Sir Isaac Newton was the man who finally built a complete
model of astronomy and physics, in doing so he brought together
all the work of Galileo and Kepler and introduced many new
concepts.
Newton was to unify physics and astronomy by realising celestial
mechanics (Kepler's work) and terrestrial mechanics (Galileo's
work) were in fact all governed by the same rules of physics.
Newton is often credited as being the true founder of modern
physical science as well as experimental investigation, and
although others laid down the foundations there can be no dispute
that it was he that was ultimately responsible for the actual
arrival of the modern era of these disciplines.
He was the first scientist ever to be honoured with a knighthood
for his work and, along with Einstein, stands well ahead of the
rest in terms of bringing an understanding of the universe in
which we live.
The surprising thing about Newton is that before he was about 25 years old no one, not even himself it appears, had any clue of the genius that lay dormant with him. It was, ironically, discovered when Cambridge University was closed due to the plague and he went home. In a little over two years, until the university reopened, he produced so much work that it is believed that his initial understanding of all his future work was generated in this period.
Newton became reluctant to share his work following the high degree of public criticism he often received for it (as most revolutionaries do). This criticism was particularly harsh on his work on optics (with Robert Hook as the chief critic) and this together with having failed to get his work on calculus published led Newton to become very withdrawn. For example his work Opticks was finished by around 1692 but was not published until 1704 after Hooks death.
Newton's AchievementsHis many great achievements include:
- Laying down the foundations for differential and integral calculus, realising that the integration of a function was simply the inverse of differentiating it. (It should be noted that there was and continues to be a dispute over whether it was Newton or Leibniz that discovered calculus, it seems that they discovered it independently. Newton almost certainly laid down the principles before Leibniz, but he had failed to get the work published). For a detailed account of this click here to download an excellent article by S. Subramanya Sastry on this (.pdf format).
- Newton also worked on optics, reaching remarkable conclusion that white light is not a single entity, as had been assumed since Aristotle's work on the subject, but suggested that white light is actually made up of many different rays, each of differing colour. He had noticed this when he saw light passed through a prism (he probably first noticed this effect while looking through his telescope).
- He realised that this would mean all refracting telescopes would suffer from chromatic aberration. This led him to construct the world's first ever reflecting telescope.
- Newton also developed the world's first reflecting microscope.
- Newton's work on optics also took him to the remarkable conclusion that light was not actually waves but consisted of small particles (this is of course now confirmed, they are called photons).
This list does not include Newton's greatest work which we are interested in (detailed below) and is by no means exhaustive of his other accomplishments, it serves merely to point out that the breadth of his work remains unsurpassed.
Undoubtedly his greatest work was in physics and celestial
mechanics, which culminated in his Universal Law of Gravitation.
It was in 1686 that Edmond Halley persuaded Newton to divulge his
new physics and its relation to astronomy and in 1687 he
published Philosophiae Naturalis Principia Mathematica
(usually referred to only as Principia), this still
stands as the greatest work ever written in the field of
mathematical physics, and indeed in any discipline of science. It
is further recognised as one of humanity's greatest achievements
in abstract thought.
So how did Newton change our understanding of the universe?
This was the first complete analysis of motion and dynamics. The importance of these laws should not be understated, these principles still dominate our world today.
Before going on here it is important
that the reader has a fair grasp of scalars and vectors and
understands that velocity and acceleration are both vectors,
click
here for a very brief explanation.
The reader should also be aware that acceleration may be
described as a 'change in velocity'.
'An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force'.
It should be fairly clear then that this is essentially
Galileo's Law of Inertia, and it is indeed often referred to as
such. Just to review then this means that in the absence of an
external force a body shall continue to move at a constant
velocity.
Therefore any acceleration (change in velocity in either
magnitude or direction) of a body signals that it is being acted
upon by a force. It is important to remember forces cause
accelerations not motion.
From Galileo's projectile work Newton surmised that if the
vertical component has a constant acceleration (which Galileo
proved by experimentation) then this acceleration must be due to
a vertical force acting upon the object.
This was a crucial realisation by Newton (of course to
us now it is elementary, it is simply gravity) because what it
meant was that in a situation where this force did not exist,
such as in deep space, the natural motion in any direction would
be a steady speed in a straight line (constant velocity).
'The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.'
Here Newton is describing that when an external force acts on
an object the acceleration (remember a vector) that is caused
will be proportionate to that force and inversely proportional to
the mass of the object.
If we use units of kilograms (kg) for mass and metres per second
squared (m/s2) for acceleration then the force can be
measured in Newtons (convenient!), as long as these units are
used the second law can be expressed as:
This sounds reasonable, if we take a horizontal force acting
on two different bodies, one twice the mass of the other, we
would expect the smaller mass to accelerate at twice the rate of
the body with the larger mass.
But what about free falling objects? It must be remembered that
Galileo had already firmly established that all free falling
bodies accelerate at the same rate. So from Newton's second law
if two bodies are falling, one twice the mass of the other then,
there must be a force twice as strong being applied to the larger
mass. This is obvious when you think about it, all we are
discussing here is the difference between
mass and weight and what we are saying is that the larger
mass must weigh twice as much as the smaller mass.
Clearly Newton's second law demonstrates that mass and weight
must be proportionate, with the acceleration due to gravity being
the constant:
force (weight) = mass x acceleration due to gravity (which is
constant for all objects at any one location).
Newton's second law was crucial to his work because it introduces quantitative measures.
Newton's Third Law'For every action, there is an equal and opposite reaction'.
This is actually very straightforward, Newton had already
established that a force was necessary for an object to change
its velocity, but here he is first suggesting that the force
itself must be the result of an interaction between objects.
Then taking this one step further he realised that as this force
is produced not only is the action of this force present (i.e.
the change in velocity that is felt by the original body) but
there is also an equal and opposite (magnitude and direction)
reaction present.
As an example consider a person firing a gun, the
action of firing is the bullet flies forwards but there
is an equal and opposite reaction in the recoil of the
gun. Of course from Newton's second law although the forces are
the same it does not mean the accelerations are the same (that is
dependent on the relative masses). Another example of the equal
and opposite reaction would be a man pushing on a wall, an equal
force to that which is exerted by the man on the wall (action),
will be exerted by the wall on the man (reaction), but in the
opposite direction.
We could test this theory by reducing the friction in the
example. If the man now puts on roller skates and pushes on the
wall he goes backwards! Why? Clearly because of the equal and
opposite force the wall has made upon the man. It is also
interesting to note that the force that produces the acceleration
can be generated by either the action (e.g. in the case of the
gun) or the reaction (e.g. in the case of the wall).
It is important to realise these interactions are going on all the time, as you are reading this you are probably sitting on a chair, and as your body exerts a downward force on this chair, the chair is exerting an equal upward force on your body.
There is one other crucial point to note, at the end of Newton's writing it states that:
'This law also takes place in attractions'
This is central to his idea of the universe, this means that this law applies not only to contact interactions but also to distance interactions, i.e. gravity.
The principles of these laws cannot be overstated, they represent an important step in our understanding of how the universe works and were crucial to Newton's development of the Universal Law of Gravitation.
Continue to Newton's Universal Law of Gravitation
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