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The law states that"everly body continues in its state of rest or uniform motion along a straight line, unless it us acted upon by some external force to change that state".
Thus is often paraphrased as "zero net force implies zero acceleration", but this is an oversimplification. As formulated by newton, the first law is more than a special case of the second law. The significance of the first law is to establish frames of reference for which the other law are applicable, such frames being called Inertial Frames.
The net force on an object is the vector sum of all the forces acting on the object. Newton's First law says that if this sum is zero, the state of motion of tge object does not change. Essentially, it makes the following two points.
• An object that is not moving will not move untill a net force acts upon it.
• An object that is in motion will not change it's velocity untill a net force acts upon it.
The first point seems relatively obvious to the most people, but the second may take some thinking through, because we have no experience in everyday life of things that keep moving forever(except celestial bodies). If one sides a hockey puck along a table, it doesn't move forever, it slows and eventually comes to a stop. But accourding to Newtons law, this is because a force is acting on the hockey puck and sure enough, there is frictional force between the table and the puck, and that frictional force is in the direction opposite the movement. It is thus force which cause the object to sliw to a stop. In the absence of such a force, the pucks motion would not slow. Newtons first law is just a restatement of what Galileo had already described and Newton gave credit to Galileo.
However, a key difference between Galileo idea and Aristotle's is that Galileo realised that force acting on the body determines Acceleration, not velocity, and hence the body will maintain its velocity.
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