Thursday, April 19, 2018

Is angular momentum conserved if you move off at a Tangent?


Lets imagine a binary system of two astronauts in space connected to one another via light rope.


The rope is taut and they're spinning round and round with their axis of rotation being the the axis perpendicular to the their centre of mass.


Now, my question is this. Lets say they each let go of the rope; they move off at tangents.


Is angular momentum conserved ? And what is their subsequent motion ?


My hypothesis : They will move off at constant velocity at a tangent to their circular motion. I believe that because they now under the effects of no external force by the rotational analogue of newton's second law (dL/dt = T) their angular momentum will be of equal magnitude but opposite sign.


Does this make sense ?




Answer



That's correct; there's no change to the angular momentum here with respect to the center of mass. Remember that $L = r \times p$. If $p$ is constant (as it is once they both let go of the rope), then you can see that $r_\perp$ is constant also, and $r_\parallel \times p$ is guaranteed to be zero.


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