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Sir Isaac Newton
FRS (pronounced /ˈnjuːtən/; 4 January 1643 – 31 March 1727 [OS: 25 December 1642 – 20
March 1726])[1] was an English physicist, mathematician, astronomer, natural philosopher,
alchemist and theologian. His Philosophiæ Naturalis Principia Mathematica, published
in 1687, is considered to be the most influential book in the history of science. In this
work, Newton described universal gravitation and the three laws of motion, laying the
groundwork for classical mechanics, which dominated the scientific view of the physical
universe for the next three centuries and is the basis for modern engineering. Newton
showed that the motions of objects on Earth and of celestial bodies are governed by the
same set of natural laws by demonstrating the consistency between Kepler's laws of
planetary motion and his theory of gravitation, thus removing the last doubts about
heliocentrism and advancing the scientific revolution.
In mechanics, Newton enunciated the principles of conservation of momentum and
angular momentum. In optics, he invented the reflecting telescope and developed a theory
of colour based on the observation that a prism decomposes white light into a visible
spectrum. He also formulated an empirical law of cooling and studied the speed of sound.
In mathematics, Newton shares the credit with Gottfried Leibniz for the development of
calculus. He also demonstrated the generalised binomial theorem, developed the so-called
"Newton's method" for approximating the zeroes of a function, and contributed to the
study of power series.
In a 2005 poll of the Royal Society asking who had the greater effect on the history of
science, Newton was deemed much more influential than Albert Einstein.[5]
NEWTON’S LAWS
Newton's First Law
Newton's First Law states that an object will remain at rest or in uniform motion in a
straight line unless acted upon by an external force. It may be seen as a statement about
inertia, that objects will remain in their state of motion unless a force acts to change the
motion. Any change in motion involves an acceleration, and then Newton's Second Law
applies; in fact, the First Law is just a special case of the Second Law for which the net
external force is zero.
Newton's First Law contains implications about the fundamental symmetry of the
universe in that a state of motion in a straight line must be just as "natural" as being at
rest. If an object is at rest in one frame of reference, it will appear to be moving in a
straight line to an observer in a reference frame which is moving by the object. There is
no way to say which reference frame is "special", so all constant velocity reference
frames must be equivalent.
Newton's Second Law
Newton's Second Law as stated below applies to a wide range of physical phenomena,
but it is not a fundamental principle like the Conservation Laws. It is applicable only if
the force is the net external force. It does not apply directly to situations where the mass
is changing, either from loss or gain of material, or because the object is traveling close to
the speed of light where relativistic effects must be included. It does not apply directly on
the very small scale of the atom where quantum mechanics must be used.
Data can be entered into any of the boxes below. Specifying any two of the quantities
determines the third. After you have entered values for two, click on the text representing
to third to calculate its value.
F = ma
Newton's Second Law Illustration
Newton's 2nd Law enables us to compare the results of the same force exerted on objects
of different mass.
Limitations on Newton's 2nd Law
One of the best known relationships in physics is Newton's 2nd Law
but, though extremely useful, it is not a fundamental principle like the conservation laws.
F must be the net external force, and even then a more fundamental relationship is
The net force should be defined as the rate of change of momentum; this becomes
only if the mass is constant. Since the mass changes as the speed approaches the speed of
light, F=ma is seen to be strictly a non-relativistic relationship which applies to the
acceleration of constant mass objects. Despite these limitations, it is extremely useful for
the prediction of motion under these constraints.
Mass and Weight
The mass of an object is a fundamental property of the object; a numerical measure of its
inertia; a fundamental measure of the amount of matter in the object. Definitions of mass
often seem circular because it is such a fundamental quantity that it is hard to define in
terms of something else. All mechanical quantities can be defined in terms of mass,
length, and time. The usual symbol for mass is m and its SI unit is the kilogram. While
the mass is normally considered to be an unchanging property of an object, at speeds
approaching the speed of light one must consider the increase in the relativistic mass.
The weight of an object is the force of gravity on the object and may be defined as the
mass times the acceleration of gravity, w = mg. Since the weight is a force, its SI unit is
the newton. Density is mass/volume.
Weight
The weight of an object is defined as the force of gravity on the object and may be
calculated as the mass times the acceleration of gravity, w = mg. Since the weight is a
force, its SI unit is the newton.
For an object in free fall, so that gravity is the only force acting on it, then the expression
for weight follows from Newton's second law.
You might well ask, as many do, "Why do you multiply the mass times the freefall
acceleration of gravity when the mass is sitting at rest on the table?". The value of g
allows you to determine the net gravity force if it were in freefall, and that net gravity
force is the weight. Another approach is to consider "g" to be the measure of the intensity
of the gravity field in Newtons/kg at your location. You can view the weight as a measure
of the mass in kg times the intensity of the gravity field, 9.8 Newtons/kg under standard
conditions.
Weightlessness
While the actual weight of a person is determined by his mass and the acceleration of
gravity, one's "perceived weight" or "effective weight" comes from the fact that he is
supported by floor, chair, etc. If all support is removed suddenly and the person begins to
fall freely, he feels suddenly "weightless" - so weightlessness refers to a state of being in
free fall in which there is no perceived support. The state of weightlessness can be
achieved in several ways, all of which involve significant physical principles.
Click on any of the examples for further details.
Newton's Third Law
Newton's third law: All forces in the universe occur in equal but oppositely directed pairs.
There are no isolated forces; for every external force that acts on an object there is a force
of equal magnitude but opposite direction which acts back on the object which exerted
that external force. In the case of internal forces, a force on one part of a system will be
countered by a reaction force on another part of the system so that an isolated system
cannot by any means exert a net force on the system as a whole. A system cannot
"bootstrap" itself into motion with purely internal forces - to achieve a net force and an
acceleration, it must interact with an object external to itself.
Without specifying the nature or origin of the forces on
the two masses, Newton's 3rd law states that if they arise
from the two masses themselves, they must be equal in
magnitude but opposite in direction so that no net force
arises from purely internal forces.
Newton's third law is one of the fundamental symmetry principles of the universe. Since
we have no examples of it being violated in nature, it is a useful tool for analyzing
situations with are somewhat counter-intuitive. For example, when a small truck collides
head-on with a large truck, your intuition might tell you that the force on the small truck
is larger. Not so!
Small truck,
large truck
Newton's Third Law Example
Newton's third law can be illustrated by identifying the pairs of forces which are involved
in supporting the blocks on the spring scale.
Presuming that the blocks are supported and at equilibrium, then the net force on the
system is zero. All the forces occur in Newton's third law pairs.