The pulley is a wheel with a cord, rope or belt running round it as shown in Fig. 44. It is used to transmit power and also to change the direction of it. A pulley can be either fixed or movable. A compound pulley makes it possible to raise a heavy weight with a very small force, not by increasing the energy, but by transposing velocity into power.
Fig. 44. A FIXED PULLEY
Fig. 45a. AN INCLINED PLANE
Fig. 45b. ONE OF THE USES OF AN INCLINED PLANE
The inclined plane is any hard smooth surface set at a slant to the force to be overcome. A barrel can be rolled up an inclined plane against the force of gravity, as shown in Fig. 45, while it could not be lifted straight up to the same height.
The wedge is simply an inclined plane on a small scale. It is useful where a great force must be exerted through a small distance, as in splitting a stick of wood, as shown in Fig. 46.
Fig. 46a. A SIMPLE WEDGE
Fig. 46b. TWO WEDGES FORM A PRINTER’S QUOIN
A screw is also a modified form of an inclined plane. By means of a screw great pressures can be exerted in a small space and here again a powerful force is had with a corresponding loss of velocity. It is shown in Fig. 47.