Lesson Plan: Orbital Speed Physics • 9th Grade

This lesson plan includes the objectives, prerequisites, and exclusions of the lesson teaching students how to calculate the orbital speed of an object moving in a circular orbit given its orbital radius and the mass of the object it orbits.


Students will be able to

  • know that for an object to move in a circular orbit around a large object of mass 𝑀 at a radial distance 𝑟, it must have an orbital speed 𝑣 given by 𝑣=𝐺𝑀𝑟,
  • calculate 𝑣, 𝑀, or 𝑟 using 𝑣=𝐺𝑀𝑟.


Students should already be familiar with

  • the basic terminology of orbits: orbital speed, orbital period, and orbital radius,
  • how to use simple formulae to find the basic properties of a circular orbit (i.e., using 𝑇=2𝜋𝑟𝑣 to find the orbital period from the orbital speed and radius),
  • the idea that for an object moving in a circular orbit, the acceleration of the object is given by 𝑎=𝑣𝑟, where 𝑣 is the linear speed of the object and 𝑟 is the radius of the orbit,
  • the idea that the acceleration due to gravity at a point near a massive object is given by 𝑎=𝐺𝑀𝑟.


Students will not cover

  • noncircular orbits,
  • Kepler’s third law of planetary motion.

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