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A Rotating motor compressing a spring

  1. Aug 28, 2015 #1

    I've recently came across this video (), where the authors use a motor to compress springs and therefore achieve locomotion. I've been thinking why is there a resulting downward net force. But i can't really figure it out.

    Thank you for time :)

    See the video from 1.16 minutes and further!
  2. jcsd
  3. Aug 28, 2015 #2


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    Is there a "resulting downward net force"?

    After running the motor for several minutes the robot is still pretty much at the same height. It certainly hasn't accelerated vertically until it hits the ceiling. So what does that say about the average vertical net force?
  4. Aug 28, 2015 #3
    It think it can never hit the ceiling because even if the force was enough to produce the maximum displacement of the string the elastic force resulting wouldn't be enough to get the robot to the ceiling. And watching more closely the springs are at angle so there is horizontal displacement too
  5. Aug 28, 2015 #4
    I sort of get your point, but there must be a force for something to move in earth like conditions. I would like to understand where that force comes from...Why the rotating motor produces that force, is that related to the torque produced by the little masses attached to the motor
  6. Aug 28, 2015 #5


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    The motor accelerates the small masses up and down. Newtons laws say this will cause a reaction force that accelerates the motor/robot down and up. eg when the mass accelerates upwards the motor/robot accelerates downwards and vice versa.

    Averaged over a long time the whole assembly does not move vertically (eg it's still on the table not on the ceiling).

    Are you asking how it moves horizontally?
  7. Aug 28, 2015 #6
    Thank you :) , i got the ideia. I also would like to know why it also moves horizontally, i guess it is because the springs are at an angle, but i'm not sure...
  8. Aug 28, 2015 #7
    When a mass moves along a circle, it enjoys a pseudo force radially outwards ( centripetal force) . You can resolve this force into vertical and horizontal components. And thus you can explain the whole thing.
  9. Aug 28, 2015 #8
    Thank you for your contribution, the pseudo force you are referring, i think you meant centrifugal force. That may explain it because the motor is not attached to the table, but still, it is not that clear for me :)
  10. Aug 28, 2015 #9
    Yeah... its centrifugal force. It was a silly mistake.
  11. Aug 28, 2015 #10
    This may help you.
  12. Aug 28, 2015 #11
    In this case doesn't the centripetal force cancel out the effect of the centrifugal force? Imagine if i have a wheel in the air suspended by a string, if i give the wheel a rotation, according to your explanation the string should oscillate. Is that right ?
  13. Aug 28, 2015 #12


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    Regarding horizontal motion...

    The small weights move back and forth so you might expect the robot to just oscillate back and forth. However max friction between the spring and desk isn't constant, it varies as the robot bobs up and down. It so happens that friction is always lower when the small mass is going in one direction and higher in the other.

    I believe this is a variation of the "stick slip" mechanism that fools many people into thinking they have made a so called "reactionless engine".
  14. Aug 28, 2015 #13


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    This is rather certainly the proper explanation. It is an interesting gadget, but I think the mystery is solved.
  15. Aug 28, 2015 #14
    Centrifugal and centripetal force don't arise in the same frame. Centrifugal force can be experienced in non inertial frame whereas centripetal force plays it role in inertial frame. You have to choose a particular frame while calculating resultant force.
  16. Aug 29, 2015 #15
    Thank you all, it will take some time until CWatters explanation sinks in in my head, but it seems that mystery is solved :D
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