GENERAL SCIENCE 



coming from the hook, touches. The force arm is the 

 whole diameter of the pulley and the weight arm is one- 

 half the diameter. If the diameter is four inches, the 

 force arm will be four inches and the weight arm two 

 inches. The mechanical advantage of a single movable 

 pulley, then, is 4 -T- 2, or 2. Hence the force required to 

 lift a 2oo-pound weight is 100 pounds. The other 100 

 pounds of the weight is held by the strand of rope fas- 

 tened to the hook. If two persons held the ends of a rope 

 with a loo-pound weight hanging on it, each one would 

 be holding 50 pounds, or one-half of the 100 pounds; 100 

 Ib. -T- 50 Ib. = 2, the mechanical advantage. 



Since the strand of rope fastened to the hook does not 

 move when the force lifts the weight, the 

 force will move twice as fast as the weight. 

 When the weight is lifted three feet, the 

 force will have moved six feet. Hence the 

 work done by a force of 100 pounds is equal 

 to the work accomplished on a weight of 200 

 pounds when it is moved 5 feet: 100 X 10 = 

 200 X 5. 



(c) Combinations of Pulleys. When one 

 or more fixed pulleys are combined with one 

 or more movable pulleys, the combination 

 is called a block and tackle. Heavy weights 

 can be lifted by such combinations with a 

 small force. Two or more pulleys may be 

 fastened in one block, and when this block 

 is securely attached, it is called the fixed block of pul- 

 leys. When a block of pulleys is attached to the weight 

 which is to be moved, this is called the movable block of 

 pulleys, or simply the movable pulleys, because they move 

 with the weight. The fixed pulleys are used only to change 



BLOCK AND 

 TACKLE 



