(2006? Rare classmates live upstairs. Recently, she is decorating the house and needs to carry the decoration materials, but some materials are not convenient to move because the corridor is too narro

(2006? Rare classmates live upstairs. Recently, she is decorating the house and needs to carry the decoration materials, but some materials are not convenient to move because the corridor is too narrow. (1) Winding downwards from the crown block;

(2) useful work done on materials w = GH = 765 n×10m = 7650 j;

(3) The moving pulley is supported by two ropes, and the pulling force moves twice as far as the distance between the object and the moving pulley.

Therefore, the mechanical efficiency η=W has w total = =GhF? s = GhF2h = G2F = 765 N2×450n = 85%；

(4) According to the given data analysis, the mechanical efficiency is always less than 1.

So, the answer is: (1);

(2) The useful work done by the pulley block when lifting materials is 7650J；;

(3) The mechanical efficiency of pulley block is 85%;

(4) According to the given data analysis, the mechanical efficiency is always less than 1.