B Help with motor for pulling uphill

1. Oct 14, 2018

Tyler Schaffler

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. Oct 14, 2018

stockzahn

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.

Attached Files:

• Sled_01.jpg
File size:
8.8 KB
Views:
82
3. Oct 14, 2018

Cutter Ketch

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.

4. Oct 14, 2018

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.

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