The concept of force is vital to Isaac Newton's formulation of classical physics. The force is the first derivative of momentum with respect to time. That is, it's the rate of change of momentum with respect to time. While in many cases the mass is constant, leaving the force with only one term, the product of mass and acceleration, this can by no means be assumed, as there are also many cases of changing mass in the physical world. An alternate, though equivalent, view of force, is as a position-respective directional derivative (or gradient) of the potential energy. Using these two views of force, many physical problems can be solved which would be otherwise intractable. In most introductory physical problems, the force equation is the easiest way to find a solution. Newton's Third Law, F = dp/dt, indicates that the sum of the forces on an object equals...