Mechanical efficiency calculation problem

Mechanical efficiency computation chainb lock is an examination questions type, as long as the students grasp the mechanical efficiency calculation problem of the problem solving skills, accurately identified which force to do the work is useful work, which total energy is doing the work force, will be able to correct the problem solving fast. In this paper, through the analysis of the general topic, for example, and the mechanical efficiency, and the students to manual hoist and summary mechanical efficiency calculation problem of the problem solving skills. A using block and tackle, the vertical case 1 in small construction site, simple lifting equipment commonly used vertical lifting construction materials, its working principle of pulley block as shown in figure 1. One will be the total weight is 4000 G n bricks uniform to 10 m high above, takes 40 s, hoist provide pulling force F 2500 n. : (1) the useful work in the process; (2) the work and power of the force F; Analysis (3) the mechanical efficiency of pulley block. (1) work is equal to the size of the forces acting on the object and the object in the direction of the force on through distance by figure 1. The figure 1 shows that useful work is the effect on the objects of the work done. As shown in figure 2, because the object in the vertical direction to do uniform motion in a straight line, so F = G, namely there W = F, h = Gh. (2) due to mechanical tension is through the direct effect on the rope of the free end, so the work force F do is work all the time. So the power of pull force F can be used.