Can questions like the one given in the following pic be solved by taking the reference frame answhere in the middle of the string and not on the fixed pulley?(http://cnx.org/content/m14731/latest/pq8.gif) A somewhat similar method has been given in http://cnx.org/content/m14783/latest/" [Broken] ( Search for Horizontal pulley system ) I'm unable to comprehend the following line: "There is an important subtle point here. The positions of blocks are on either side of the reference point (not on the same side as usually is the case). If positive direction of reference x-direction is towards right as shown in the figure, then velocities of two blocks are : " Why does acceleration get -ve? pl help.