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Homework Help: General motion of a particle

  1. Apr 8, 2012 #1
    a particle moves in the xy plane such that at a time t it is at a point (x,y) where


    prove particle moves in a circular path with constant angular velocity. prove that the acceleration of the particle at time t is in the direction of the radius from the particle to the centre of its path.

    so i have




    not sure how to show angular is constant.

    i can take |V| and that s constant but im not using angular velocity.

    for the second part:

    if if i take direction of A= (d2y/dt2)/(d2x/dt2)=tan(at) where d2y/dt2 is second derivative of y wrt t

    does this answer the question as motion is circle centre (o,0) so any radius will have tan (at) as its gradient?
  2. jcsd
  3. Apr 8, 2012 #2


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    hi jiboom! :smile:

    (try using the X2 button just above the Reply box :wink:)
    that's fine :smile: … now use v = rω (and r is constant) :wink:
    erm :redface:

    isn't it obvious that A is parallel to (x,y)? :wink:
  4. Apr 8, 2012 #3
    now you point it out, yes it is:)

    i was going to use v = rω but as i am taking |v| am i not just getting angular speed not velocity?
  5. Apr 8, 2012 #4
    think im being thick.

    is it that as angular velocity only depends on the angle we have |ω|=ω

    just as v=|v| for 1-d motion
  6. Apr 8, 2012 #5
    sorry but im now confused again.

    the question actually says
    y=2sin ωt



    so |V|=2ω

    so using v=rω only gives me r=2????
  7. Apr 8, 2012 #6


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    No, you knew from the original position vector that the mass was moving in a circle with radius 2: if [itex]x=2 cos(\omega t)[/itex]and [itex]y= 2 sin(\omega t)[/itex], then [itex]x^2+ y^2= 4 cos^2(\omega t)+ 4 sin^2(\omega t)= 4[/itex].

    The calculation you have done is the derivation of [itex]v= r\omega[/itex]!
  8. Apr 8, 2012 #7
    so what do i need to say to justify angualr velocity is constant?
  9. Apr 8, 2012 #8


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    hi jiboom! :smile:

    (been out for the evening o:))

    yup, x=2cosωt y=2sin ωt means r = 2 and angular speed = ω !
    again, it's obvious!

    go to sleep on it :zzz:, and it'll all be obvious in the morning!
  10. Apr 8, 2012 #9
    lol. wont be sleeping til i get this.

    i dont have much here to play with.


    now is ω angular velocity or angular speed?

    the v i have is not angular velocity and not constant

    r is 2

    now in your first post you said to use v=rω but then i just get r=2 so that does not show
    angular velocity = constant

    think its going to be long night unless i get a big nudge here,its not obvious to me due to a lack of understanding about velocity v speed |v| angular velocity ω and angular speed | ω|
  11. Apr 8, 2012 #10


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    what's the difference??

    we're in 2D, so they're the same
    but you know that v is constant (from the formula), so it does :smile:

    i think you're over-thinking this

    v is velocity, v is speed, v = |v|

    ω is angular velocity, ω is angular speed, ω = |ω|

    v = rω

    v = ω x r :wink:
    Last edited: Apr 9, 2012
  12. Apr 8, 2012 #11
    i see what you are saying but im thinking along these lines:


    then how can i then say |v| is constant? |v| is 2ω so is only constant if ω is but that is what im trying to show

    and i dont understand

    v = ω x r

    lhs is a vector,but rhs is vector product of 2 scalars ?
  13. Apr 8, 2012 #12
    tiny-tim has gone off-line. can you help explain what they were getting at?
  14. Apr 9, 2012 #13


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    hi jiboom! :smile:

    (just got up :zzz:)
    v = |v| is constant because the coordinate formula for v shows it is
    no, ω and r are both vectors, the rhs is a vector product of two vectors

    (technically, ω is a pseudovector :wink:)

    v = ω x r is the definition of the angular velocity vector for a rigid body
    Last edited: Apr 9, 2012
  15. Apr 9, 2012 #14
    im afraid im totally lost here.

    v = |v| is equal to 2ω so how are you saying this is constant?

    im trying to show ω is constant which you seem to be saying,forgive me if im misunderstanding, is equal to ω

    so i dont see how the constancy of v follows without assuming what im trying to show, ie ω is constant.

    v = ω x r: still dont see what the vector is for ω. i thought the r was radius,but i now think its the postion vector of the particle.
  16. Apr 9, 2012 #15


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    hi jiboom! :smile:
    it follows from the following coordinate equations …
    … what is |v| ? :smile:
    ω is the vector of magnitude ω pointing along the axis of rotation

    and yes, r is the position vector of the particle


    v = ω x r is an equation you need to be completely familiar and happy with

    you need to study it in your books or wikipedia or other internet site (google "angular velocity" and "rigid body") until you are :wink:
  17. Apr 9, 2012 #16
    as i say |v|=2ω, but if ω is the angular velocity then i cant say this is constant without assuming what i want to prove.

    i should say,i have changed that a in my op to ω as that is the letter the question uses.
  18. Apr 10, 2012 #17
    anyone ?
  19. Apr 12, 2012 #18
    hi. dont give up on me just yet :)

    i do appreciate your help and just need one last shove to get there.

    all i need convincing off now is that the omega=angular speed =|v| is constant.

    does this follow from the question? if this is so,then for sure i am happy |v|=constant => angular velocity is.
  20. Apr 12, 2012 #19


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    in your original post, you wrote …
    so |V| = … ? :smile:
  21. Apr 12, 2012 #20
    hi,thanks for coming back.

    forget the a,the letter used in the question was omega.

    i know what |V| is,it is just 2omega


    if this omega (a in my original post) is the angular speed i cant say |v| is constant UNLESS i know omega in the question is constant?

    maybe i should be clearer, when in question i have

    R=(2cos wt,2sinwt)

    as this w is not said to be constant in the question i could have w=w(t) and should really have

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