Newton's First Law is often written as "An object at rest will remain at rest unless acted on by a force. An object in motion will remain in motion unless acted on by a force." but this may lead to confusion. A better way of saying it is "If the net force on an object along an axis is 0N, there is no acceleration on that axis, and vice verse. If the net force on an object along an axis is not 0N, there is a nonzero acceleration on that axis, and vice versa."

This law can be counter-intuitive, because we often think of objects as "naturally coming to rest" without forces acting on them, but this is a misunderstanding. In reality, objects with no net forces acting on them will continue with their same velocity forever and ever until a nonzero net force changes their motion. This is related to the idea of "inertia" which means an object's tendency to keep whatever velocity it has and to resist changes in its velocity. The more inertia an object has, the more difficult it is to change the velocity of that object. The amount of inertia an object has is based only on its mass (it is more difficult to change the velocities of more massive objects) so mass can be thought of as a measurement of inertia.

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