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Energy calculation in Simple harmonic motion

  1. May 4, 2013 #1
    Hello
    why we use cosine and sine in simple harmonic motion?
    why we use particularly cosine with potential energy and sine with kinetic energy of simple harmonic oscillator?
    regards
     
  2. jcsd
  3. May 5, 2013 #2

    mfb

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    The general solution for position and velocity is a sum of a cosine and a sine. If the starting conditions are not relevant, you can choose the origin of time (t=0) such that one term is a cosine and the other one is a sine.
     
  4. May 5, 2013 #3
    thanks sir
    but why we use these sine and cosine?
    what these sine and cosine represent?
     
  5. May 5, 2013 #4

    mfb

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    What do you mean with "represent"? They are the solutions to the differential equations which describe the motion of the system.
     
  6. May 5, 2013 #5

    ZapperZ

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    Unless you specifically know the gender of the person you are responding, please do not assume that all of us here are males. You are insulting the many female physicists, physics students, and participants in this forum.

    Zz.
     
  7. May 5, 2013 #6
    Mathematically, as mfb said, the summation of a sine and cosine is the solution to the differential equation that governs simple harmonic motion. For a mechanical oscillator, this equation is mx''+kx = 0, and the general solution is x(t) = c1*cos(wt+p)+c2*sin(wt+p). If you want a more intuitive understanding, you must realize that sine and cosine provide us a way to quantify circular motion. Conveniently, we can express the movement of an oscillator as a function of cosines and sines that have a constant angular frequency.

    Here is a good animation that will hopefully make things more clear:
    In that animation, picture the oscillator as being the up and down movement of that rod as it goes around the circle, which is simply the sine wave.
     
    Last edited by a moderator: Sep 25, 2014
  8. May 5, 2013 #7
    Or you can say that the very definition of a Simple Harmonic Motion is that where the position, velocity, acceleration of the particle vary sinusoidally with time
     
  9. May 5, 2013 #8
    Yes ma'am.
     
  10. May 5, 2013 #9
    !!!!!! are you going to check every post for such 'serious' infringments ??
    What about dorks ????
    Assuming someone is male is not an insult to anyone unless someone wants to take it as an insult.
    Have there been many complaints about this?
    Come on look at the questions!!!!
     
  11. May 5, 2013 #10
    Back to the issue....Potential energy is to do with force x displacement...if the expression for displacement involves Cos(ωt) (x = ACosωt ?) then it is no surprise.
    Kinetic energy is to do with velocity and v = dx/dt so if x is a Cos function then v is a Sin function v = ωASin(ωt).
    Hope this helps sir/madam
     
  12. May 5, 2013 #11

    sophiecentaur

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    Would "Oh wise one" be appropriate?
    This gender thing is, indeed, a general problem. The word "they" tends do be used for "he or she" and it is a bit gramatically unsatisfactory. I wonder whether we could have a ruling on this from the boffins in PF Towers?
     
  13. May 5, 2013 #12

    sophiecentaur

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    Sine and Cosine are Mathematical Functions. They do not actually represent anything but 'describe' how variables are related to each other. In this case it's how one variable (a general displacement) varies with another variable (time).
    I could give an alternative verbal description of a harmonic oscillation: the displacement varies regularly on either side of a mean position, as time progresses, and this variation is smooth, is faster whilst the displacement is at the mean position and blah blah, etc. etc. . . . . . but the mathematical description is more accurate and allows you to calculate and predict in detail.

    I know Maths can often annoy people because it is difficult to grasp but without it, Science is very limited. You need to 'join the Maths club' at some level, at least, of you want to advance knowledge to any useful level.
     
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