Newton
Why do objects move in the ways
they do?
Isaac Newton
• Born in 1642 (by
coincidence, the same
year of Galileo’s death.
• Born very prematurely in
a small town in rural
England (never expected
to survive)
• He never knew his father
(he died three months
before Isaac was born)
• Isaac went
away to
grammar
school at 15
• He returned to
the farm at 18,
but was a
terrible farmer
• An uncle
persuaded his
mother to allow
him to go to
college which
he did in 1661.
Newton’s Home
• He paid his way
through Cambridge by
waiting tables and
cleaning rooms for
wealthier students and
faculty
Plague
• Just as Newton had finally been awarded a scholarship
at Cambridge in 1665, the Plague invaded England. To
escape its effects, Cambridge was shut down and
Newton retired for two years in the countryside.
A Productive Time
• Newton’s two years in the
countryside were possibly two
of the most productive for one
person in the history of science.
• During this time Newton wrote
that he first understood the law
of gravitation, the law of
optics, and invented calculus,
as well as create his laws of
motion.
• A legend is that gravity unveiled
itself when an apple fell on his
head!
• All this while only 23 and 24
years old.
Newtonian Motion
• Newton’s famous laws on motion were
collected in a book he reluctantly wrote
called “The Principia”
• It took much convincing from Edmund
Halley before he would publish his physics
findings.
Newton’s First Law
• 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.
• We can see that this
law was an extension
of what Galileo said,
so Newton’s first law is
also known as the Law
of Inertia
Inertia
• Inertia is the tendency of an object to
remain at rest or in motion unless acted
upon by an unbalanced force.
• Inertia is measured as the mass of an
object (in Kg)
• Inertia Demos: Cards n’ Coins; Table
Cloth, Heavy Ball Light Ball, Car Crash
Newton’s Second Law
• F=ma
• Newton gave the world a way to talk
quantitatively about Force
• He was able to relate an object’s Inertia to
its Acceleration, based on the Force
applied to it.
• F is force in the above equation and is
measured in Newtons (named for him not
by him)
F=ma
• Think: It takes a force (a push or pull) to make
an object accelerate
• m stands for mass
• a is acceleration
• Force is the product of these.
• The unit of mass is the kilogram, the unit of
acceleration is m/s2.
• The unit of force is therefore the product of these
units: kg m/s2 (referred to as the Newton)
Newton vs. Aristotle
• According to Newton,
and his law: F=ma, an
object needs a net
force greater than
zero in order to have
any acceleration
(change in velocity)
• An object at rest or
traveling at a constant
velocity has no net
force acting on it (no
acceleration)
• The airplane stays at
a level height and a
constant velocity due
to the fact that all
forces on it are
balanced.
The Newton
• The Unit of force used in SI is the Newton.
• Since f=ma and m=kg, and a=m/s²
»F=kg•m/s²
• The unit kg•m/s² is known as the Newton
Example:
• What is the force on a
100 kg object
accelerating at 9.8m/s²?
• F=ma F=100kg•9.8m/s²
= 980 kg•m/s²
or 980 N
Newton’s Third Law
• For every action there is an equal and
opposite reaction.
• This explains why a rocket fires up into
space, why astronauts can move about in
space using jets of air, and why an object
can rest on a table top.
Newton’ Paraphrase
"We are like dwarfs sitting on the shoulders
of giants. We see more than they do,
indeed even farther; but not because our
sight is better than theirs or because we
are taller than they. Our sight is enhanced
because they raise us up and increase our
stature by their enormous height"
Newton
• “If I have seen further it is by standing on
the shoulders of giants”
(meaning Archimedes, Aristotle, Kepler,
Brahe, Copernicus and Galileo etc.)
Personality
• Newton was a classic introvert
• He could concentrate on one topic for
many hours
• He loved his pet
• He was extremely modest (he refused to
publish his work until coaxed)
• He also had many strange habits…
• He published his work
in Latin only.
• Here is a picture of
his one of is notebook
pages.