Pete,
Imagine another scenario - pull the 200 lb load half way up the tower. Now tie
the other end of the gin pole rope off to the base of the tower. Now climb the
tower to the point where the load is hanging. Make a red mark on the side of
the tower to signify the position of the load. Now continue climbing until
you
reach the base of the gin pole. Imagine that the bottom of the gin pole
pipe is flush with the bottom of the holding fixture that clamps it to the
tower. Loosen the set screw on the holding fixture a little until the gin pole
pipe starts to slip downward thru the holding fixture. Let the gin pole drop
exactly 1 foot, then re-tighten the set screws (don't try this at home -
its a
thought experiment). Now climb down the tower until you reach the load.
Measure
the distance between the loads current position and the red mark (position
before
you dropped the mast). You should find that the load has dropped 2 feet
(twice
as far as the gin pole). Now imagine that you do the experiment all over
again,
except this time tie the far end of the gin pole rope to the load
instead of the base of the tower (I will leave the logistics of this to you).
In this case, when you drop the gin pole 1 foot in the holding fixture, the
load
will drop 1 foot instead of 2. That is the difference. The first case (Stan's
example) is a 2:1 mechanical advantage, hence the doubling of the force on
the
gin pole. The second case (your example) is a 1:1 mechanical advantage hence
the
gin pole only sees a force equal to the weight of the load (I am neglecting
the rope for simplicity sake). Essentially you are both correct, but your
examples
are not the same as illustrated above. As far as adding the 3:1 mechanical
advantage to the pull rope - Stan hasn't convinced me yet that this will
lighten the load on the gin pole. I think it will lighten the load on the
ground
crew, but I am not sure that we are talking about the same configuration (help
me here Stan).
73 de Mike, W4EF..............
At 01:41 AM 4/3/99 +0100, you wrote:
>Mike, I think Stan's wrong here. Imagine a situation where you tie the
>down-pull rope to the up-pull rope, and tie them both to the gin pole. How
>much load is the gin pole supporting? The weight of the load, plus the
>weight of the rope. The multiplication provided by the block and tackle
>only applies to the pull required on the end of the rope, and is a function
>of the distance times the force. In a 3:1 block and tackle you are pulling
>3X the length of rope at 1/3 the force. The load on the fixed pulley at
>the top of the gin pole is exactly the same.
>
>73, Pete Smith N4ZR
>n4zr@contesting.com
>
>Loud is.
>
*********************************
* Michael C. Tope *
* Jet Propulsion Laboratory *
* Section 333 *
* M.S. 238-737 *
* Tel (818) 354-1335 *
* Fax (818) 354-2825 *
* email: mtope@jpl.nasa.gov *
*********************************
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