Get more lessons like this at http://www.MathTutorDVD.com Learn what instantaneous velocity is, why it is important, and how to calculate it in physics. We begin by discussing average velocity, and demonstrate that the instantaneous velocity at any point in time is the slope of the line tangent to the position-time graph at that point. Specifically, we take the slope of this line as the limit when the change in time approaches zero. This is explained to be the basic definition of the derivative in calculus 1. Because of this, instantaneous velocity is used heavily in physics, mechanics, and calculus.