Direction of force in special relativity If a spring is compressed, you can release either end and it will expand outward at the same speed irregardless of which end is released (just in opposite directions). However, if the spring is moving in the direction of its length, relative to some inertial observer, the result is no longer symmetrical - if the spring is released so as to expand opposite to the direction of travel it will move (relative to the spring) faster than in the opposite case because of the velocity addition formula.
The same situation would arise with two electrons fixed next to each other - if the system is traveling east and you release the west-side electron it will travel (relative to the system) faster than if you release the east-side one, as observed by the inertial observer.
Is there a simple intuitive model for this? Do forces work differently in different directions - or perhaps we should say inertia varies by direction - or does it just boil down to the mathematics?
 A: I assume you want to know the reason for the observed asymmetry. When you look at mechanics from a classical perspective and from the special relativity perspective, you notice a few changes when the system you observe is moving with a velocity close to that of light.
The theory of special relativity says that the speed of light is a constant, irrespective of how fast you are moving or what frame you are observing from(as long as motion is uniform and not accelerated. Under this 'postulate', it becomes clear that time and space can 'vary' depending on how fast you or the thing you are observing is moving. These effects of time dilation and length contraction are not mathematical tricks, but real effects.
In classical mechanics, the force is equal to $$F = \frac{dp}{dt}$$, in which $p = mv$ (m is mass, v is velocity).
But in relativity momentum instead becomes $P = \gamma mv$, where gamma is the Lorentz factor. And the force becomes $$F = \frac{dP}{d\tau}$$, where $\tau$ is the proper time, i.e. time as measured by the moving object. 
Owing to the $\gamma$ factor, which depends on velocity of the object and the direction, you basically get the effect that you have mentioned. It happens due to the relative velocity and the electrons direction.
Of course, I have oversimplified a bit, but I think I can conclude that this effect actually happens because of the relativistic definition of momentum and force.
