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Attached is a Q from an AS mechanics paper. I'm perplexed about part (c) in which we are told that the tensions in the two cables are equal.

My physics brain says take moments about the CoM of the beam so eliminating the mass of the beam and making the moments of the two cables equal and opposite, which reduces the problem to a very simple one, and gives the correct answer.

Markscheme warns about 'correct' answers which ignore the mass of the beam and offers 4 different options for taking moments, none of them around the CoM of the beam. Can anyone tell me why this method is invalid?

My physics brain says take moments about the CoM of the beam so eliminating the mass of the beam and making the moments of the two cables equal and opposite, which reduces the problem to a very simple one, and gives the correct answer.

Markscheme warns about 'correct' answers which ignore the mass of the beam and offers 4 different options for taking moments, none of them around the CoM of the beam. Can anyone tell me why this method is invalid?