Monkey on a rope, I'm the monkey. Real world.

Factual Questions
21

Absolutely right. I withdraw my objection and smack myself for not noticing this before I opened my mouth. :smack:

[QUOTE=md2000]
I was about 220lb (diet starts next week) and the skinny guy at the other end could not have been more than 190lb.
When I reached for a handhold, missed, and slipped and dropped, he was yanked about 5 feet into the air. The only reason he did not keep going was a practical one - the rope was looped through a ring at the top of the escarpment, and running on a thick folded blanket up there (to prevent chafing and abrasion). So friction stopped him… and me.
[/QUOTE]

That strikes me as something wrong with your setup. My girlfriend is half my weight and has no trouble belaying me without ever leaving the ground. Gym walls and outdoor hitches are meant to have enough friction to prevent that situation.

22

The "relative to you"part is a red herring. If you pull down 1 foot on one end, the other goes up 1 foot. Its a 1 to 1 relation, no mechanical advantage. Altho there might be some help since pulling down on the rope lightens the weight, I think.

23

If you pull 1 foot of rope thru the pulley you have 51 ft on the down side, 49 on the up side, not 49 and a half.

24

But if you pull one foot through your hands, you’re only pulling six inches through the pulley.

25

Hmm- but the six inches through the pulley is all the rope has moved along its length. If I moved the rope on the other side down six inches relative to the ground and the pulley, don’t I go up six inches?

26

You are mixing up where you are measuring the length of rope.
6 inches at the load = 3 inches pulled past the pulley.

27

Yes. But why is that relevant?

28

Because it is.

29

Not to the physics of the situation it isn’t.

Hang your weight in cabbage from a rope that goes over a pulley. Pull the rope down a foot. Cabbage goes up a foot.

Work done = 1 you-foot

Hang your own weight from that rope and pull the rope down a foot relative to you. You go up six inches.

Work done = 1/2 you-foot

To disprove this, a sound explanation based in Newtonian physics which explains the reality described by the OP, you need more than “because it is”.

30

No I don’t

31

We can’t help people with willful ignorance here…

32

This is all that really needs to be said on the matter, rather than relying on intuition or rules of thumb based on lengths of rope that may or may not apply to this particular situation.

33

If it was, me, I think this drawing might be more accurate:


Before:                          After
  O                                O
 | |                              | |
 | |                              | |
 | |                              | |
 | |                              | |
 | |                              | |
 | |Me                            | |
                                  
                                          \_|O   Aaaaaaaaaaaaahhhh!
                                          / |
34

Ah. In that case, you are doing no work at all, but merely converting some quantity of gravitational potential energy into an equal amount of kinetic energy.

35

well I don’t know about slm, but in my case ignorance fought! In fact I’m kind of embarrassed I couldn’t figure that out on my own.

36

Call her out on the weight lie. :smiley:

37

There’s a REAL easy way to answer this.

If you have to pull 1 foot to raise 1 foot, there’s no mechanical advantage.

A stationary person would have to pull one foot to go up one foot.

But in your case, if you pull one foot of rope past you, you’ve only risen 6 inches. The rope you’re pulling came down 6 inches; you went up 6 inches. Ergo, mechanical advantage of 2.

38

I see Chronos and ZenBeam beat me to it. I particularly like Chronos’s point about each segment of the rope holding half your weight.

Not at all a red herring. A person on the ground wouldn’t get any mechanical advantage lifting a dead weight, but the person hanging on the rope gets advantage by lifting his own weight.

He has to pull two feet of rope to get one foot higher. What clearer evidence of mechanical advantage could we want?

Brilliant rhetoric. I’m convinced. Ignore my previous comments.

39

It depends on how far you fall. If the belayer keeps little slack, the weight difference can be pretty substantial without going flying. If there’s a lot of slack, the faller can gain enough momentum to lift someone.

40

Nothing wrong. Works as advertised. A significantly heavier weight falls several feet before stopping; the yank on the rope lifts him from a lying back position to a few feet in the air before he stops (friction). Then as I get handholds and pull myself up a bit, the slack allows him to lower to the ground. perhaps there was a bit too much slack but I did not want to be “hanging” off the rope. Added to that, I was lunging up (and sideways) for a hold, so he had no time to reel in the added slack.

But this is the answer to the OP’s question. In the real world, there is a LOT of friction, it’s designed into the system for precisely the reason you mention (and I demonstrated). The goal is to stop motion within a few feet, not to experiment with physics. Whereas, in the reel world, there is no friction and the heaviest load sinks to the ground.