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Doug Miller
 
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Default Figuring loads / block & tackle theory

In article , (Harry K) wrote:
(Doug Miller) wrote in message
. com...
In article ,

(Harry K) wrote:
(Doug Miller) wrote in message
gy.com...
In article ,

(Harry K) wrote:
"Greg O" wrote in message
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"The Other Harry" wrote in message
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The question was (and is), what happens to the load?

If the entire pot arrangement weighs 40 pounds, does half
of that load go to the top hook and half of it go to the
anchor hook?

--
Harry

I am assuming you have a rope attached to the pot, that runs up to a

hook
or
pulley attached to the ceiling,, then back down to an cleat.
If the pot weighs 40 lbs, the top hook in the ceiling, (or window

frame,
whatever!) with the pulley will be carrying 80 lbs. The tension,

(weight)
felt by the rope at the cleat will be 40 lbs.
Greg

Wrong. The tension on both sides will be equal (20 lbs) and the top
hook will feel 40lbs. There is nothing being added to the 40 lbs to
increase it to 80. I think you have confused the effect of a pulley
which, when rigged right, will cut the lifting force by 1/2.

It appears that *you* are the confused one here. A *movable* pulley will

cut
the lifting force in half. A *fixed* pulley only changes the direction in
which the force is applied -- and this situation is entirely analogous to

a
fixed pulley.

--------40----top-------------
/\
/ \
20/ \20
/ \
load/40 \anchor
------------bottom-------

Nope. You have a major problem he on the left side, a 40-lb weight is
suspended on a rope that has only 20 lbs tension. Doesn't work that way.

Suppose the anchor on the right is replaced by an un-anchored weight. What


weight is required on the right to balance the 40 lb weight on the left?
According to your diagram, the answer is 20 lbs. Now do you see your

error?

--------------------top--------------
\anchor /anchor or pulley
\ /
\20 /20
\ /
\ /
\/
40
load with pulley
--------------------bottom----------

Not the same situation.

No. My first diagram is wrong in that the 20lbs should be 40. The
second is correct. Or am I misunderstanding your second part?

Permit me to clarify. I agree that your second diagram is correct. My point

is
that it's not the same situation as the first diagram, and thus the loads
in the second diagram *must* be different from the loads in the first. You

now
state correctly that the loads in the first diagram should be indicated as

40,
not 20, and I wish to emphasize that this is loadS plural, i.e. in both
segments of the rope -- thus the load on the top anchor in the first diagram
is in fact 80 pounds, not 40 as you stated in your text.


No, it is 40. The second 40 is sort of a ghost 40lbs as it is the
-same- 40 lbs only extended to a second anchor.


Wrong.

Try it, lift 40 lbs
directly and then run the rope over a plley and pull on it, you will
still only see 40 lbs. See me exlanation to Greg O.

I *did* try it, as I described in an earlier post. Go back to the original
post in this thread. The question is how much load is placed on the beam or
whatever that the pulley hangs from. With a weight hanging free on one side,
and the rope it hangs from tied off to a stationary object on the other side,
the load placed on the pulley's support is approximately double that of the
weight, depending on the angle of the anchored segment of the rope. The
mathematics behind this has been clearly (and correctly) described by others
in this thread, and I won't repeat it here. You can look it up if you want to
understand why this doesn't work the way you think it does.

Your explanation to Greg is just as flawed as your explanation here.

Harry K