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Recirculating Purchase

Posted in Puzzle

In the picture above, our friend Alison is pulling on the fall of a purchase. She is also pulling on me, and I am the object she is trying to move. Disregarding friction and angles, what is the total mechanical advantage? Hint: notice that Alison is standing on a skateboard that is in line with the pull.

If you think you know the answer, send it to, or leave a comment below (please don’t give away the answer by posting it online). The winner, randomly selected from the pool of correct entries, will receive an autographed copy of The Rigger’s Apprentice. Entries must arrive by noon Pacific Time, Monday, June 11th. The winner will be announced on Wednesday, June 13th.

As always, I invite you to catch up on all the back-articles and -puzzles on my Blog. The current article is called Not Falling Far, Part 2: The Bounce Test.

In other news, time is running out to sign up for our 3-day  “Rig Your Boat” workshop, here in Port Townsend.

Fair leads,

Brion Toss

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  1. Tom Kiley
    Tom Kiley


    June 6, 2018
  2. stan gibbs
    stan gibbs


    June 7, 2018
  3. Jesse

    Looks like a 7:1 with a change of direction. Hard to see how it’s rigged.

    June 7, 2018
  4. Tom Garner
    Tom Garner


    June 7, 2018
  5. Jaime Torres
    Jaime Torres

    The purchase advantage on the block set the gentleman is holding is 7 to 1. She is pulling on the carabiner he is holding with a 7 to 1 advantage. Because she is on a “frictionless” skateboard, she is just acting as a connector between the gentleman and the block set.

    She neither loses nor gains advantage because of that connection.

    its as if was the man pulling in t he working end of the rope. He feels the 1/7th of the load on his left hand as he feels on his right hand (excluding drag, friction, and all the other incalculables)

    June 7, 2018
  6. paul


    June 7, 2018
  7. Frank Ficken
    Frank Ficken

    Because she is on the moving board, she really doesn’t exert any force except what comes from holding your hand, essentially, you’re pulling on the line yourself. It’s a bit hard to see all the lines on the blocks in front of you, but it looks like a 1:7 advantage.

    June 7, 2018
  8. mike

    None to none.

    June 7, 2018
  9. Nathan Bossett
    Nathan Bossett

    Say each arm is pulling 1 lb.
    Guy feels 1 lb from arm + 6 lb from block, total 7
    Woman is pulling with 2 lb (woman+skateboard are needless complications: you could have two 1 lb hanging weights with blocks to make the force horizontal and the mechanical situation would be the same.

    June 7, 2018
    • Nathan Bossett
      Nathan Bossett

      Oh. I see it says to leave the answer in comments below, but then it says not to post the answer online. Confusing…. this is online.

      June 7, 2018
  10. Peter Hennessey
    Peter Hennessey

    Ok, starting with a six part purchase rove to advantage with a turning block, that give 7-1 advantage, plus one more part in the handhold, in my book that makes 8 to 1 advantage, and a lot of fun judging by her smile!

    June 8, 2018
    • Peter Hennessey
      Peter Hennessey

      P.S. The fact she’s standing on a moving platform serves to elimnate any out of balance forces, making the pull on each hand equal and opposite. So, 7/1 in the left hand plus 1/1 in the right hand equals 8/1. QED.

      June 9, 2018

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