248 Our Surroundings 



Its main use is in changing the direction of the force applied, 

 but the amount of force needed is no less than without a pulley. 

 For example, by means of a fixed pulley, a weight may be lifted 

 vertically by a pull in any convenient direction. 



The single movable pulley is fastened to the weight and moves 

 with it. One end of the rope is fastened to a support. This 

 corresponds to the fulcrum of a lever. The rope then passes 

 through the pulley, which corresponds to the weight. The effort 

 is applied to the free end of the rope. Being a modified lever of 

 the second class, it always gives a mechanical advantage of greater 

 than 1. That is, it requires less effort, measured in pounds, than 

 the weight to be lifted. 



A combination of one or more movable pulleys with one 

 or more fixed pulleys is called a block and tackle. With its 

 help, very heavy weights can be moved with comparatively small 

 effort. 



The Wheel and Axle. The wheel and axle is another varia- 

 tion of the lever. It consists of a cylinder, the axle, to one end of which 

 is fastened a wheel or a handle. Around the axle is wound a rope, 

 one end usually being fastened to the axle and the 

 other being connected to the weight to be lifted or 

 moved. Turning the wheel or the handle rotates the 

 cylinder, causing the rope to wind up on it and 

 thereby lifting or moving the weight. The wind- 

 lass, used to hoist water from wells, is a familiar 

 form of wheel and axle. 



The steering apparatus of an automobile is an 

 application of the wheel and axle. A slight pull on 

 WHEEL AND ^ e steer i n g" wheel exerts great turning force on the 

 AXLE steering rod, which corresponds to the axle. The 

 Why ie 1 v s er? is a steer i n g ro< ^, instead of winding a rope, exerts its 



force on rods connected with the front wheels. 

 The Inclined Plane. The inclined plane is a flat surface 

 set at an oblique angle to the plane of the horizon. The effort 

 required to move any weight upward along such a surface is less 

 than that required to move it vertically. If a boy attempts to lift 

 a barrel of apples into a wagon and finds his strength is not 



