Isaac Newton

Isaac Newton, was an English physicist. However, during his day, physics was known as natural philosophy. Newton is recognized as one of the most influential scientists of all time. Newton contributed to understanding of optics, invented calculus, and laid the foundation of mechanics. The mechanics laid out by Newton will dominate physical thought for about the next 3 centuries. 

Indeed, Isaac Newton laid the foundations of classical mechanics, in 1687, with Principia Mathematica (published with financial help from astronomer Edmond Halley), where he  proposed his three laws of motion. These laws of motion accounted for the relationship between bodies, the forces acting on those bodies, and the subsequent motion of those bodies, as a consequence of these forces. 

Newton's laws of motion: 

1) Objects in motion, stay in motion, unless acted upon by an outside force. 

2) Force, is equal to mass times acceleration. (F=ma)

3) For every action there is an equal and opposite reaction of the same magnitude.

First law

Before Galileo, it was commonly held that the natural state of an object was to be at rest. A force here, or some other influence, was required to keep a body in motion. For Galileo, this was not the case. He held that the natural state of an object is to be in motion. Rest was just a special case of motion, with zero velocity. Newton is going to take Galileo's assertion, his principle of inertia, as his first law of motion.

Newton's first law declares that an object at rest will remain at rest and an object in uniform motion tends to stay in that uniform motion, unless acted upon by some unbalanced external force. 

When considered in practice, because of Gravity's pervasive influence, there are hardly any circumstances where an object can be at rest with no force acting upon it at all. It should be noted that even isolated stars are moving under the influence of distant gravitational forces.

If the velocity of an object is changing, the object is being acted upon by outside forces. An example of this is planets moving in elliptical orbits around the Sun. Their speeds and directions of motion are constantly changing. The first law, therefore, states that a force must be acting upon them.

Second law

Mechanics are placed on a quantitative footing with the second law. it shows us how to calculate the motion of an object that is acted upon by a given unbalanced force. In particular, it relates the unbalanced force acting on an object to the acceleration of that object. 

The two are related through the concept of inertia. This is the property of a body that causes it to resist attempts to change its state of motion. Inertia is identified with the mass of the body. It is a measure of how much matter the body contains. The greater the mass, the greater the inertia. 

The acceleration of an object is directly proportional to the unbalanced force acting upon it. It is the same direction as the force. It is inversely proportional to the object's mass. 

It can be argued that Newton's second law is the most useful of the three laws. If the force is known, then the object's subsequent motion can be predicted in all circumstances. Conversely, it may be used to infer the forces acting on an object by observing its motion. It can tell us only how a body responds to force. It says nothing about the origin and nature of the forces itself. 

Third law

The third law, concerns the forces that act between two bodies. 

When two bodies interact, the force exerted by the first body on the second body is equal in magnitude, and opposite in direction to the force exerted by the second body on the first. 

Newton's law of universal gravitation:

Newton also gives us his law of universal gravitation. Gravity, in the world of Newton, was an attractive force between two bodies, that is:

• proportional to the product of their masses

• inversely proportional to the square of the distance between their centers.

This first great unification in physics is Newton, who unifies the celestial and the terrestrial. Newton realized that it was the same physical phenomenon that made an apple fall that kept the moon in orbit around the Earth.

Despite the fact that general relativity is applicable to understand gravitation, as a consequence of the curvature of spacetime as a result of the presence of an uneven distribution of mass, Newton's law of universal gravitation is still applicable in many applications. General relativity is used, instead of Newtonian gravity, in the case where:

• there is a need for extreme precision

• when dealing with very strong gravitational fields

• extremely massive and dense objects

• very close distances

Otherwise, Newton's law of universal gravitation remains applicable.