Thanks for your responses. I have attached a file below. Please have a look at it. Try to increase the length by simply typing a number in the Length field and hitting the 'enter' key. Take a look at the pendulum. When length reach large values (eg. L = 2.0 m), the fixed end of the string begins to oscillate as well. I could not figure out why this is. In the properties of string, I have set x = y = 0 (so they are fixed !) and Pos(x) and Pos (y) are functions of x and y.  Could you able to tell me why the fixed end of the string is moving for L >>. I am trying to understand EJS. Thank you.

PS. Looks like my xml file is not uploading..