(2004?Suqian) The mechanical efficiency of a pulley block is determined using the device shown in the figure. Pull the spring loaded dynamometer upward, so that the object A weighing 5.4N rises 0.2m at a constant speed.
The division value of the dynamometer is 0.2N, so the number of the dynamometer is 2.4N.
This pulley block consists of 3 sections of rope to bear the weight of the object, so s=3h
η=
=
=
| 5.4N x 0.2m |
| 2.4N x 3 x 0.2m |
= 75%
So the answer is: 2.4; 75%.