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B Help with motor for pulling uphill

  1. Oct 14, 2018 at 12:46 AM #1
    Hello all!

    I was curious how to figure out which motor to use to help pull a sled up an inclined surface of about 55 degrees. It would be pulled on snow and has a weight of about 300lbs. Would it make a difference if the motor was on the sled vs pulling from the top of the hill? How do you figure out the required power and torque for this? I haven't done physics in forever and would love any help that could be provided.

    Thanks
     
  2. jcsd
  3. Oct 14, 2018 at 2:34 AM #2

    stockzahn

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    Homework Helper
    Gold Member

    Below a schematic with the forces acting on the sled. For pulling it upwards with constant speed, the force ##F= G\,sin\alpha+R## , where

    ##G = mg## (##m## = mass of sled, ##g## = gravitational acceleration)
    ##R = N\,\mu=G\,cos\alpha\,\mu## (##\mu## = friction factor)

    To find the necessary power ##P## you have to multiply the force with velocity ##v## you want to pull the sled upwards (##P=Fv##). The power also can be expressed as the torque ##T## multiplied with the rotational speed ##\omega## (##P=T\omega##). Depending on the radius ##r## of the wheel you want to use for the rope/wire to pull the sled, you find the relation between the pulling speed and the rotational speed (##v=r\omega##). This should give a good estimation for the necessary power and torque needed for the motor, but of course you have to considerate margins and losses, so the machine to use must be stronger/more powerful.

    Both is possible, I think there are other technical considerations to make like

    - is the sled pulled upwards everytime at the same location
    - is it possible to attach the motor to the sled and what are the consequences (the sled gets heavier, ergo you need another motor)
    - do you want the motor stay outside all the time (otherwise you have to carry it up before you want to use it)
    - ...

    That doesn't sound like to be homework, but if it is, please use the template according to the forum rules next time.
     

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  4. Oct 14, 2018 at 12:28 PM #3
    You wouldn’t want to put the motor on the sled for two practical reasons: the motor is more weight that needs to be lifted, and it is much easier to get power to a motor in a fixed location. However, the choice doesn’t change the physics in any fundamental way.
     
  5. Oct 14, 2018 at 2:52 PM #4

    berkeman

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    Staff: Mentor

    What's at the top of the hill? What are you planning on using for a power source? Can you just use your Jeep with a winch to pull the sled up the hill?

    And as @Cutter Ketch says, adding the weight of the motor and batteries to the sled is kind of counter-productive. :smile:

    http://image.fourwheeler.com/f/3663...1+ar0/129_1107_03+winch_anatomy+jeep_winching

    129_1107_03%2Bwinch_anatomy%2Bjeep_winching.jpg
     
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