Net Force = ma
The net force changes but to look at that then we need to look at more than F=BIlsinO
Magnetic Force on each section is constant
Top View
B
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--> [] Horizontal two sides are perpindicular, So F=BIl o=90
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End on
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--> __ End of coil parallel to field so O=0, So T=nBIA. There would be a force on each side, one up and one down
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Rotated
Top View
B
-->
--> | Horizontal two sides are perpindicular, So F=BIl o=90
-->
End on
-->
--> | End of coil parallel to field so O=90, So T=nBIAcos90=o. There would be a force on each side still, one up and one down but no torque as the two forces are both acting through the centre of rotation. Hence net turning force is zero