I performed a lab where I had to find the acceleration of two masses that were connected by a string and hanging on a pulley. The equation used was: http://home.earthlink.net/~copper017/Random/PhysicsLab/acceleration.jpg [Broken] After performing the calculations I got the following data: http://home.earthlink.net/~copper017/Random/PhysicsLab/data.jpg [Broken] Looking at my data, I noticed that as M1 and M2 became closer in mass, the percent difference between the experimental acceleration and the theoretical acceleration became larger and larger. I don't quite understand why that would happen. My lab instructor tried to explain it to me, but I didn't get it. So I'm hoping that someone here could help. Thank you.