Physically Incorrect

This is the solution to this problem.

The answer is 3Mg. We’ll solve this in 3 steps. First, let’s look at the weight hanging from the pulley and draw a force diagram for it, applying Newton’s 2nd law along the vertical (up-down) axis:

Note that A = 0 so MA = 0 as well, where M is the block’s mass. The block is acted upon by two forces, its weight (-Mg) and the tension in the rope T. Newton’s 2nd law tells us that T = Mg. Next, let’s look at the pulley:

Because the rope is massless, it “transmits” the tension throughout its length. This means that it’ll be pulling the pulley downwards at either side by the same amount. Once again, since the pulley isn’t moving (A=0), the downwards force must cancel the upwards tension of the rope, T’. This tells us that T’=2T=2Mg=20 Kg. Finally, examining the weight between the pulley and the scale,

we find that the tension in the rope leading to the scale, T”, is equal to T”=T’+Mg = 3Mg = 30 Kg, which is the weight the scale will register.