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Each supporting strand of a block and tackle increases the mechanical advantage by
Weegy: Let's look at a simple system. A block with three wheels is attached to the ceiling and a block with three wheels is attached to a load to be lifted off the floor. [ As well as the block attached to the ceiling, we have a hook there also near to the block and we start by knotting one end of our rope onto that hook. We now run the free end of the rope through the bottom block, back up through the top block, back down to the bottom block and so on, finishing up with the free end of the rope coming out of the third wheel at the top and hanging down. Now for the physics. Physics says that a well oiled pulley wheel cannot alter the load in a rope passing over it. Therefore, we have to accept that the load in any part of the rope is the same. Now we count how many lengths of rope are going to be lifting the load. I see six. Three wheels in the bottom block and two lengths of rope to each wheel. As these six ropes are all pulling upwards when we pull down on the free end, and the load in each rope is the same, the load in the rope is one sixth of the weight to be lifted. This is the pull required on the free end just to hold the load off the floor. A sightly greater pull will overcome any friction in the pulleys and start the load moving upwards. The mechanical advantage of the system we have here is 6. An interesting variation is when we have four wheels in the bottom block and pass what was the free end hanging down through the fourth wheel. We now have the free end to be pulled upwards and it will be contributing to the lifting forces on the weight. There are now eight lengths of rope lifting the weight and the pull required will be just a tad more than one eighth (1/8) of the weight to be lifted. Read more: ] (More)
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Asked 5/11/2011 1:54:35 PM
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