phillip1882 wrote:If the spring is not at equilibrium, and you drop it, it will oscillate. This is because gravity again acts on every atom in the same way, which means there is no relative force upon the atoms in the spring's rest frame. The only forces in that frame are those pushing it towards equilibrium (from its own extension), so it extends/contracts as appropriate, and oscillates.
my problem is that it is ACCELLERTING thus there either is a force acting upon the spring outside the system, which should cause it to behave in a different manner, or there is no force.
No, it should not cause what you think. Lets imagine a more simple system.
Take two balls, with a slack string between them. If you pull one ball, the string will eventually get taut and will pull the other ball too, but this takes a little while to occur.
If you pull both balls at once, at the same rate (i.e. same force on each), the string remains slack and both balls remain stationary
in the rest frame of either.
This is basically the same system - there is a force on the system, but if you just look at the string between them, you can't tell because it remains slack.
The difference here is similar to the gravity system. In one case, there is a force on
one ball, which makes the string taut because only one ball moves. Thus you observe the effect of the force on the system, even if you are in the rest frame of any part of it. In the other case, there is an equal force on
both balls. Although there is a force on the system, the system does not change (in its own rest frame), but it is still subject to a force.
The system thus does not 'behave in a different manner' because of the force. The balls remain the same distance apart, the spring remains slack. This does not mean there is no force.
okay let's take two scenarios and see if you can tell me the difference between the two.
let's say I'm accelerating upward in a spaceship at the rate of g. i hold a spring. the spring stretches toward the force as expected. i let go. it oscillates during free fall, as expected, and falls to the ground.
The spring would actually stretch away from the force, if you're holding it in your hand, because it's being pulled from one end. This is
not like gravity, because the force causes the system to change internally (it only acts on one part), as opposed to acting on the entire system at once and thus not being noticeable in the system's rest frame.
now let's say instead I'm coasting at a constant velocity in a spaceship, but there is a powerful magnet pulling both me and the spring downward. i let go. this time the spring won't oscillate during free fall but stays stretched out.
This scenario is not clear to me. Is the magnet exerting an equal force upon every atom (note: a magnet will not actually do this, but we may make up a system where it does. We must nevertheless understand that this system is
not physically meaningful in the context of magnets)?
If you drop the spring (which is already stretched) and the magnet pulls the entire spring equally, this is exactly equivalent to gravity, and the spring
does oscillate, as described earlier.
would you agree? now which one models gravity?
Assuming I understand what you mean in the magnet scenario, this one models gravity, but the spring
does oscillate unless it's at equilibrium when you drop it.
