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Canine weight pull physics questions

  1. Jul 18, 2011 #1
    My name is Derek Cox, I am new to the forum and I am not an engineer or anything close. I have a degree in flight management and I am a commercial plumber that works on large scale construction projects.


    Here are my questions but first I will give you some background on the questions. I am not looking for exact calculations unless they are easy. I am really looking for an easy method of calculating this out.

    I own a few dogs that compete in weight pull competitions. The dogs are trained to pull. Generally the dogs pull on a track with rails of angle iron and the cart has v-groove casters, the dog works on carpet. The other option is a cart with pneumatic tires; the cart and the dog are on carpet. The dogs pull 16 feet and then the pull is complete. It is amazing at the amount of weight some of these dogs can pull. http://www.youtube.com/watch?v=7CDuL3JzpZA&feature=related"
    So my first question: Does friction increase exponentially as weight increases.

    For example, 2 dogs with the same strength per pound one weighs 50 pounds and one weighs 100 pounds. The small dog can pull 500 pounds (10 times its own weight) versus the big dog can only pull 800 pounds (8 times its own body weight).
    If this is true is there a formula to use for this application?

    Second question: If the track is set on an incline of 2 inches over a 16 foot span what does the load force or friction factor increase the force to pull it?

    Example: A dog pulls 2000 pounds on a track as mentioned above (2” over 16’) does that take the same force to pull as 3000 pounds on a level surface?

    Also is there a formula for this question?



    I know there are some different factors that may need to be known. Just let me know I will do the best I can.
    Thanks for all the help.
     
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Jul 18, 2011 #2

    tiny-tim

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    Welcome to PF!

    Hi Derek! Welcome to PF! :smile:

    I think the friction is pretty nearly negligible.

    If you double the size of the dog (eg its height), then you multiply the weight by 8, bit you only multiply the area of each muscle by 4, so we would expect the force-to-weight ratio to be less.

    The dog needs to supply energy.

    (Gain in energy equals https://www.physicsforums.com/library.php?do=view_item&itemid=75", and work done equals force times distance)

    Energy is both https://www.physicsforums.com/library.php?do=view_item&itemid=132" (depends on change in height, = 32mh).

    (h in feet and v in feet per second)

    So on a sloping track, the dog need to do extra work to supply the potential energy (in addition to the kinetic energy).
     
    Last edited by a moderator: Apr 26, 2017
  4. Jul 18, 2011 #3
    Thanks for the reply. From what I am reading there is really no general answer. It would be specific to each situation right?
    You may have to dumb it down for me. I am smart, just not that smart. :)
    Derek
     
  5. Jul 18, 2011 #4
    No, I wouldn't expect friction to increase exponentially as weight goes up...not on typical surface, let alone devices design for rotating under load, like casters!

    Clearly, the casters and the rails are the ones supporting the dead weight due to the mass of those blocks, this weight is due to gravity and it points down...the dog, on the other hand, is simply trying to start moving such mass in the horizontal direction...

    The thing is that, as explained by Newton's First Law of Motion:

    On the flip side, if an object is not moving, it will tend to stay put unless an external force is applied to it...this stillness is what the dogs are trying to disturb.

    To do that, the dog needs to do some work and start putting some kinetic energy into the mass (giving it velocity), but to go from zero speed to some speed, it takes acceleration 'a', and hence, it takes a force F=ma (Newton's Second Law of Motion) to do so.

    In any case, while the load is on a level surface, the dog has nothing working against him and hence he can take his time while stepping without worries about the weight falling back...every little work to make the weight move is good...

    One the other hand, when the load is on a slope, in addition to trying to start moving the weight, you have the dog trying to move the weight up!...the slope.

    When you put that weight on the slope, the weight can be decomposed into two component, again, one directly onto the tracks (most of it) and another one trying to pull the weight down the slope...it is this component that does not give the dog a rest because it is always there...the dog cannot take a break while stepping or anything, because the weight falls back....

    In any case, for the slope as described by you, this pull back will come down to about 1% of the weight, so, a 2000 pounds weight, will produce a 20 pound pull back.
     
  6. Jul 19, 2011 #5

    tiny-tim

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    Hi Derek! :smile:

    (just got up :zzz: …)
    Once you know the force the dog can exert, then there are equations which will tell you exactly how the cart will move.

    But finding the force from the size and weight of the dog, the size of its muscles etc, is too complicated and impractical … the only sensible way is just to test the dog and see. :wink:
     
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