In physics, inertia refers to the resistance a physical object has to a change in its state of motion. Things at rest don’t start moving on their own, and planets continue to circle the sun without a means of propulsion. The phenomenon of inertia is the subject of Isaac Newton’s first law of motion, which states, “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” If a force—for example, friction—is not present, the object will continue as it was, moving in the same velocity or
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