1) x.0s must have the dimension length, not time
2) If you want to solve for w.n you have to provide a guess value for this variable. The other assignments you mad in the solve block should be done outside in front of it as these are no guess values
3) You solve for a single variable so why are you trying to assign the result to a vector with four variables?
Unfortunately the solve block can't find a solution if you correct the above so I guess you may want to check your equation or the values you assign. After all the RHS of your equation always yields 0 (because you had set x.0s and x'.0s to zero, so the expression never can be 5 mm.
Can it be that you intended to set x.0s to 50 cm instead of 0?
