Newton's first law: law of inertia
Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare. (Every body perseveres 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.)[1]
This law is also called the law of inertia.
This is often paraphrased as "zero net force implies zero acceleration", but this is an over-simplification. As formulated by Newton, the first law is more than a special case of the second law. Newton arranged his laws in hierarchical order for good reason (e.g. see Gailili & Tseitlin 2003).[2] Essentially, the first law establishes frames of reference for which the other laws are applicable, such frames being called inertial frames. To understand why this is required, consider a ball at rest within an accelerating body: an aeroplane on a runway will suffice for this example. From the perspective of anyone within the aeroplane (that is, from the aeroplane's frame of reference when put in technical terms) the ball will appear to move backwards as the plane accelerates forwards (the same feeling of being pushed back into your seat as the plane accelerates). This appears to contradict Newton's second law as, from the point of view of the passengers, there appears to be no force acting on the ball which would cause it to move. The reason why there is in fact no contradiction is because Newton's second law (without modification) is not applicable in this situation because Newton's first law was never applicable in this situation (i.e. the stationary ball does not remain stationary). Thus, it is important to establish when the various laws are applicable or not since they are not applicable in all situations. On a more technical note, although Newton's laws are not applicable on non-inertial frames of reference, such as the accelerating aeroplane, they can be made to do so with the introduction of a "fictitious force" acting on the entire system: basically, by introducing a force that quantifies the anomalous motion of objects within that system (such as the ball moving without an apparent influence in the example above).
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 the object does not change. Essentially, it makes the following two points:
An object that is not moving will not move until a net force acts upon it.
An object that is in motion will not change its velocity (accelerate) until a net force acts upon it.
The first point seems relatively obvious to most people, but the second may take some thinking through, because we have no experience in every-day life of things that keep moving forever (except celestial bodies). If one slides a hockey puck along a table, it doesn't move forever, it slows and eventually comes to a stop. But according to Newton's laws, 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 this force which causes the object to slow to a stop. In the absence of such a force, as approximated by an air hockey table or ice rink, the puck's motion would not slow. Newton's first law is just a restatement of what Galileo had already described and Newton gave credit to Galileo. It differs from Aristotle's view that all objects have a natural place in the universe. Aristotle believed that heavy objects like rocks wanted to be at rest on the Earth and that light objects like smoke wanted to be at rest in the sky and the stars wanted to remain in the heavens.
However, a key difference between Galileo's idea and Aristotle's is that Galileo realized that force acting on a body determines acceleration, not velocity. This insight leads to Newton's First Law—no force means no acceleration, and hence the body will maintain its velocity.
The Law of Inertia apparently occurred to many different natural philosophers independently. Inertia of motion was described in the third century BC in the Mo Tzu, a collection of Chinese philosophical texts, and the 17th century philosopher René Descartes also formulated the law, although he did not perform any experiments to confirm it.
There are no perfect demonstrations of the law, as friction usually causes a force to act on a moving body, and even in outer space gravitational forces act and cannot be shielded against, but the law serves to emphasize the elementary causes of changes in an object's state of motion:
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Sat Jan 19, 2008 3:40 am
