.wikipedia.org/wiki/Barycenter)?

    If so I always understood that both are undergoing all rotational motion? Still unsure about the concept I'm afraid.
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    (Original post by Sm0key)
    The title is intentionally simplistic so as to helpfully convey a dilemma I've come across during some electromagnetism reading (specifically when reading over the torque that results on a rectangular current loop in a uniform B field). I'm used to computing and thinking that I physically understood torque...

    Whilst I'm happy with the nut and bolt calculation RE displacement cross force applied, I realised atet I don not know what the physical justification is for the existence of torques.

    For example taking this thought experiment:

    There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?

    Returning to the source of my doubt, the current loop, why should the loop rotate about the central axis of the loop - what determines that this will be the rotation axis?
    For the rod example - you are applying an external force, so the centre of mass will accelerate. You are also applying an external torque, so the object will rotate. In this case, it's obvious where it will rotate around (presumably around it's centre of mass).

    For the torque of a dipole question, you still have enough symmetry to 'see' where it will rotate around: There's two external forces that form a couple, and the loop is symmetric. (I can't work out how to actually describe what axis it is beyond a VERY unhelpful "it's the perpendicular bisector of the point of action of the two forces).

    In general, you may not be able to use symmetry arguments to determine the axis of rotation. In these cases you are going to have to resort to fairly high-powered dynamics methods I suspect - either using Euler's Equations or some other method.
 
 
 
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