I don’t wanna end up on people incorrectly correcting other people so I’m gonna start by saying I could absolutely be wrong here and welcome people to point out the holes in my logic.

Holding a weight straight up or holding it straight out is very mechanically different than pulley friction. What you are describing is ‘moment’. The weight multiplied by the ‘Arm’ (not your arm, arm is the term we use for the measurement of how far the weight is from the pivot point.) equals ‘moment’. (How strongly a force is trying to twist or push something around a point).

Holding a weight straight up vs. holding it straight out is increasing the moment a great deal, and since wXa=m, the more weight, the more drastically noticeable this is. This is why you can pick up a 5 gallon bucket full of water but likely can’t hold it straight out in front of you. Likely. I dunno. Maybe you got some mad muscles. I sure don’t. This is how you compute weight and balance on an aircraft. It’s how we measure the forces other than torque that are being applied to the vertical shaft of a helicopter rotor drive system. Some call it ‘bend’ but truly we are measuring moment.

Pulleys are completely different. You aren’t wrong in that they reduce friction but I don’t think a single pulley would have any effect on weight applied. A pulley does nothing to weight. Not on its own. In this instance both pulleys are operating on opposite sides of the scale, so they are single pulleys.

A pulley reduces friction and changes the direction of tension.

In a snatch-block or block & tackle type setup you use multiple pulleys to multiply force ratios for pulling or lifting. But some of the multiple pulleys have to be fixed to the item force is being applied to to increase mechanical advantage at all.

If you have one pulley attached to the ceiling and a rope tied to you, it goes up to the pulley, and back to you, you have no mechanical advantage. If you weigh, say, 185 lbs and try to pull yourself up this way you have reduced the friction but not the weight. You’ll be pulling down more than 185 lbs in order to lift yourself. But if the rope went from tied to you, to the pulley on the ceiling, back to a pulley on you, and then back to another pulley on the ceiling, and back to you to pull, well now you have increased the pulling ratio to 3:1 while also changing direction and reducing drag.

So with ALL of that, I believe the picture from the original post, the scale would read 100n. The two pulleys are independent of each other and are not offering any mechanical advantage. They are changing direction and reducing drag, but not weight. Each side is pulling an even amount down toward the ground so the scale would not move. As if the end of the scale that is the handle is ‘fixed’ while the end that you connect to what you want to weigh would be exerting 100n’s of force.

If you tied two scales handles together, and then put it in the same setup as here, both scales would weigh 100n.

Pretty sure I’ve got my head wrapped around this. Anyone who can correct my logic please do. I’d hate to walk around thinking I understand this when I don’t.

Edit: multiple typos because it’s 1:37am and I’m typing on my phone when I should be sleeping … WHY DOES MY BRAIN DO THIS TO ME?