Newton’s 1st law (N1L) states that a mass remains in its current state of motion in the absence of a (net external) force. While the common intuition is that all objects have a natural tendency to slow down or stop moving, implying that a force is required to maintain motion, the truth is actually quite the opposite: all objects have a natural tendency to remain at rest (if currently at rest), or remain moving at its current speed in its current direction (if currently moving), unless compelled by a (net external) force.
This reluctance to change of motion is called inertia. The larger the mass, the more difficult it is to change an object’s motion. Mass is thus a measure of an object’s inertia.
Since all masses have inertia, all masses have this inherent ability to maintain its current state of motion (whether stationary or moving). Any change in motion must therefore be the result of forces disrupting the status quo.
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xmPhysics 