What is Newton's first law of motion?

Newton's First Law of Motion is that an object in motion tends to stay in motion, and an object at rest tends to stay at rest. Basically, if nothing stops the moving object, it will keep moving. If nothing causes the object at rest to move, it will stay not moving and at rest.
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Conceptually this law says that objects want to continue doing what they are already doing. If the object is moving, it will continue to move at the same speed. If the object is not moving, it will  stay at rest.  To change what an object is doing, you must apply an unbalanced force to the object.  When this unbalanced force is applied, the objects motion will change.  This means that an unbalanced force applied to an object will always cause the object to accelerate - since acceleration is defined as a change in velocity.

You experience the first law when you are a passenger in a car and the driver brakes suddenly.  Your body wants to keep moving forward and the seat belt restrains your motion.  Likewise, when you are turning it feels like your body is pushing into the side of the car. Actually  your body wants to keep moving forward while the car is turning underneath you, making it feel like your body is what is moving.

Practically speaking, all objects are subject to a variety of forces - gravity, wind, air resistance, electrostatics - which are either trying to slow down or speed up the object so we seldom experience constant motion.

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Newton's laws of motion are three physical laws that frame classical mechanics. They describe forces acting on an ideal object and its motion due to those forces. The first law originally stated: "Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." This is commonly restated in the following form: "First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it." In other words, with of a non-zero net force, the center of mass of a body either is at rest or moves at a constant velocity. An important corollary is thus that an object at rest will stay at rest, unless a force acts upon it; and an object in motion will remain in motion unless a force acts upon it. The laws of motion were first compiled by Sir Isaac Newton in his work Philosophiae Naturalis Principia Mathematica, first published on July 5, 1687. Newton's first law is a restatement of the law of inertia. Newton gave credit to Galileo who had already described inertia.

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An object in uniform motion will remain in motion so long as there is no force opposing it.  An object at rest will remain at rest so long as there is no force acting upon it.

Essentially a bowling ball sitting still will remain in its position so long as nobody or anything touches it.  On a frictionless surface a bowling ball in motion will stay in motion so long as it does not come into contact with anything, and something does not come into contact with it.

Within an atmosphere (ie. not in a vacuum) a situation where there are no forces acting on something isn't possible.  An object moving upwards will slow in it's upwards velocity, and reverse and become negative because gravity will always be acting on it.  That same object will eventually come to a stop because it has to eventually make contact with the surface.

Furthmore, an object moving anywhere in an atmosphere will always have a form of resistance acting on it, be it gravity, air friction, surface friction, an obstacle etc.

The first law is the most obvious law, it more or less goes withtout saying.

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What is Newton's first law?

Newton's first law of motion, also known as the law of inertia, describes how an object or a body behaves while at rest or in linear motion. The law states that the body at rest or in motion will remain at rest or in motion at constant velocity, or speed in a straight line, unless an external force acts on it. For example, a ball simply placed on the ground will remain at rest until someone decides to kick it, which is when it will begin to move.

Larger and heavier objects, or rather objects that have more mass, have more inertia than objects with less mass, meaning more force is necessary to move them. Fast-moving objects also have more inertia that slow-moving objects. For example, a rock won't be moved by the wind on a windy day, but a piece of paper will easily be blown away; the rock has more mass than the piece of paper and therefore has more inertia.

Despite being called Newton's first law, in honor of the great scientist Sir Isaac Newton, the law of inertia was actually first deduced by Galileo Galilei, who, through his experiments with balls, concluded that a body in motion will continue moving unless a force stops it and causes it to come to rest. Newton simply refined Galileo's deduction and named it his first law of motion.