# SHM -> F = -kx

1. Aug 22, 2008

### linyen416

SHM --> F = -kx

SHM assumes that F is the only force acting on the system, so if we have a mass held between two springs on a linear air track, the F = -kx
force refers to the restoring force from the springs?

Is that the only thing it refers to? What about air resistance as the gliding object travels horizontally along the air track?

I've noticed also that the weight of the gliding object introduces friction so it's not exactly a closed system, could I explain this by saying that this is because F = -kx which acts horizontally, is perpendicular to the force of the weight? So F = -kx does not accoutn for the friction introduced by weight?

Or am I wrong to say that becase the friction introduced by the weight is acting horizontally as well?

I'm confounded.

2. Aug 22, 2008

### tiny-tim

Hi linyen416!

Yes, the friction is horizontal.

But it's very small, so the motion will be very very nearly SHM (just like an ordinary pendulum, for example).

3. Aug 22, 2008

### linyen416

Re: SHM --> F = -kx

so I shouldn't explain the 'unlcosed-system' quality by saying that the wt force acts in a different vector componenet as the force from springs, but rather I should explain it by saying that the friction is introduced by the weight pushing down, so it's not exactly closed?

4. Aug 22, 2008

### linyen416

Re: SHM --> F = -kx

so to clear things up:
F = -kx refers to restorative force from springs
F = -bv refers to the air resistance as the glider moves along

and so if we use our amplitude -time graphs to calculated b, we should find that as m increases, b increases because the non-closed system causes extra friction to be introduced, therefore extra damping other than just air restistance.