I don´t understand this: Accordingo to the second Newton´s Law, F=ma. You have a spring on the floor with constant k with an horizontal base over it. You drop a ball from different heights, h. Clearly, when h is great, then the string will be compressed a long distance. If you drop the ball from a not too big height, so it wont be compressed that much. This means that the force changes depending on the height, right? But, the acceleration for the two balls is the same, g, so the forces acting on them must be the same. So, what happens?