Hi, So, I have a doubt regarding the equations for vertical oscillations on a spring. My book says the net force on the block is: F = k(d+y) - mg. If we define d the distance at -kd = mg. I, don't understand, the reason being: When the block is moving downwards, if its performing simple harmonic motion, it is accelerating upwards. This means that the upward force, that is, that provided by the string, must exceed mg. In this case, the book's formula holds. However, as soon as the block goes up through its equilibrium position (the one after the mass was hung) the acceleration should be downwards meaning mg is greater than the force provided by the string. Shouldn't the equation, thus read: F = mg - k(d+y) Thanks in advance, Peter G.