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Homework Help: Moments help needed

  1. Apr 24, 2007 #1
    Hi all,

    just having a little trouble getting my head in to the following problem.

    (See attached picture)

    The picture shows a robot leg, assuming the rest of the leg joins onto something rigid, I need to calculate the torque the motor is required to have to resist the 333g making it collapse.

    I'm at a loss with this, any help appreciated!

    many thanks


    Attached Files:

  2. jcsd
  3. Apr 24, 2007 #2


    User Avatar

    Staff: Mentor

    Thread moved to Homework Help forums. Welcome to the PF, tombot. Please keep in mind the PF rule that homework and coursework problems are to be posted here in the Homework Help forums, and not in the general forums.

    On your question, is the 135 degree angle joint free to pivot? If so, I don't see how a motor at A is going to do anything to keep everything above the 135 degree joint from coming down. Is there more to this problem than is shown?
  4. Apr 25, 2007 #3
    The leg is one of 6 made for my hexapod robot, the 135 degree angle shown does not pivot.

    Imagine that there is an identical leg connected to the one in the diagram, and 3 leg pairs in this fashion. At any given time the robot will have a tripod in contact with the ground.....
    _______ ________/
    / \ goes to /
    | | |

    Heres a pic of my robot: http://www.tombot.net/beam/pics/radar_built4.jpg [Broken]


    Last edited by a moderator: May 2, 2017
  5. Apr 25, 2007 #4
    hmmm the sketch didn't translate to the board..... hopefully the robot pic will suffice?
  6. Apr 25, 2007 #5
    I have been thinking about the problem some more...

    I have resolved the force into x and y components, making the assumption that the force will travel down the top of the leg to the 135 degree angle. The force of course is split equally due tot he angle, thus 167g into the x and 167 into the y from the 135 degree point of the leg.

    I am not sure if I am proceeding in the correct way? any pointers? :uhh:
  7. Apr 25, 2007 #6


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    Staff: Mentor

    The picture helped. I'm not sure of the easiest way to procede, but from picturing how the robot will fall if the motor torques are not enough, I would be inclined to draw the diagram showing two opposing legs with two motors, and the horizontal piece connecting the two legs. I know the problem is left-right mirror symmetric, but it's hard for me to picture how to draw the free body diagram without something connected to the horizontal bar on both ends. Does that help at all?
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