# What Does the Law of Inertia Apply To?

The law of inertia is a fundamental physics principle used to describe how an object behaves in motion or at rest, particularly when forces are applied to it. The object can be anything that has mass.

Inertia may refer to the amount of the object’s mass (how hard it is to change the object’s state), its momentum or the general principles of inertia described by Isaac Newton. The law of inertia describes how stationary objects remain stationary unless an external force acts on the object. Additionally, the law describes how an object in motion maintains its current direction and speed unless a force is applied to it.

Sometimes misunderstood, the force of friction is an external force to objects that is consistent with the law of inertia. Objects that move through the air or on a surface encounter friction, which slows down and eventually stops the objects. Once stopped, the static friction force of the object prevents it from further movement. To start an object moving again, an external force that is greater than the static friction force is required. In space, where there is no air friction, it is possible for objects to move without slowing or changing direction unless they interact with the gravitational forces of other objects.

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