Skip to main content

Chapter 11 Noninertial Frames

So far we have studied motion with respect to inertial observers. For these observers, Newton's second law of motion takes the familiar form, “the rate of change of momentum equals the net force”, or \(\vec F = m\vec a\) for a constant mass particle.

Observers that have a non-zero acceleration with respect to an inertial observer are called non-inertial. When the second law of motion is written in a non-inertial frame, the equation of motion is modified. The new equations of motion contain forces not present in the inertial frame are called fictitious or inertial forces. We will study these modifications in this chapter.

A particularly important application of these modified equations is to the frames fixed to the Earth, also called Earth-based frames. Since Earth is rotating about its axis, all observers on the Earth are accelerating with respect to any fixed inertial frame. This makes all Earth-based frames non-inertial.

Newton's second law of motion in rotating frames that have a constant angular velocity with respect to an inertial frame, two inertial forces arise. These inertial forces are called centrifugal and Coriolis forces. We will study their implications in some detail in this chapter. But, first we will study a simple situation of a frame accelerating in a straight line with respect to an inertial frame.