1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can we consider this one direction?

  1. Aug 14, 2011 #1
    [PLAIN]http://media.tiscali.co.uk/images/feeds/hutchinson/ency/osci0001.jpg [Broken]

    I don't understand the concept of a complete osillation perfectly..I see that the vibrating object passes through the rest position two times in two different diection??How does the object have the same phase relative to the rest position..

    Thanks in advance
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 14, 2011 #2
    When I teach this, I stand at the board with a marker in my mark and move my hand up and down regularly. This makes a dark mark on the board. Then, while still raising and lowering the marker, I walk parallel to the board. The marker traces out a sine wave. Can you picture that?

    In the same way, a marker attached to the pendulum would trace out a sine wave on a piece of paper that moved under it. This http://www.youtube.com/watch?v=MhG5UceWxLk" sort of gets the idea across, especially if you imagine the pen moving as smoothly as a pendulum.

    I think the answer to your question is clear if you focus on the sine wave associated with the pendulum movement. In the situation drawn on the left side of the diagram you posted, the wave would start at a crest at A, pass through the axis, be at a trough at B. cross the axis again and end at a crest at A. That would be one complete oscillation and one complete wavelength (being crest to to descending node to trough to ascending node to crest).

    In the the situation drawn on the right, you start at the axis at A, go to the crest at B, back through the axis (A), to the trough at C and once more to the axis at A. That again is one wavelength (ascending node to crest to descending node to trough to ascending node).

    Does that answer your question?
     
    Last edited by a moderator: May 5, 2017
  4. Aug 14, 2011 #3
    And here is a vid of oscillations within oscillations within oscillations which is all plainly started and eventually returns to the beginning release of the pendulums. Can you see the many patterns until the return to the original configuration? Fascinating. http://www.wimp.com/pendulumwaves/" [Broken]

    And synchronizing 5 metronomes that are rate adjusted to the same frequency but NOT released at the same time, do all sync up with a master energy buffer(they are coupled by the common plank on two rollers.) Again fascinating to watch the separate oscillations synchronize together. http://www.youtube.com/watch?v=W1TMZASCR-I"
     
    Last edited by a moderator: May 5, 2017
  5. Aug 15, 2011 #4
    I think two factors you are missing are 'starting point' and the 'direction', whether the ball is moving to the left or right?
    In the first picture, ball's motion was from A to B to A. From the 'starting point' A, the ball moves to the 'right'. That is the start of a cycle or oscillation. Everytime the ball moves to the 'right' from point A is start of a new cycle.

    In the second picture, ball starts from A then moves to the 'right' to B, then from B to C through A. But this time ball move to the 'left' from A, so this is NOT a start of a new oscillation. Again everytime the ball moves from A to the right is a start of a new cycle.

    Note, starting point can be any point between B and C, and direction can be either left or right. As long as you are consistent with 'starting point' and 'direction' you can not miss a cycle.
     
    Last edited by a moderator: May 5, 2017
  6. Aug 15, 2011 #5
    Well First, Thanks very much,
    I can't really imagine what you are talkin bout...I can't just explain my problem in the right way..i
    The definition of a complete oscillation is "The motion of a vibrating body through the time interval between passing through one point two consecutive times in one direction"
    I think I understand this,but my problem is with the phrase "In one direction"..how come?? If we consider the mass to move
    1- From a to b (to the right)
    2-From b to a (to the left)
    3-from a to c (to the left)
    4-From c to a (to the right)
    then the mass passed through the point a two times in two different directions during its motion!!!From b to a (to the left) -- and From c to a (to the right)
    Thats completely contradictory

    Hope you got a clear picture of my problem now
    Thanks very much for ur efforts
     
  7. Aug 15, 2011 #6
    yes , I can imagine
    I don't find the answer in the sine wave of a pendulum..They still different directions
    Thanks very much
     
  8. Aug 15, 2011 #7

    DaveC426913

    User Avatar
    Gold Member

    I don't see what the problem is.

    One complete oscillation is:
    b > a > c > a > b (or, if you like, a > c > a > b > a)

    It passes through a twice, yes, but in two different directions, so that's not a complete oscillation. An oscillation has not been completed until it passes through a in the same direction, which means, as above, b > a > c > a > b (or, if you like, a > c > a > b > a).


    misr, what do you think is one complete oscillation?
     
  9. Aug 15, 2011 #8
    I think it from a--b--a--c---a
    Right??but in this case it passed in two different directions

    a > c > a > b > a
    those are two different direction :(
     
  10. Aug 15, 2011 #9

    kith

    User Avatar
    Science Advisor

    Using your notation and expanding your example a bit:

    ...
    0-From c to a (to the right)
    1-From a to b (to the right)
    2-From b to a (to the left)
    3-from a to c (to the left)
    4-From c to a (to the right)
    5- From a to b (to the right)
    ...

    So the complete oscillation -according to your definition- has taken place between 1 (starting at a with a swing to the right) and 4 (ending at a with a swing to the right).

    Another notation would be:
    ... (->a) (->b) (a<-) (c<-) (->a) (->b) ...
     
  11. Aug 15, 2011 #10

    DaveC426913

    User Avatar
    Gold Member

    Correct. So what's the problem?
     
  12. Aug 17, 2011 #11
    O really?I think "one direction" doesn't mean that the body passes through the rest position in the same direction...because it certainly passes through the rest position in two different directions

    but it means that the body returns to move in the same direction (having the same phase realative to trhe starting point

    so am i on the right way|
     
  13. Aug 17, 2011 #12

    DaveC426913

    User Avatar
    Gold Member

    Yes. A single oscillation includes passage through A twice, once in each direction.

    Just like a sine wave. See attached.
     

    Attached Files:

    Last edited: Aug 17, 2011
  14. Aug 24, 2011 #13
    Yeah Got it now ..its my text book again confusing me
    Thanks very much this is very helpful
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook