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Jenny’s building site brainteaser

By David, 18 April 2018 Brainteaser

Jenny’s hard at work on a building site. This problem will tax her brain as well as her brawn!

Jenny needs to get some concrete blocks to the top of a building site. She has one full block and one half block.

Jenny has a rope with a pulley all set up. She can stand on the ground and pull down on the rope, and the other end will go straight up! She starts by attaching the half block to one end of the rope, and then pulls the half block all the way up.

When she tries to get the full block up, she runs into a problem. No matter how hard she pulls on the rope, the block won’t budge. She’s just not heavy enough to move it!

Jenny thinks about getting a crane or adding more complicated pulleys to make the job easier. But then she has a brainwave! She has everything she needs right here.

How can Jenny lift all the concrete?

Scroll down for the answer!

 

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Brainteaser answer

With half a block already at the top, Jenny has everything she needs to get the full block up. She connects the full block to the rope on the other side from the half block. The difference between the two is half a block, a weight that Jenny can move. When the full block is up, she then lifts the half block back up again.

There’s also another solution that will work if Jenny is feeling strong. Jenny’s having difficulty lifting the full block because she isn’t heavy enough. If she lowers the half block and then attaches herself to it somehow, she’ll be heavy enough to lift the full block. It will take twice as much strength as the other method, though!

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3 comments

  1. Thankyou Double Helix.

      Reply
  2. If she anchors the rope to above the floor where she wants to lift the block, and attach the pulley to the large block, she can multiply the force and so lift the block up.
    cheers

      Reply
    1. Good one! She’ll have to pull up rather than down, but she’s pretty strong, so it shouldn’t be a problem.

        Reply

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