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 frame and a frame moving with a uniform velocity. For that, the 2nd order derivative of x' w.r.t. t' should be [{1-(u/c)^2}^(1/2)]*(d2x/dt2), since m'=m/[{1-(u/c)^2}^(1/2)]. But it's not coming. May be, I'm making some mistake. Please help me out.