(1) calculated from the data in the table: mechanical efficiency★ = w? W total = 0.2J0.27J = 74.1%;

(2) As can be seen from the figure, the pulley in the second experiment is a moving pulley. Due to the gravity of the moving pulley, the extra work is increased and the mechanical efficiency is reduced.

(3) From the drawing, it can be seen that the pulley block has different rope winding methods compared with the second and third experiments, which makes its friction force different, which is the main factor that causes its different mechanical efficiency;

Using pulley block can save labor and change the direction of force.

So the answer is:

( 1)74. 1%;

(2) The moving pulley has gravity (or the moving pulley can do more extra work under the same useful work);

(3) friction; It can save labor and change the direction of force.