Relativity of spring "constant" Let equal-magnitude, oppositely signed charges be at rest at the Origin and on the y-axis of IRF K. They are held apart by a compressed spring. The force exerted by the spring on either charge is equal and oppositely directed to the electrostatic force. Viewed from IRF K’, which moves in the positive x-direction of K at speed v, the charges and the spring move in the –x’ direction at common speed v. But according to the general field transformations the Lorentz force on either charge in K’ is less than it is in K by a factor (1-v^2/c^2)^(1/2). Is the spring constant actually a function of the spring’s motion relative to an IRF? And if so, what is the general rule for arbitrary spring orientations in frame K’?