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Help With Science Fair; Calculating Force

  1. Jan 26, 2013 #1
    Calculating Force; A Nut Pushing Down on a Screw

    Hello, I'm new to Physics Forums, and this question is not really about schoolwork/coursework, however because it is somewhat school related, I was afraid to post it anywhere else.

    Anyway, I am doing a science fair project where I am designing/building a machine, and to know if it will work/be efficient, I have to calculate a force.


    1. The problem statement, all variables and given/known data

    If a nut, affixed to external pillars in a way that would prohibit it from spinning, but allow it to move vertically, is exerting 9.8 N (its mass is 1 kg) downward onto a screw with a thread of 45°, with how much force would the screw spin? If you need any further information/variables, just ask and I will quickly provide.


    2. The attempt at a solution

    Even though I am a very advanced student (I skipped 7th grade), I am only in 8th grade, and my school doesn't offer physics of this level, so I don't even know where to start.

    If you do this problem for me, would you please explain the process of solving it, if it isn't a problem? Also, if you don't want to outright do this problem for me, could you at least point me to some sources that would show me how to solve the problem? I fully understand middle school physics, I am currently completing Algebra 1, and I have a basic understanding of trigonometry.


    Thank you for even taking the time to read this, and I would be very grateful if you would help me in any way.
     
    Last edited: Jan 26, 2013
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  3. Jan 26, 2013 #2

    tms

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    This is the correct area to post such a question (although you might want to try the engineering section if you have no luck here), but as a matter of policy people will help you, but they will not do your work for you.

    From your description, it sounds like you want the weight of a nut to turn a screw. Have you actually tried it? I'm skeptical that it would work, since, even with the threads at 45 degrees, half of the force will just push the screw down onto whatever is holding it in place. Friction will be your enemy here: between nut and screw, between screw and its supports, and even between nut and it supports. Do everything you can to reduce it. In the real world, screw gears always (as far as I know) work the other way around: the screw turns and moves the nut (or a gear).
     
  4. Jan 26, 2013 #3
    Yeah, that's been my main concern, it's why I set the thread to 45°. But how can I test it without actually building it? To build it I need to spend money, and before I spend any money I want to know if there is even a shot at it working, that's why I'm asking about it. By the way, The screw is attached to a generator. Also, can you think of any other gear or system that allow me to achieve my goal?
     
  5. Jan 26, 2013 #4
    Calculating a Force; A Nut Pushing Down on a Screw; Science Fair

    Hello, I'm new to Physics Forums, and I am doing a science fair project where I am designing/building a machine, and to know if it will work/be efficient, I have to calculate a force.


    1. The problem statement, all variables and given/known data

    If a nut, affixed to external pillars in a way that would prohibit it from spinning, but allow it to move vertically, is exerting 9.8 N (its mass is 1 kg) downward onto a screw with a thread of 45°, with how much force would the screw spin? If you need any further information/variables, just ask and I will quickly provide.


    2. The attempt at a solution

    Even though I am a very advanced student (I skipped 7th grade), I am only in 8th grade, and my school doesn't offer physics of this level, so I don't even know where to start.

    If you do this problem for me, would you please explain the process of solving it, if it isn't a problem? Also, if you don't want to outright do this problem for me, could you at least point me to some sources that would show me how to solve the problem? I fully understand middle school physics, I am currently completing Algebra 1, and I have a basic understanding of trigonometry.

    P.S. If you are skeptical of the screw gear setup working (like I am), can you think of another gear or system that would allow me to achieve my goal?

    Thank you for even taking the time to read this, and I would be very grateful if you would help me in any way.

    This thread is reposted from Homework/Coursework, to view the original thread discussion go here --> https://www.physicsforums.com/showthread.php?p=4244298&posted=1#post4244298
     
  6. Jan 26, 2013 #5

    tms

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    If you're trying to drive a generator, then I don't think it will be a great success even if it works perfectly, because the screw is of finite length, and when the nut gets to the end the generator will stop. Perhaps you'd better explain just what you're trying to accomplish.
     
  7. Jan 26, 2013 #6
    Well I was planning on having the nut's thread composed of two half circles that would be attached to servos that would release the screw at the end of a run, and then the nut would be raised back to its original position by a system of pulleys, at which point the servos would grab the screw again.
     
  8. Jan 26, 2013 #7

    tms

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    What is your machine supposed to do?
     
  9. Jan 26, 2013 #8
    Produce energy.
     
  10. Jan 26, 2013 #9

    tms

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    That doesn't give anyone much to go on.
     
  11. Jan 27, 2013 #10

    Simon Bridge

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    Re: Calculating a Force; A Nut Pushing Down on a Screw; Science Fair

    You have a vertical screw threaded 45deg, and a nut sits on top.
    If the nut can rotate, then it's weight alone may make it drop down the screw.
    That the idea?

    You handle it like a mass sliding down a slope with friction... only there's a rotation involved. Thus, the effect of gravity and the reaction from the thread will create a torque.

    You can also do it in terms of mechanical advantage - google for "physics of screws".
     
  12. Jan 27, 2013 #11
    Re: Calculating a Force; A Nut Pushing Down on a Screw; Science Fair

    You've got the idea until the second sentence. The nut can't rotate, and the screw can. The goal is to make the nut's weight spin the screw.
     
  13. Jan 27, 2013 #12

    Simon Bridge

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    Re: Calculating a Force; A Nut Pushing Down on a Screw; Science Fair

    Same physics :)
     
  14. Jan 27, 2013 #13
    Well the goal is to have a nut, connected to these rods that wouldn't allow it to spin, pushing down on a screw attached to a generator, making the screw turn and power the generator.
     
  15. Jan 27, 2013 #14

    phinds

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    OK, I get that, but you say that the generator is to then raise the nut so the process can begin again, yes? If so, that's a perpetual motion machine, which I assume you know to be impossible, and if it is not so, what am I missing about your description?
     
  16. Jan 28, 2013 #15

    berkeman

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    Staff: Mentor

    Two threads merged into one in the HH forum.
     
  17. Jan 29, 2013 #16

    Simon Bridge

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    The standard approach would be to tie string to the screw and nut, wind the other end around an axle, use the axle to run a generator, drop the weight, axle turns, generates power.

    To return the weight, just crank the axle the other way with something.

    In your case, only half the stored gravitational potential energy would be available to turn the generator. I think the static friction will stop it from working anyway - unless you are going for some fancy low-friction materials and you say you are on a budget?
     
  18. Jan 29, 2013 #17
    Well, I did originally plan to have the generator power servos that would raise the weight through a set of pulleys, but then I quickly realized that was obviously impossible. So now I've been thinking to use human input to raise them. What I was thinking was having the whole assembly attached to an axle, which would then go through a stand, and on the other side of the stand there would be handles attached to the axle, so a person could flip the whole assembly after the weight got to the end of he screw.
     
  19. Jan 29, 2013 #18
    I don't really see where the screw comes in...
     
  20. Jan 29, 2013 #19

    Simon Bridge

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    It's heavy.
     
  21. Jan 29, 2013 #20
    Well at that point I could just have the weight.
     
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