# 2: Conic Sections

- Page ID
- 6797

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A particle moving under the influence of an inverse square force moves in an orbit that is a conic section; that is to say an ellipse, a parabola or a hyperbola. We shall prove this from dynamical principles in a later chapter. In this chapter we review the geometry of the conic sections.

- 2.1: The Straight Line
- We start off, however, with a brief review (eight equation-packed pages) of the geometry of the straight line.

- 2.2: The Ellipse
- ellipse is the locus of a point that moves such that the sum of its distances from two fixed points called the foci is constant. An ellipse can be drawn by sticking two pins in a sheet of paper, tying a length of string to the pins, stretching the string taut with a pencil, and drawing the figure that results. During this process, the sum of the two distances from pencil to one pin and from pencil to the other pin remains constant and equal to the length of the string.

- 2.3: The Parabola
- We define a parabola as the locus of a point that moves such that its distance from a fixed straight line called the directrix is equal to its distance from a fixed point called the focus.

- 2.4: The Hyperbola
- A hyperbola is the locus of a point that moves such that the difference between its distances from two fixed points called the foci is constant.

- 2.5: Conic Sections
- A plane section of a cone is either an ellipse, a parabola or a hyperbola, depending on whether the angle that the plane makes with the base of the cone is less than, equal to or greater than the angle that the generator of the cone makes with its base. However, given the definitions of the ellipse, parabola and hyperbola that we have given, proof is required that they are in fact conic sections.