I'm just uncertain (haha), what IS newton's Second law exactly? F=ma, or F=dp/dt, aren't these defining equations for F itself? what is a force? what would you mean a "force" acts on a object, would it be equally probable to define say, d^3Q/dt^3 = m dx^3/dt^3? does newton's second law have anything to do with the fact that coulomb, gravity etc can be expressed as a second order differential equation? is it a definition, or an accident?