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Chapter 9: Newton's Universal Law of Gravity

Difficulty Level: At Grade Created by: CK-12

We know gravity as the force pulling downwards on everyday objects.  The principles of gravity, however, apply on a much larger scale, and were discovered from study of the solar system.  This chapter will cover Kepler's Laws of Planetary Motion, Newton's Universal Law of Gravity, and the mechanics of circular orbits.

Chapter Outline

Chapter Summary

  1. Kepler’s Three Laws of Planetary Motion are:
    1. The orbital paths of the planets about the sun are ellipses with the sun at one focus.
    2. If an imaginary line is drawn from the sun to a planet as the planet orbits the sun, this line will sweep out equal areas in equal times. (A planet moves faster when it is closer to the sun and slower when it is farther away from the sun.)
    3. The square of the time \begin{align*}T^2\end{align*}T2 for the orbital period of a planet about the sun is proportional to the cube of the average distance \begin{align*}r^3\end{align*}r3 between the sun and the planet. That is, \begin{align*}T^2 \propto r^3\end{align*}T2r3 or \begin{align*}T^2 = kr^3\end{align*}T2=kr3 where \begin{align*}k\end{align*}k equals \begin{align*}\frac{4\pi^2}{GM}\end{align*}4π2GM and \begin{align*}M\end{align*}M is the mass of the central body \begin{align*}\left(T^2 = \frac{4\pi^2}{Gm} r^3\right)\end{align*}(T2=4π2Gmr3). If \begin{align*}T\end{align*}T is expressed in years and \begin{align*}r\end{align*}r in astronomical units than \begin{align*}k = 1\end{align*}k=1 and \begin{align*}T^2 = r^3\end{align*}T2=r3
  2. The Universal Law of Gravity The force \begin{align*}F\end{align*}F between two objects is directly proportional to the product of their masses, \begin{align*}m_1m_2\end{align*}m1m2, and inversely proportional to the square of the distance, \begin{align*}r^2\end{align*}r2between their centers:

    \begin{align*}F = \frac{Gm_1m_2}{r^2}\end{align*}F=Gm1m2r2

    where \begin{align*}G\end{align*}G is the universal gravitational constant equal to \begin{align*}G = 6.67 \times 10^{-11} \frac{N*m^2}{kg^2}\end{align*}G=6.67×1011Nm2kg2.

  3. The gravitational acceleration near a massive body of mass \begin{align*}m\end{align*}m is \begin{align*}g = \frac{Gm}{r^2}\end{align*}g=Gmr2 where \begin{align*}m\end{align*}m is the mass that creates the gravitational acceleration and \begin{align*}r\end{align*}ris the distance from the center of the planet to a point outside the planet.
  4. The electrostatic force between two charged bodies is \begin{align*}F = \frac{kq_1q_2}{r^2}\end{align*}F=kq1q2r2.

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Difficulty Level:
At Grade
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Date Created:
Jun 27, 2013
Last Modified:
Jan 14, 2016
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