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spring/shock absorber has "reactance"?

Discussion in 'Electronic Basics' started by [email protected], Jan 20, 2005.

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  1. Guest

    They say that the mechanical analogues of capacitors are springs, and
    of inductors are shock absorbers. And this does have a strong
    intuitive appeal.

    But do springs/shock absorbers have any kind of frequency-dependent
    behaviors?
     
  2. richy

    richy Guest

    inductors are not shock absorbers, as they do not dissipate energy like
    shocks do.
    Shocks would be simulated by a resistive element.

    Answer to second is Yes. They are "tuned" to your cars mass, and expected
    road conditions.
     
  3. TimPerry

    TimPerry Guest

    cross-posted to the universe at large.

    its time to refer back to the "water" analogy.

    or maybe electricity is like tiny little fireflies trapped in a still...they
    try to excape but are slowed by the spiral condensor... then they fall to
    the ground drunk exausted and happy...
     
  4. John Larkin

    John Larkin Guest

    If you treat

    Capacitance = mass
    Inductance = spring
    Resistance = damping (shock absorber, viscoscity)

    then identical differential equations will describe both systems.

    A parallel L-C circuit has a resonant frequency where it's easiest to
    excite. A mass hung on a spring is the same, it twangs at a resonant
    frequency if whacked. Jump on the fender of a car with bad shocks; it
    will bounce at the resonant frequency.

    John
     
  5. Yes, a car suspension has all three derivatives. The mass and its
    spring are supported parts, definining a resonant frequency (taking
    the mass of the car as infinite). The shock absorber with its viscous
    drag adds dissipation sufficient to overwhelm the sharp resonance, or
    else you have to buy new shocks.

    John Polasek
    If you have something to say, write an equation.
    If you have nothing to say, write an essay
     
  6. These analogies are meaningful because of identical mathematical
    description. As part of the formalism, charge is equivalent to position and
    rate of charge change is current and analogous to rate of change of
    position.

    Capacitors store potential electrical energy C*V^2/2 as springs store
    potential mechanical energy k*x^2/2. Inductors store kinetic electrical
    energy L*I^2/2 as masses store kinetic mechanical energy m*v^2/2.

    When you make a lagrangian formulation based upon these energies, the
    equations for electrical and mechanical motions are identical and frequency
    dependence is identical.

    Bill

    Bill
     
  7. Though it is quite possible to use several different physical pairs
    for C and L anologs, I confirm that for mass and springs,
    Bill is spelling out a pairing that I know is in use: i.e. the spring
    is not the L as you might suppose, but the C.

    Brian Whatcott Altus OK
     
  8. is this a job for the goatse man ?
     
  9. For a mass-spring system, if we assume a rectifying force which is
    dependent on position, then

    F = -k*x

    By newtons famous law,

    F = m * a

    So, if x is a function of time, we have

    -k*x(t) = m * x''(t)

    thus,

    x''(t) = -k/m * x(t)

    The solution is, of course,

    x(t) = sin(sqrt(k/m) * t)

    where sqrt(k/m) is called the 'angular frequency'

    For electronics, if we say that

    k = 1/C, and m = L, then

    v(t) = sin(t/sqrt(LC))

    This makes the resonant frequency w = 1/sqrt(LC), which we know to be
    the case.

    By this, we can say that the spring is the equivalent to the capacitor,
    and the mass is equivalent to the inductor.

    Another way to look at it is that the fundamental correpondence is mass
    and charge. The spring creates a rectifying force, just like the voltage
    across the capacitor induces the charges to move. Once the mass is in
    motion, its inertia keeps it going, which is what F = ma is all about.
    An inductor opposes motion, and then wants to keep the motion going,
    just like inertia.

    Thus, the real correspondence is voltage across the capacitor to tension
    in the spring, and the movement of charge throught the inductor to the
    inertia of the mass.

    For more information, you can consult "The Feynman Lectures", volume I,
    chapter 23. He uses a cool technique to derive the equations of damped
    oscillation for both mass-spring and inductor-capacitor systems.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  10. John Larkin

    John Larkin Guest

    If you jump on the fender of a car with bad shocks, the entire car
    oscillates. It's the mass of the car and the stiffness of the springs
    that determine the resonant frequency, typically a couple of Hz; if
    you take the cars's mass as infinite, absolutely nothing will happen
    if you jump on it.

    John
     
  11. Don Kelly

    Don Kelly Guest

    To complete your analogy - treat current as force and voltage as velocity.
    (nodal modal)

    You can also use

    current+velocity
    Voltage =force
    Inductance=Mass
    Capacitance =compliance
    resistance =damping
     
  12. Guest

    | inductors are not shock absorbers, as they do not dissipate energy like
    | shocks do.
    | Shocks would be simulated by a resistive element.

    There is a very resistive element to shock absorbers, but there is some
    that I suppose coule be said to be inductive. It's just a very low Q.
     
  13. Guest

    | If you jump on the fender of a car with bad shocks, the entire car
    | oscillates. It's the mass of the car and the stiffness of the springs
    | that determine the resonant frequency, typically a couple of Hz; if
    | you take the cars's mass as infinite, absolutely nothing will happen
    | if you jump on it.

    Nor will you be able to get off the car :)
     
  14. Guest

    If you make a basic cart out of just a box and four wheels, the ride
    would be rough.

    If, to improve the ride, you are given the choice of using either a set
    of springs or a set of what most people call "shock absobers" you would
    use the springs.

    Why?

    Because the springs absorb the shocks.

    The telescopic devices which most people call "shock absorbers" are not
    shock absorbers. They are dampers.
     
  15. Actually, you'll be sucked into the resulting black hole...

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  16. Airy R.Bean

    Airy R.Bean Guest

    Reactance is characterised by the storage of energy.

    In the case of the capacitor, you might think that your
    AC source is the only voltage source in your circuit, but
    after the first 1/4 cycle, the capacitor acts as a voltage source
    and starts to give back the energy that it has stored.

    The combined result of the two voltage sources, your
    AC excitation and the capacitor itself, accounts for
    the out-of-phase current waveform.

    (This bothered me for years! How could the current
    be non-zero if the AC driving voltage was zero?!)

    The same analogy applies to springs and to shock absorbers;
    the spring stores energy when stretched; the shock-absorber
    stores energy when compressed. Both the spring and shock
    absorber will return energy at some time and this exhibit reactance!
     
  17. Guest

    Actually, it's:

    Capacitance = spring
    Inductance = mass
    Resistance = damping (shock absorber, viscoscity)



    Slick
     
  18. Guest

    Your statement is complete rubbish. If you don't know the science,
    don't make it up.
     
  19. Greg Locock

    Greg Locock Guest

    wrote in
    This is some mad person's view of the world. To those of us who were
    brought up in the mechanical field then it is perhaps simpler to re-state
    the basics - dampers dissipate energy, springs and masses store energy.

    So, I doubt that shock absorbers are anything other than resistors,
    whichever choice of voltage or current you think represents displacement.

    The tradition by which control theory people attempt to reduce mechanical
    systems to electrical analogues is, to my mind, counter productive and
    basically a bit stupid.

    Of course anyone who wishes to demonstrate the converse is more than
    welcome to build the electrical analogue to a (non linear) suspension
    model, with 500 DOF, and solve it.
     
  20. Dear phil-news-nospam:

    Shock absorbers are not as similar to resistive elements as they could be.
    They are not linear with "current". Doesn't muck with resonant frequency
    much...

    David A. Smith
     
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