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Sled Dogs and Components of Force

  1. Apr 14, 2016 #1
    1. The problem statement, all variables and given/known data
    This is a real world problem, which I have been thinking about for some time now.
    The problem is this: I have a weighted sled which requires a pulling force to move it, and I have a team of dogs generating that pulling force. I am trying to figure out the differences between two different ways of configuring the team.
    Terms being used are
    "gangline" -central rope, all the dogs and the sled are attached to this
    "tugline" -two to three foot line coming off of the gangline in pairs, and attached to the harness of each dog

    Configuration 1: Each pair of dogs is pulling on either side of the gangline, with the pulling force being applied via the tugline at an angle of about 30 degrees off of the gangline. Essentially, each dog is running forward, but also pulling outward at a bit of an angle to the direction of travel, in order to run alongside the gangline.

    Configuration 2: Each pair of dogs is pulling on either side of the gangline, with the pulling force being applied via the tugline, with a spacer bar perpendicular to the gangline which allows the tugline to be parallel to the gangline. Each dog is basically running straight forward all the time.

    2. Relevant equations
    Components of force. Fx = F sin(30), Fy = F cos(30)

    3. The attempt at a solution

    My intuition says that configuration 2 is the more efficient setup, if all the force being applied by the dogs can be applied in the direction of travel, the Y axis (think, traveling up the trail), there are no X components of force on either side of the line cancelling each other out.

    I am trying to figure out if the X component of force each dog applies in configuration 1 is entirely wasted effort and does in fact cancel itself out. I do not think it does, thinking: if you hold up both ends of string with a weight in the middle, and move your hands apart along the X axis, the weight still progresses along the Y axis.

    I have used configuration 1 for many years and it definitely works, but I would like to know if I am limiting my efficiency by a significant amount.

    I haven't used many specific values in this question because in a real world setting the entire system is pretty dynamic; sled weight, pulling force required, and amount that each dog is pulling can vary quite a bit, but if you guys need more info or any clarification let me know.

    TL;DR
    Is having sled dogs pull at an angle significantly less efficient than having them pull in a straight line?

    Thanks
    Hopefully I posted this in the right section.
     
  2. jcsd
  3. Apr 14, 2016 #2

    PeroK

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    The dogs are definitely wasting effort in configuration 1 for the reasons you outline and configuration 2 should definitely be more efficient, as all the force is directed in the required direction.

    In general, the forward force will reduce with the cosine of the angle. At 30° you have an 87% forward pull. At 60°, however, it would only be 50%. A rough estimate, therefore, is that you are losing about 13% of your dogs' effort.

    Regarding your question about lifting a weight. It would be much harder to lift a weight with two people pulling out at an angle. As the weight rises and the angle becomes greater, it will become harder to generate any upward motion. Eventually, almost all the force from each side is simply pulling against the other.


    .
     
  4. Apr 14, 2016 #3
    i feel that the sled dogs pulling at small angles so that the y-component is more will be more efficient than the other way which you are proposing-
    the arguments are that its not a theoretical framework which actually works -as mostly it is an idealization- in practical terms the two parallel pullings may not impart the twice-pullforce as the moment of the forces about the centre will act and a part will get lost in producing the torqoe to the contraption attached to the central rope. and this is vital as live pullers or dogs may not be adjusting their pulls exactly equal in magnitude and direction.
    there are some practical advantages of the earlier ways of pulling at an angle- the freedom to puller dogs generate more force and as the terrain is not ideally flat the trained dogs could manuver to totake slightly new paths if it is desirable.
    on the whole i think the configuration one will be found more efficient if suitable experiments be done to test the methodology.
     
  5. Apr 14, 2016 #4

    haruspex

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    I don't understand the point you are making here. Perhaps you are thinking of the transverse bars somehow being fixed at right angles to the path of travel, but in practice it would be able to rotate somewhat, so the pair of dogs would not need to stay exactly level. The force exerted by each dog would still be straight ahead.
     
  6. Apr 15, 2016 #5
    yes i was thinking of the actual working of 'spacer bar' which is perpendicular to gangline and the tugline having parallel arrangement -if pulled by an exactly equal force- i was worried about this ideal condition
    being achieved in practice.
     
  7. Apr 15, 2016 #6

    haruspex

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    My point is that even if you try to fix it right angles the gangline can twist, so it won't remain at right angles to the direction of travel.
     
  8. Apr 15, 2016 #7
    yes i agree with you , thats why i raised the 'practicability' of the contraption being used and pull by the pair not being added straight along tugline.
    moreover i have a hunch that local tryout might have been made with efficiency in view-we need those info-as its an age old practice.
     
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