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Is the popular 4x4 winching strategy of a single block correct?

  1. Apr 6, 2015 #1

    SWB

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    Question – A single block reduces the force on the pull by ½ and double the length of the rope pulled and double the pulling force? This is the typical quote made on all popular 4x4 winching sites. I have found this





    http://www.hardworkingtrucks.com/winch-pulling-power/


    http://www.gowarn.com/warn-winches/winch-tips.aspx


    http://cdn.shptrn.com/media/mfg/437...ostPeopleDontKnowAboutWinching.pdf?1289591024


    Having considered the above I feel pulley being used just to change the direction of a force. Considering friction this would in fact reduce the pulling force of the electric winch and not increase the pulling force.


    The Question – Is the popular 4x4 winching strategy of a single block actually providing a doubling of pulling power (not considering friction) or not?
     
  2. jcsd
  3. Apr 6, 2015 #2

    Simon Bridge

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    Welcome to PF.
    The video is correct. Notice that the winch is on the mass being pulled.
    You can use a free body diagram to work it out.
    Done simply, if the winch reels in length L of rope, the fwd moves L/2 forward, so the work is FL/2 as opposed to FL without the setup.
     
  4. Apr 6, 2015 #3

    SWB

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    Hi Simon
    If the winch was not attached to the car but anchored a couple of meters from the snatch block (pulley) would the same forces apply.
    My point being the lenth of the rope is only doubled as the winch is anchored to the car. No mechenichal advantage is being derived.
     
  5. Apr 7, 2015 #4

    Simon Bridge

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    That's correct.
    The video kinda skips over that part: they are selling the block not teaching physics. Notice they never actually say there is any advantage if the anchor is not the car... except being able to winch at an angle etc.
     
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