1. The problem statement, all variables and given/known data I'm writing a paper on one of my lab experiments and I'm not sure my physic concept is right? The question asks if the mass of a cart affects the extension of the spring. A spring is attached to a cart which is attached to sting through a pulley and connected to a hanging object of mass W (a picture is attached). From my results, the spring stretched slightly more with the cart than with out the cart attached to the spring. 2. Relevant equations I know that at equilibrium all forces equal 0. Σ Fx = Σ Fy = 0 Fs= kx F=mg (with cart) Σ Fx = Fs –Th Σ Fy = Tv + Fn - Mgw -Mgc (without cart) Σ Fx = Σ Fx = Fs –Th Σ Fy = Tv - Mgw 3. The attempt at a solution If k is constant and Fs= kx and F=mg then mg=kx. Then if the cart was attached, mass will increase, therefore x (the spring extension) will increase, which explains why the spring stretched more when the cart was attached. Is this somewhat right, because he said i have the wrong concept? Can anyone help me? Am I missing something?