(1) useful work w useful =Gh=5N×0. 15m=0.75J, mechanical efficiency η= W useful 2 W total 2 = 1J 1.35J =74%, pulling force F3 = w3s3 =/kloc.

So the answer is: 0.75; 74,3. 1.

(2) Through the analysis of the data in the table, if only the labor-saving fact is considered, the first slope is the most suitable, and the mechanical efficiency should also be considered when determining the slope, because the higher the slope, the higher the mechanical efficiency.