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Homework Help: Upwards Circular Movement

  1. Nov 6, 2014 #1
    1. The problem statement, all variables and given/known data
    Hello there,

    So I have a problem that I can't seem to figure out on my own. I have no background in physics (I haven't taken a physic course as of right now,) but my robotics team requires some math.

    I need to find the equation for moving something with a mass of 1.5kg upwards (towards the sky, for clarification) in a circular path, as shown. The radius is approximately .32 meters:

    2. Relevant equations
    The equations I have tried to combine in an attempt to solve it are the equations for centrifugal force [F=m(v2/r)] and the force of gravity (9.8m/sec2.) I am uncertain whether or not I am searching in the right area, but I hope someone here can help.

    3. The attempt at a solution
    So far, I have tried using centrifugal force with the force of gravity located inside the equation (I believe the equation I tried was [F=m(v2/r)(9.8m/sec2)], where m= 1.5 and v= .5m/sec.

    When numbers are plugged in, I got [F=1.5(.5m/sec2/.32m)(9.8m/sec2)]

    If I did the math correctly, the answer should be 22.972m/sec2. (I rounded to the thousandths place for all division)

    I am not sure if this is correct or not, so any help would be appreciated.
  2. jcsd
  3. Nov 6, 2014 #2


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    You need to clarify how the force is to be applied. Typically, for a motion like that, what you are looking for is a torque rather than a linear force.
    Where does the given speed come from? Is it desired to have it moving steadily at this speed, or is it to start from rest and achieve that speed, or start from rest and average that speed?
    Centrifugal force is a 'fictitious' force. It only arises if you take your reference frame to be that of the spun object. Normally one uses an 'inertial' frame (one that is not accelerating). In an inertial frame there is no such thing as centrifugal force. Instead, in order that the object rotates about an axis, the resultant of the actual forces must be a 'centripetal' force.
    Even if those two forces are relevant, you need some logical basis for how to combine them. If two (actual) forces act in the same direction (which they don't here) you could simply add them. What makes no sense at all is to multiply them together!
  4. Nov 7, 2014 #3
    Once again, I have not taken a physics course at all, so this is relatively new to me.

    The speed is just a "filler variable," of sorts, because a certain speed is not required for this. The given speed is via motors and gears, and again, speed is not important with what I am dealing with. One of these motors provides 1.529N of force...

    I understand centrifugal force is not real, however, all websites I have been to trying to figure this out has given me "centrifugal" force to work with what I have searched in Google.

    So, if I am understanding correctly, I would need to change centrifugal to centripetal force, and maybe even cut it out altogether?

    My new attempt at an equation would have to be:

  5. Nov 7, 2014 #4


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    Then perhaps we can assume the speed is arbitrarily slow (i.e. the motor is only just powerful enough) and thus ignore any centripetal/centrifugal aspects. It depends on your answer to my next question:
    I should have asked exactly what you meant by that. Are you trying to find what force (or torque) is needed? Or how the object will accelerate? Or how quickly it will get there? Or how fast it will be moving at the top? Try to tell us as much as you can about what you are trying to do.
    Motors that provide motion via a shaft are rated in terms of the torque they provide, not the force. If I put a gear wheel on the shaft then the linear force depends on how large the gear wheel is, but the torque is fixed by the motor. Assuming it is a shaft at the centre of the illustrated arc, the force at the object is torque / radius.
    If it is a linear motor (like a piston), you need to say how it is connected to the object moving in the arc.
  6. Nov 8, 2014 #5


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    There isn't ONE equation for moving something. There are a whole set of "equations of motion". To work out which equation(s) you need we need more information about the problem. Are you trying to work out how much power or energy is required to raise the object? or perhaps you know how powerful the motor is and want to know how fast it will raise the object?

    The image you posted suggests your object moves around the outside of some sort of track or guide? Is that the case?

    Motors don't usually generate a "force" (they normally generate a torque)... is it a linear motor? or a regular round motor with some kind of linear gearbox?
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