(20 13? Tieling) As shown in the figure, Xiao Qiang uses a pulley block to move the object A at a constant speed of 6m along the horizontal ground within 30s. It is known that object a weighs 800N and is affected by. (1) An object with a gravity of 800N advances at a constant speed of 6m on a horizontal plane. Although it is powerful and has a distance, it is F⊥S, so it does no work, that is, the gravity work of A is 0J.

(2) Because the weight is borne by three ropes, s=3h=3×6m= 18m,

W=Fs=25N× 18m=450J，

Power of pulling force P=Wt=450J30s= 15W,

(3) The useful work done to overcome sliding friction is: w ′ = fs = 60n× 6m = 360J.

Mechanical efficiency of pulley block: η = w ′ w×100% = 360J450J×100% = 80%.

So the answer is: 0; 15; 80%.