Why is W = 3/√2 kg for equilibrium in Feynman lecture 2.5?

[haruspex]

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You show forces on two bodies that in principle can move in relation to each other, the plank and the weight. Each FBD should only show one body and the forces on it.
You do not care about forces on the fulcrum, weight or pulley, so just show the plank and the forces on it.

You show the force of gravity on the plank but I see no force from the support or from the rope. You also show an upward arrow in the middle of the plank, but there is no such force here.

 

[new90]

ok

 

[new90]

i añready finis the problem thanks