PULLEYS, INCLINED PLANES, AND WEDGES 45 



wedge will lift a weight as many times greater than the force, 

 as the base or length of the wedge is times as long as the 

 vertical face or thickness This may be 

 stated as a rule as follows: 



To find the force required to lift a certain 

 weight multiply the weight by the greatest 

 thickness of the wedge and divide by the 

 horizontal length. 



FIG. 28. Lifting 

 Power of Wedge. 



On the inclined plane previously described 

 the force acts in a direction parallel to the 

 plane; that is, the cord attached to the 

 ball pulls up the plane. In 'Fig. 28 a weight W is being lifted by 

 driving two single wedges. To raise the weight we must strike 

 or push on the face of either one of the wedges, as at F on the face 

 ab. This force acting parallel to the base ac of the wedge causes 

 a pressure P in a direction at right angles to the base. 



Problem on the Wedge 



A single wedge is 2 ft. long and 4 in. thick. What force must 

 be applied to it to lift a weight of 600 Ibs., neglecting friction? 



The Principle of the Screw. The screw possesses great 

 industrial utility in pressing bodies together 

 or in raising weights, and may be classed 

 among the simple machines. The screw is 

 an inclined plane, and the effect of a screw is 

 produced when such a plane moves spirally 

 around a cylinder. This movement may 

 be illustrated by cutting out a wedge- 

 shaped piece of paper and wrapping it 

 about a round stick or bolt. The sloping 

 side draws a thread on the stick as in 



FIG. 29. Principle 

 of the Screw. 



.Fig. 29, This thread is called a helix (Fig. 30). 



