@Mac Dude 

Thank you for answering me!

I have now deleted the part with x:=0. But is my calculations correct now (please see screenshot below)? I'am a little bit unsure about my initial conditions...

The question/problem i working with is a pendulum with a massless, elastic and unbendable rod. This exact problem focuses on the part where there are only translation in the y-direction, and I shall calculate the equation of motion in the y-direction.