Newton's First Law of Motion

Newton's first law of motion is

Every body continues in its state of rest, or uniform motion in a straight line, unless compelled to change that state by forces impressed upon it.

Newton's first law of motion states that a body subject to zero net force does not accelerate (i.e., it moves in a straight line at a constant speed). However, this law is only valid in special frames of reference known as inertial frames. In fact, we can think of Newton's first law as the definition of an inertial frame. Namely, an inertial reference frame is one in which a body subject to zero net force does not accelerate. There are an infinite number of different inertial reference frames all moving at constant velocities with respect to one another. (See Section 1.5.4.) It is impossible to identify an exact inertial reference frame. The best approximation to such a frame is the so-called International Celestial Reference System (ICRF), whose origin is the center of mass of the solar system, and whose coordinate axes are defined with respect to extremely distant point radio sources (mostly quasars) whose positions can be measured to great accuracy via very long baseline interferometry (VLBI).^1.1