Newton&#39;s Three Laws of Motion

Wednesday 28 March 2012

Newton's Three Laws of Motion

06:32 juilite

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Let us begin our explanation of how Newton changed our understanding of the Universe by enumerating his Three Lawsof Motion.

Newton's First Law of Motion:

I. Every object in a state of uniform motion tends to remain in thatstate of motion unless an external force is applied to it.
This we recognize as essentially Galileo's concept of inertia, and this is oftentermed simply the "Law of Inertia".
Newton's Second Law of Motion:
II. The relationship between an object's mass m, its accelerationa, and the applied force F isF = ma.Acceleration and force are vectors (as indicated by their symbols beingdisplayed in slant bold font); in this law the direction of the forcevector is the same as the direction of the acceleration vector.
This is the most powerful of Newton's three Laws, because it allows quantitativecalculations of dynamics: how do velocities change when forces are applied.Notice the fundamental difference between Newton's 2nd Law and the dynamics ofAristotle: according to Newton, a force causes only a change invelocity (an acceleration); it does not maintain the velocity as Aristotleheld.
This is sometimes summarized by saying that under Newton, F =ma, but under Aristotle F = mv, where v is the velocity.Thus, according to Aristotle there is only a velocity if there is a force, butaccording to Newton an object with a certain velocity maintains thatvelocity unless a force acts on it to cause an acceleration (that is,a change in the velocity). As we have noted earlier in conjunction with thediscussion of Galileo, Aristotle's view seems to bemore in accord with common sense, but that is because of a failure toappreciate the role played by frictional forces. Once account is taken of all forcesacting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that arefound to be in accord with the observations.
Newton's Third Law of Motion:
III. For every action there is an equal and opposite reaction.
This law is exemplified by what happens if we step off a boat onto the bank ofa lake: as we move in the direction of the shore, the boat tends to move inthe opposite direction (leaving us facedown in the water, if we aren'tcareful!).