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My friend is an instructor at an outdoor adventure centre which has lots of zip wires. The instructor's harness has a shorter line that attaches to the zip wire than the normal harness does and she claims that you go much faster with it. Is this true? I can't think of any reason as to why it would be MUCH faster but apparently their speed gun proved it.

Edit: Diagram for clarity

enter image description here

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  • $\begingroup$ Did they prove it with different people in the same harness, or same person in different harnesses? If so, I can think of 2 possibilities. 1) long line could lead to a greater lag angle (how far behind the trolley the person is), which could cause binding of the trolley's pulleys. 2) the line itself has huge wind drag, which is unlikely to be a significant cause. $\endgroup$ – Carl Witthoft Feb 6 '14 at 19:51
  • $\begingroup$ Could you add a diagram? $\endgroup$ – Fergus Feb 6 '14 at 19:58
  • $\begingroup$ @CarlWitthoft I thought the angle could be different and therefore make a difference but after thinking about it some more, I'm not actually sure there would be a difference in the angle. Somebody should work out the math to see if there is any difference in the angle. $\endgroup$ – Brandon Enright Feb 6 '14 at 20:46
  • $\begingroup$ Added a diagram for clarity. I don't think anyone has been speed checked in both harnesses so technically no valid conclusions can be drawn. It's more of the average speed in the shorter harness is more than the average speed in the longer one and just the word of the instructors who have used both. The only thing I can think of as to why is could be quicker would be due to better/less worn pulleys in the instructor harness $\endgroup$ – Josh Roberts Feb 6 '14 at 20:55
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    $\begingroup$ In my experience of these, there is more friction in the trolley than on the person, and there is some swinging backward and forward. A shorter line will make more rapid swinging, and maybe a quicker restoration of the sliding down conditions? $\endgroup$ – Joce Jun 12 '14 at 12:38
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My first guess is that if feels faster since your eyes are closer to the line zipping by. This the same effect that makes it look like you are going slower down the highway compared to distant buildings than to stuff at the edge of the road.

The shorter line would have slightly less wind resistance, but that should be a insignificant effect in the scheme of things.

Someone mentioned the angle of the dangle would be different between someone hanging from a long teather and a short teather, but there should be no difference for the same person as long as the weight of the teather is small compared to the weight of the body.

The weight of the person could matter though. One clue is you said "she" for the instructor. Women, especially physically fit women likely to be such instructors, will be lighter on average than the average customer. She could indeed be zipping faster than most others, although this has nothing to do with teather length. One flaw in your diagram is that you don't show the zip line sagging from the weight of the person. The more the sag, the less of a downhill push the person gets. Think of the limiting case where the line sags so much that you stop at a dead spot before the other end where you are lower than both ends.

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  • $\begingroup$ You should maybe mention that the weight has nothing to do with how much she is accelerated by gravity. It matters for the roll in the zip line which will have a higher roll-resistance with a heavier person for mechanical reasons. $\endgroup$ – Philipp Feb 6 '14 at 21:54
  • $\begingroup$ @Philipp: Right, accelleration due to gravity is the same regardless of weight. If the friction is proportional to weight, then that cancels out too. I don't know how significant friction of the pulley is. They could be made with reasonably insignificant friction for this purpose, but maybe they aren't. How much that varies due to weight is another question. $\endgroup$ – Olin Lathrop Feb 6 '14 at 23:03
  • $\begingroup$ This sounds like something that should be tested. I wonder if it could be to do with the period of oscillations being shorter with a shorter line, though I don't really see how this would make much difference. $\endgroup$ – Benjohn Jul 14 '14 at 12:30

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