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    thread highjacked by bandits ....
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    (Original post by TeeEm)
    It looks harder than it is ...

    it is from a very old paper (50 years or so ...) and took me ages as I had similar difficulties to model it
    (it was the "show" that help me see it)
    Having thought about this a bit, the derivation seems straightforward (or a bit easier maybe) if we choose an origin at the (accelerating) c-o-m of the door, as this allows us to state that dL/dt = external torque about the c-o-m, for L calculated w.r.t to this origin, and the only external torque is that due to the hinge.
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    (Original post by atsruser)
    Having thought about this a bit, the derivation seems straightforward (or a bit easier maybe) if we choose an origin at the (accelerating) c-o-m of the door, as this allows us to state that dL/dt = external torque about the c-o-m, for L calculated w.r.t to this origin, and the only external torque is that due to the hinge.
    You are probably right ...

    I cannot confirm at present ....

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    I always find it strange how I could have done it pretty routinely 2 years ago, but would need to read up on a tonne of stuff to do it now.
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    (Original post by AntiBabylonista)
    I always find it strange how I could have done it pretty routinely 2 years ago, but would need to read up on a tonne of stuff to do it now.
    any better?

    http://www.thestudentroom.co.uk/show....php?t=3870889
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    (Original post by TeeEm)
    You are probably right ...
    In fact, having tried to read up on the details of torques in accelerating frames (been a long time since I've had to do so), I think I overstated the requirements. The calculation will work if:

    1) we choose any frame such that the door is not accelerating in this frame. You did this by setting the origin at the hinge, and then calculated T,L,I relative to that. You could have chosen the origin at the accelerating c-o-m of the door equally well though.

    2) we choose a frame where the door *is* accelerating, but we then calculate T,L,I relative to the accelerating c-o-m of the door. In this case, we can use the usual relation \tau = dL/dt where \tau is the torque due to the external forces about the accelerating c-o-m. (So in this case, we could choose an origin stationary on the ground relative to the train, but we would have to work with messy time-dependent position vectors for the location of the c-o-m and forces)

    What we can't do though is to choose a frame where the door is accelerating, then blindly calculate torques about, say, the end of the rod, as you have done - we would get the wrong answer without introducing an additional "fictional force" torque term. This would, of course, be painfully messy compared to the sensible option 1) above.
 
 
 
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