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Inertial Frames: Instructor's Guide

Main Ideas

  • Inertial frames are “not accelerating”.
  • The Earth is not an inertial frame.

Students' Task

Estimated Time: 10 min, including wrap-up

  1. Students should first be asked to write down some properties of inertial frames.
  2. After having a few minutes to respond, but without further discussion, students should be asked whether the classroom is an inertial frame.

Prerequisite Knowledge

  • Familiarity with Newton's Second Law.


Activity: Wrap-up

Discuss student answers, emphasizing that the notion of inertial frame is context dependent: The classroom works fine for the Newtonian mechanics of small objects, with gravity viewed as an external force, but is noninertial due to rotational acceleration.


  • (Newtonian) Inertial frames are precisely those in which Newton's laws are valid.
  • What is a noninertial frame in special relativity?
  • Do the laws of physics apply in noninertial reference frames?
  • What is a “law” of physics?

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