Ok I've done an experiment in which both a standard Atwood's pulley was used, two different masses suspended on either side of the pulley. And an accelerating cart, where the cart was on a horizontal surface, and a weight was attached on the other side of the pulley hanging down. The mass was kept constant by transferring weight from one mass to the other. From the experimental data I made graphs of force vs acceleration, and was able to calculate the moment of inertia. Here is my question, I found that the moment of inertia found for the accelerating car is considerably smaller than for the Atwood’s pulley. But I really don't know how to explain the significance of this. Anyone know???