Answer to Question #168485 in Physics for April

Question

A pulley system as shown in figure 4 has an ideal mechanical advantage of 5 and is used to 

lift a load of 1000 lb. If the effort moves 10 ft, how far does the load move? If this work was 

done in 15 s, what horsepower was developed? If the machine was 80 % efficient, how much 

effort would be needed to lift the load?

Answer 1

If the effort moves 10 ft, the load moves 5 times less because the mechanical advantage is 5:

$$H_L=H_E/5=2\text{ ft}.$$

Find the work:

$$W=mgh.$$

The power is

$$P=\frac Wt=0.2423\text{ hp}.$$

The effort needed will be

$$F=\frac{m}{\eta\text{M.A.}}=\frac{1000}{0.8·5}=250\text{ lb}.$$