Q:

What is Newton's first law?

A:

Newton's first law states, "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it." A more colloquial way of saying it is "an object in motion stays in motion."

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Newton's first law, one of his three laws of motion, describes inertia. For example, if a ball is set in motion, it keeps going unless a force stops it from going farther. A number of forces could stop it, such as friction as the ball rolls across the surface. Other things could make it go faster, such as a fan blowing behind it or gravity pulling it down a slope. Similarly, an object that is not moving stays still unless something acts upon it to move it, such as a cat's paw swiping at the ball. Newton describes how velocity changes in his second law.

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The work-energy theorem is a generalized description of motion that states that the work done by the sum of all forces acting on an object is equal to the change in that object's kinetic energy. This principle of work and its relationship to kinetic energy is a core mechanical physics concept.

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The law of inertia states that 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. The law of inertia is sometimes referred to as Newton's first law of motion.

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According to NASA, Newton's three laws of motion are: an object in motion tends to stay in motion, force equals mass times acceleration and for every action there is an equal and opposite reaction. Together, these three laws form the basis for classical mechanics.