Mentor's Note: This post and another have been merged at the member's request in order to attach their results. 1. The problem statement, all variables and given/known data See the attached picture for the lab setup. It is all about releasing a cart of 1.212 kg attached with a string and hanging masses. I recorded the Initial and final velocity, initial and finial position, and the tension force . Ps. I follow the instructions on the lab manual to do calculations. It did not say I have to consider potential energy. 2. Relevant equations The work done by tension is supposed to equal to the change in kinetic energy of the cart, but I don't know why I can't get that result. So I wonder which mass should I use to calculate the Change in Kinetic Energy of the Cart? The mass of the hanging mass only? The mass of the cart only? Or the total mass of the system? 3. The attempt at a solution Work done by tension = Tension force x (final position - initial position) Change in Kinetic Energy of the Cart = 1/2 MVfinal2 - 1/2 MVinitial2 I appreciate your help! Data collected with computer: Calculations The problem is that Change in KE and the Work Done by Tension have huge difference!