Hey, mike, you're confusing the issue! If you have 500 pounds hanging on a
block and tackle, it doesn't matter how many ropes, the screwhook at top is
still holding 500 pounds (and now I'm confusing the issue).

--

Christopher A. Young
Learn more about Jesus
www.lds.org
www.mormons.com


"Michael Daly" wrote in message
...
On 8-Mar-2004, (The Other Harry) wrote:

I could hang it off a hook and anchor it to the window
sill. By my logic, half the weight would go to one place,
the other half would go to the other place.

Am I wrong?

Unfortunately, yes. You've got the general idea, but you're
applying it wrong.

The key is to support the _load_ with several lines through
block and tackle. The lines that run from the top block
to the lower divides the load.

The one that you pull on does not support the load if it
comes off the top block. It only supports the load if it
pulls up from the bottom block. So, if you're pulling
down on the rope, ignore it in determining how many lines
divide the load. If you're pulling up, include it.

One pulley at the top isn't going to help you. It results
in only one supporting line, hence the rope has to be pulled
with full weight of the load.

If you use two, single-sheave blocks, you can arrange it to
halve the load. Run the rope from an eye down thru the
bottom block, back up to the top block and then down to be
pulled on. This results in two lines supporting the load and
half the force in the rope.

Note that the object supporting the top block and eye will
have to support the force in _three_ lines - the two lifting
the load and the one you're pulling down on. This means
that the support is carrying up to 1.5 times the weight of
the load (depending on the angle you're pulling at).

Mike