F=ma

First of all, if people don't move, the system is unstable, the force on the wood block is F=(m*g-m/2*g)=mg/2, and the relative speed between people and the rope is zero.

Secondly, when people climb up, the speed relative to the rope is U. No matter how big the U is, the force on the rope is mg, because the force exerted by people on the rope must be balanced with gravity to maintain a constant speed.

Therefore, the resultant force of wood blocks is: f = mg/2+mg = 3/2 * mg.

The acceleration of the rising speed of the square is 3/2*mg, and the speed is equal to 3/2 * mg * t.