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Calculating torque required to rotate objects

  1. Oct 13, 2014 #1
    I don't have any background in any physics past high school, so I was wondering if someone could help me figure something out.
    I'm trying to build a robot that involves shooting off a foam bullet from a Nerf gun at different angles. I want to have a motor attached to the center of the gun, turning it around so it can shoot between 0 degrees (straight) and 90 degrees (up). Now I want to figure out what kind of motor I need to do this, but for that I need to know the torque required to rotate the gun.

    To make calculations easy, I think it would be easiest to assume that the gun is a perfect rectangular prism weighing 1.00 KG (it won't be that exact weight of course, but I thought it would be faster if I was shown how to do the calculations first then I could plug in the real numbers) 30 CM long, 10 CM wide, and 5 CM thick. The motor would have a shaft with a 2 CM radius (might look something like this: http://www.engineersgarage.com/site...yg_imageupload/28714/Servo-motor-insight1.jpg) and would be placed in the center of the side of object.

    To give you a picture of what it all looks like: At 0 degrees the gun is parallel to the ground and oriented straight like someone would hold a gun, so the object has a depth of 10 CM (it's width), 5 CM wide (it's thickness), and 30 CM long. At 90 degrees, it's now facing up so it has a depth of 30 CM (it's length), 10 CM long (it's width) and is still oriented so that it's 5 CM wide.

    To recap:
    -I'm assuming the gun is a perfect rectangle
    -1 KG mass
    -30 CM long, 10 CM wide, 5 CM thick
    -2 CM shaft radius on motor
    -Motor is connected to the center of the side of the gun
    -trying to find the torque required to rotate the object 90 degrees, against gravity

    If anyone could show me how to calculate this, or just point me in the right direction that would be helpful! Thanks.
  2. jcsd
  3. Oct 13, 2014 #2
    Well, I think the torque required depends on the angular acceleration you want for the gun.
    Torque = I * alpha, where I = mass moment of inertia and alpha = angular acceleration (consistent units)
    You should be able to find formulas for mass moments for common shapes and axis of rotation by searching.
    That might get you started.

  4. Oct 13, 2014 #3
    Thanks Terry, I'l check it out
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