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Homework Help: (just joined) I need help with my pre ap precal; angular and linear velocity)

  1. Feb 4, 2008 #1
    1.(okay just to let anyone know, I have alot of this left to do, and I don't understand much of it, like when to use certain equations and such.) pulley problem: A small pulley 6 cm in diameter is connected by a belt to a larger pulley 15cm in diameter, the small pulley is turning at 120 rpm.(from a friend I got the first two answers but I still don't understand really how to find it.)
    A.) find the angular velocity of the small pulley in radians per second.= 12.566 rad/s or 4(pie) rad/s
    b)find the linear velocity of the rim of the small pulley.=37.699 rad/s
    c)What is the linear velocity of the rim of the large pulley? explain
    d)Find the angular velocity of the large pulley in radians per second.
    e)how many rpm is the large pulley turning?

    2. I know that to find linear it's L=w(omega) * radius but other then that I am lost.

    3. I have no real clue, at first I thought I was getting everything quite well, but I have failed miserably at it, someone aid me please.
  2. jcsd
  3. Feb 4, 2008 #2
    (I have a lot of other problems, any help for anything...anyone...would help me so much, this is all due tomorrow...26 problems of stuff that I don't understand. Makes me feel so...icky if you will.
  4. Feb 4, 2008 #3
    does anyone know any other online tutors or websites that help with this kind of stuff? Like pre-ap pre-calculus help?
  5. Feb 4, 2008 #4
    (I am going to search around, see if anyone else in on right now, I am really desperate on help here...)
  6. Feb 4, 2008 #5
    (help on the site or anything? Is anyone there? I don't know if these messages would be considered flooding but I do need help....)
  7. Feb 4, 2008 #6
    okay, someone answer this, is the angular velocity the same when the two circles are atop of each other or when there is two seperate gears connected by a belt?
  8. Feb 4, 2008 #7
    the pulley looks like this
    (o_____O) like...one large circle connected to the little one by a belt..I don't know if that helps at all...I really need help.)
  9. Feb 4, 2008 #8
    Your equation L=wR -- Do you know what omega is?
  10. Feb 4, 2008 #9
    Angular velocity once found
  11. Feb 4, 2008 #10
    (still there? sorry I was looking around, trying to learn the site...also helping another person, hopefully your there...)
  12. Feb 4, 2008 #11
    for A all you do is take 120 rpm and convert it to rads/sec

    120 rev/min * 1min/60sec * 2(pie)/1 rev= 4 pie rad/sec
  13. Feb 4, 2008 #12
    (Yeah I got that far, I just get confused, my teacher isn't the best descriptive teacher....um, angular velocity is to always have what unit and what does Linear supposed to have?
    And how do you find the larger circle's linear and angular? Is the linear velocity the same as the smaller one?
  14. Feb 4, 2008 #13
    (I'm not sure but I think the angular velocity is the same when the two circles are atop of one another and the linear the same when they are seperate, but again I am not completely sure, I second guess things to much. I think..)
  15. Feb 4, 2008 #14
    Anyone there? I just have alot of questions, I know the basics, but I get it all out of order, I don't remember what belongs where and such.
  16. Feb 4, 2008 #15
    angular velocity would be in terms of radians/seconds while linear is in meters/second. The velocities of the larger pulley would be the same.

    Sorry i can't be more descriptive i'm still trying to picture the problem so far i think that to find the angular velocity of the larger pulley you need to find its circumference. you know that the smaller pulley goes around around twice in 1 second so now you just have to find how that relates to the larger pulley.
  17. Feb 4, 2008 #16
    (okay, this is what I know for fact, 1 revolution equals 2 (pie) radians. angular velocity is zero in the very very center of the circle, angular velocity is always in radians(I think) and linear is distance(cm,in,miles,meters) per time. I know how to do the work and such, but my answers are never right, I think my problem is that I don't gather the information out of the given info correctly, and I still don't know if the angular velocity is the same when the two circles are on top of each other or seperate, same as linear...that would clear up alot of my problems. plus I don't know how to find rpm...revolutions per minute...
  18. Feb 4, 2008 #17
    the picture looks like that of a bicycle
    (o___0) thats the best I can make the picture
  19. Feb 4, 2008 #18
    (I've never had to find the circumference before...only radians and such...hrmm...)
  20. Feb 4, 2008 #19


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    The rim velocities will be the same for each pulley because they are directly connected with a belt. Their angular velocities will be different, since they are different sizes.

    Exam the small pulley: (120 rev/min)*(2*3.14159 rad/rev) = howManyRadiansPerMinute

    EAch 2 pi radians is 1 revolution.
  21. Feb 4, 2008 #20
    (I am confused when you said that the velocities of the larger circle are the same...that would be impossible I thought. for the two circles to turn they would have to be going the same speed...so that would make the linear velocity of the two the same right? so to find the angular velocity....um...I dont know how to find the angular velocity if you only know the linear, but I know how to find the linear with angular....do you know the equation for that? to find the angular with the linear known?
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