Acceleration in a frame of reference moving along the x-axis with speed u
x'= (x-ut)/[{1-(u/c)^2}^(1/2)] ---->I
t'= [t-ux/(c^2)]/[{1-(u/c)^2}^(1/2)] ---->II
Find the 2nd order derivative of x' w.r.t. t'.
Actually, Newton's laws of motion should have the same form in both a stationary...