There is a very simple way to figure the mechanical advantage without
any understanding for forces et al.
With the rope/pully arrangement in place:
1) Move the rope you pull on, say 5 feet.
2) Measure how much you elevated the object being raised ,say it turns
out to be 1' .
The theoretical mechanical advantage is the ratio of the number of
feet in 1 to that in 2. In this case the mechanical advantage is
In terms of physics work = force x distance.
In a frictionless system, the work you must exert to raise an object
up the tower will be the same regardless of the mechanical advantage.
(aka "you don't get something for nothing" or "there ain't no free
You reduce the force you exert by the mechanical advantage. To
perform the same work of raising the object you must increase the
amount of rope you pull by the mechanical advantage factor. Add
friction, the force you have to apply increases, the amount of rope
you have to pull doesn't change. The extra work goes to frictional
heating of the system.
The actual mechanical advantage in a rope/pully system is (20-50%)
less due to friction.
Now look at the gin pole system. You pull on the rope 1', the tower
section goes up 1'. The theoretical mechanical advantage is 1.
73 de Brian/K3KO
"Gary McDuffie, Sr." wrote:
> On Wed, 13 Jun 2001 11:50:37 -0700, Brown, James E LRDOR wrote:
> > And what if there are no workers applying "downforce", but instead the end
> > of the line is tied off to a tree, and the load is suspended static?
> The down force is subtracted from the weight of the tree and still
> applied as down force to hold the load. The down force of the tree to
> the ground decreases by the same amount that is transferred to the
> suspended load. ;o)
> Now... Since you probably didn't have a tree right at the base of the
> tower, you are running the rope at an angle and you'll have to vector in
> the angular pull, etc. ;o}
> ag0n at arrl dot net
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