# spring/shock absorber has "reactance"?

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

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. ### richyGuest

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

3. ### TimPerryGuest

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 LarkinGuest

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. ### John C. PolasekGuest

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. ### Repeating RifleGuest

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. ### Brian WhatcottGuest

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. ### Teddy RubberfordGuest

is this a job for the goatse man ?

9. ### Robert MonsenGuest

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.

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 LarkinGuest

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 KellyGuest

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. ### Robert MonsenGuest

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.BeanGuest

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 LocockGuest

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. ### N:dlzc D:aol T:com \(dlzc\)Guest

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