pulley calculations

[bspooh]

Could someone help me out on figuring a pulley calculation.

Please neglect pulley diameters..

If I had (2) pulleys hanging from the ceiling..Their both hanging about 1 foot from the ceiling and about 15 feet apart..If If I connect a 100 pound weight with cable and then run it up over the first pulley and then run it linear to the second pulley and then drop it to the ground where I would proceed to pull down on the cable which lifts the weight..What kind of advantage is their?

would it still take 100 pounds of pulling force to lift the 100 pound weight.

I am clueless..I hope I simplified my question..

thanks,
brent

[Evan]

There is no advantage in that case. Advantage is produced when the amount of line pulled differs (increases) in relation to the line movement connected to the load.

As example you can obtain a 2 to 1 advantage with a single pulley by fastening one end of a line to the ceiling, passing the line through a block attached to the load on the floor and then pulling the bitter end of the line upwards. The bitter end will move twice as far as the load attached to the block (pulley).

[jwaggs]

You wont get any mechanical advantage if the 100 lb weight moves the same distance as the rope moves. Based upon your description you pull one foot of rope and the weight moves one foot. If you used a block and tackle and you pulled three feet of rope to move the weight one foot you would have a 3 to 1 advantage and it would take 33lbs of force to move the 100lb weight.

[bspooh]

Thanks guys, I understand now...

brent

[Evan]

Incidentally, there is a trick that produces extreme mechanical advantage using only a line and no tackle at all. If something very heavy must be moved slightly then a line can be made fast to a sutiable anchoring point, streched horizontally and fastened to the object to be moved. A stout nylon twisted line works best as it is very low stretch. Force is then applied at the center of the line at 90 degrees to the line. This can give an advantage of several hundred times.

[DR]

In Brent's question, the advantage I see is it's easier for me to pull downward with 100 pounds force than to lift the same.

Besides that, it's lots of fun to set up pulley mechanisms.

[Randy]

Another concept which is helpful in thinking about rope and pulley systems is that, neglecting friction, the tension in the rope is the same at any point. For instance, in a 2:1 block & tackle lifting 100 lb, you have to put 50 lb of tension on the rope. There are 2 segments of rope, each with 50 lb of tension, lifting the load.(This concept applies when all pulleys rotate freely and independently. Pulley diameter is not a facor.)