# HELP Magnetic fields and forces Please

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Thread starter 2 years ago
#1
http://pmt.physicsandmathstutor.com/...m%202%20QP.pdf

Im stuck on 3cii - I thought since we worked out the force in the previous question and its acting downwards you add the force to the weight?? But the markscheme takes it away saying its due to newtons 3rd law there is an upward force. However - wouldn't the two forces cancel out leaving just the weight acting downwards then??

Please could someone explain it in simple terms thank you
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2 years ago
#2
(Original post by MrToodles4)
http://pmt.physicsandmathstutor.com/...m%202%20QP.pdf

Im stuck on 3cii - I thought since we worked out the force in the previous question and its acting downwards you add the force to the weight?? But the markscheme takes it away saying its due to newtons 3rd law there is an upward force. However - wouldn't the two forces cancel out leaving just the weight acting downwards then??

Please could someone explain it in simple terms thank you
Action and reaction.

Flemings Left Hand Rule shows the force on the wire will be downwards.

However, the wire is fixed in position, the magnet is free to move. The reaction force on the magnet is net upwards, therefore in the opposite direction to Flemings LHR, which reduces the apparent weight.
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Thread starter 2 years ago
#3
(Original post by uberteknik)
Action and reaction.

Flemings Left Hand Rule shows the force on the wire will be downwards.

However, the wire is fixed in position, the magnet is free to move. The reaction force on the magnet is net upwards, therefore in the opposite direction to Flemings LHR, which reduces the apparent weight.
Oh I didn't realise the wire was clamped - okay I guess that makes some more sense now but the magnets aren't directly above the wire - they're on the sides aren't they? So why would they experience a force from the wire that reduces the weight if they're on the sides? I understand if the magnets were directly above it'd be easier to see this theory. And Also, is the upward force shared equally between the two magnets then?
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