Newton's FIRST LAW OF MOTION
In his Prncipia Newton formulated three laws that describe and predict the behavior of bodies in motion. Both Galileo and Cescartes recognized that rest and uniform motion were natural states of motion. Newton's first law, often referred to as the principle of inertia, is about uniform motion:
NEWTON'S FIRST LAW: A body remains at rest or moves along a straight line with constant velocity as long as no external force acts upon it.
If a body is in motion in a straight line at constant velocity (that is, its state of motion is a uniform state of motion), it continues to move along that line without changing its speed or direction as long as no force acts on the body. Another way of saying this is that its quantity of motion or its momentum remains the same. Thus the natural state of affairs in motion is for the body to continue whatever motion it has until an external force changes its motion.
CHANGE OF STATE OF MOTION:
NEWTON'S SECOND LAW OF MOTION
Newton's second law deals with nonuniform motion, or velocity change, and why it occurs:
NEWfON'S SECOND LAW, A body acted upon by a force will accelerate in the direction of the applied force. The greater the force, the greater the acceleration will be.
If a body's state of motion is basically defined b velocity (or, more correctly, its momentum), then acceleration is a measure of the change. Acceleratio proportional to the magnitude of the acting force-as one increases, the other does too-and inversely proportional to the body's mass-as one increases, the other decreases.
If a moving body is subjected momentarily to an external force, it will momentarily accelerate in the direction given it by the applied force. Its velocity increases to a new value but not necessarily in the same direction as the force. But if the force is applied continously, then a continuous change of velocity, or tion, takes place. For example, as a planet orbits the sun, there is a continuous change of speed and direction, or a continuous acceleration. The change must be due to some continously applied external force. In this case we know that force is the gravitational attraction of the sun.
Action and Reaction:
Newton's Third Law of Motion
Newton's third law deals with the relation between forces:
NEWTON'S THIRD LAW: A body subjected to a force rean equal counterforce to the applied force; a.ction and reaction are equal and oppositely directed.
Two forces are involved here: action and reaction. One force never acts alone. We can see equal and opposite reactive forces every day. A bird taking off from an overhead power line produces a reaction that moves the line backward; water forced out of a lawn sprinkler produces a backward reaction that rotates the sprinkler. The next section is for astronomy a classic example of action and reaction .
