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Acceleration of mass?

  1. Feb 18, 2008 #1
    Hi Folks.

    I need help calculating this simple problem.

    If I have a force of 135lbs pushing against a 39lb weight along a straight track.
    How fast will the weight accelerate at the first foot of length?
    And what is the top speed of the weight when one foot of length is reached?

    Hopefully can any of you clever guys help me calculate this.

    / Honk
  2. jcsd
  3. Feb 18, 2008 #2


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    You don't have to be all that clever do you? Force= mass times acceleration so acceleration is Force/mass. You will need to convert from "39lb weight" to mass.
  4. Feb 18, 2008 #3
    If I divide 135lbs / 39lb I get 3.46.

    How can I use this number to answer my questions?
    I just need to know the speed of the weight 1 foot from the starting position.
  5. Feb 18, 2008 #4

    Was this question so hard to get a straight answer on?
    Perhaps I'm at the wrong forum.... :-)
  6. Feb 18, 2008 #5


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    Staff: Mentor

    What are the units of that answer (hint: with english units, it is a little more difficult)?
    Sorry, just answering the problem for you doesn't help you learn as well as helping you figure out the answer for yourself. That's or goal here.
  7. Feb 18, 2008 #6


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    Honk, this seems so much like a homework problem that people are reticent about doing the problem for you.

    first, you have to differentiate between the concepts of lb force and lb mass. at most places on Earth, one pound mass, when sitting on a platform (or scale), will exert downward one pound of force and the platform will push upward on that pound mass the same amount of force, hence it doesn't accelerate.

    so you need to figure out how much mass is that when it weighs a pound. then you can use Newton's Laws and another fact from mechanics (x = 1/2 a t 2) to get what you are seeking.
  8. Feb 19, 2008 #7
    Well, I can tell you guys that this is not any homework whatsoever.
    I'm 42 years old and I just needed some help on this as I'm not a physicist.

    I'm calculating on a electric motor I'm designing but this part I need help with.
    I performed some calculations but I could not verify them myself.

    I have a stall torque of 135lb from the motor 1 foot from center, aka 135ft-lbs.
    The 39lb mass is simply the weight of the rotor. The rotor is 23.62" in diameter.
    I have used this this torque convertor:

    I converted the 135ft-lbs torque to Nm and got 183Nm.
    And I converted the mass from 39lb to 17.7kg
    Then I divided the 183Nm by 17.7kg and got 10.33.
    I have read that this number is the acceleration at meters/sec in square.

    Earth gravity = 9.81 m/s²
    My mass = 10.33 m/s²

    If I'm right here I should have slightly better acceleration than the gravity of Earth?
    Is this assumption correct ot have I missed anything here, perhaps wrong conversion.
    Maybe I should have used this force convertor instead.

    Question: But how do I calculate the speed at various distances from the starting point??
    Perhaps you can help me this time, and I assure you, it's not any homework.
  9. Feb 19, 2008 #8
    I am sorry, it is nice of other people that they were keeping quiet but you have to reliase that you want other people to do your work for you. Now, HallsofIvy basically told you the answer, you did not thank him, you didnt even say please in the first place. Yet, you want people to give you full answer and at the same time you claim that this is not "doing your homework".

    What I suggest is: Convert your imperial units into the usual meters and newtons (always use these units when calculating, check google for converter if youre lost). Then go back and look at Hallsofs equation and you get the answer.
  10. Feb 19, 2008 #9


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    First of all, even if your question isn't strictly a homework question, it is a "homework-TYPE" question. Most of the members of this forum have been "well-trained" to offer help, but not give outright solutions. The latter isn't "helping" but spoon-feeding. This is how we function here in this forum, per the PF Guidelines. So what the members have been trying to do (and in fact, HallsofIvy has given you all you need to know) is guide you in getting the answer yourself, so that next time you encounter such a thing, you'll be able to do it yourself (i.e. we teach you how to fish, rather than give you the fish).

  11. Feb 19, 2008 #10
    Then I guess this is not a forum where ordinary people can
    ask a question without being disregarded.
    I'm not a physicist or do physics calculations on daily basis.
    But I do know how to design ultra high efficient electronics and electric motors.
    My question was based on curiosity of the motor design, nothing more nothing less.

    Of course I'd like to thank the ones helping me out.
    But I didn't get an answer I could use and I thought there was more to come
    than just being disregarded.
    Luckily I have found another forum where I was helped straight away without hassle.
    This is how I define a good forum.

    Btw, I have thanked these guys a lot. :-)
  12. Feb 19, 2008 #11


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    You're 42 and design "ultra high efficient electronics and electric motors" but you can't work in SI units and seem to be unable to think of f=ma? Sorry but that doesn't add up and I'm inclined to think that if you were designing "ultra high efficient" electronics, your knowledge of physics would extend well beyond most.
  13. Feb 19, 2008 #12
    Electronics controllers and motor design have nothing to do with physics of mass.
    The design is focused on efficiency, being low resistance by smart design, and
    a controller operating at 99.5% efficiency levels.I know everything about power handling.
    The motor design is focused on lightweight, low resistance and hígh torque at the desired RPM.
    And this has nothing to do with the type of question I asked.
    I just wondered about a specific situation and therefore I needed to know more
    about physics in this field and kindly asked you guys.

    When you work on daily basis being focused on what is needed in the project, then
    there is no need or time to study other areas of interest. This is real life in real work.
    Last edited: Feb 19, 2008
  14. Feb 19, 2008 #13
    Honk, HallsofIvy provided you with all the information you would need to calculate the problem, those are the forum guidlines, I am sure members would have been more than happy to help you along the way. Honk do you have the answer to your question regarding the acceleration?
    Last edited: Feb 19, 2008
  15. Feb 19, 2008 #14
    Bye all, the other forum is much nicer and have helped me out in my needs.
  16. Feb 19, 2008 #15
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