Newton's Laws of Motion
   
The basis for modern mechanics was developed in the seventeenth century by Sir Isaac Newton. From his studies of objects in motion, he formulated three fundamental laws.

Newton's first law of motion states:- an object remains at rest (if originally at rest) or moves in a straight line with constant velocity if the net force on it is zero.

Newton's second law states:-the acceleration of a body is proportional to the net (i.e., sum or resultant) force acting on it and in the direction of that net force. This law establishes the relationship between force, mass, and acceleration and can be written mathematically as shown in Equation

F = ma

Where:-

F = force (Newton = 1 Kg-m/sec², or lbf)

m = mass (Kg or lbm)

a = acceleration (m/sec² or ft/sec²)

This law is used to define force units and is one of the most important laws in physics. Also, Newton's first law is actually a consequence of this second law, since there is no acceleration when the force is zero, and the object is either at rest or moving with a constant velocity. Equation can be used to calculate an objects weight at the surface of the earth. In this special case, F is the force, or weight, caused by the gravitational acceleration of the earth acting on the mass, m, of the object. When dealing with this type of problem, we designate the acceleration, g, which equals 9.8m/sec2 or 32.17 ft/sec2 (g is called gravitational acceleration constant). Thus, equation becomes

F = mg for this case.

Newton's third law of motion states if a body exerts a force on a second body, the second body exerts an equal and opposite force on the first." This law has also been stated as, "for every action there is an equal and opposite reaction.

The third law is basic to the understanding of force. It states that forces always occur in pairs of equal and opposite forces. Thus, the downward force exerted on a desk by a pencil is accompanied by an upward force of equal magnitude exerted on the pencil by the desk. This principle holds for all forces, variable or constant, regardless of their source.