Rule 15 of 36 · Chapter III — Force & Motion
Nothing changes its motion without a cause
Why this rule exists
Newton's first law asserts that velocity, not position, is the state a body keeps when left alone. This is subtle: it denies the intuition that motion needs continual pushing. The deeper truth is that only changes in motion — accelerations — require causes, because the laws of physics look identical in every uniformly moving frame. A body at constant velocity is already in equilibrium.
In practice
When you see steady motion or rest, expect the net force to be zero, and use that to solve for unknown forces. Draw a free-body diagram, resolve into components, and set each sum to zero. If something accelerates, hunt for the missing force; if it does not, stop adding forces — you have already found them all.
When it doesn't apply
The law holds only in inertial frames. In an accelerating or rotating frame — a braking car, a spinning planet — bodies veer without any real force, and you must add fictitious forces like the centrifugal or Coriolis to keep the bookkeeping honest.