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Reading comprehension: Newton’s Laws
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A Brief History of Newton's Laws
The Greek philosopher Aristotle dominated scientific thinking for many years. His views on motion were widely accepted
because they seemed to support what people observed in nature. For example, Aristotle thought that weight affected
falling objects. A heavier object, he argued, would reach the ground faster than a lighter object dropped at the same time
from the same height. He also rejected the notion of inertia, asserting instead that a force must be constantly applied to
keep something moving. Both of these concepts were wrong, but it would take many years -- and several daring thinkers
-- to overturn them.
The first big blow to Aristotle's ideas came in the 16th century when Nicolaus Copernicus published his sun-centered
model of the universe. Aristotle theorized that the sun, the moon and the planets all revolved around the Earth on a set
of celestial spheres. Copernicus proposed that the planets of the solar system revolved around the sun, not the Earth.
Although not a topic of mechanics per se, the heliocentric cosmology described by Copernicus revealed the vulnerability
of Aristotle's science.
Galileo Galilei was the next to challenge the Greek philosopher's ideas. Galileo conducted two now-classic experiments
that set the tone and tenor for all scientific work that would follow. In the first experiment, he dropped a cannonball and a
musket ball from the Leaning Tower of Pisa. Aristotelian theory predicted that the cannonball, much more massive,
would fall faster and hit the ground first. But Galileo found that the two objects fell at the same rate and struck the ground
roughly at the same time.
Some historians question whether Galileo ever carried out the Pisa experiment, but he followed it with a second phase of
work that has been well documented. These experiments involved bronze balls of various sizes rolling down an inclined
wood plane. Galileo recorded how far a ball would roll in each one-second interval. He found that the size of the ball
didn't matter -- the rate of its descent along the ramp remained constant. From this, he concluded that freely falling
objects experience uniform acceleration regardless of mass, as long as extraneous forces, such as air resistance and
friction, can be minimized.
But it was René Descartes, the great French philosopher, who would add new depth and dimension to inertial motion. In
his "Principles of Philosophy," Descartes proposed three laws of nature. The first law states: "that each thing, as far as is
in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to
move." The second holds that "all movement is, of itself, along straight lines." This is Newton's first law, clearly stated in
a book published in 1644 -- when Newton was still a newborn!
Clearly, Isaac Newton studied Descartes. He put that studying to good use as he single-handedly launched the modern
era of scientific thinking. Newton's work in mathematics resulted in integral and differential calculus. His work in optics
led to the first reflecting telescope. And yet his most famous contribution came in the form of three relatively simple laws
that could be used, with great predictive power, to describe the motion of objects on Earth and in the heavens. The first
of these laws came directly from Descartes, but the remaining two belong to Newton alone.
He described all three in "The Mathematical Principles of Natural Philosophy," or the Principia, which was published in
1687. Today, the Principia remains one of the most influential books in the history of human existence. Much of its
importance lies within the elegantly simple second law, F = ma.
http://science.howstuffworks.com
1) Skimming exercise: read the above text very quickly and answer the questions:
a)
b)
c)
d)
e)
f)
When did Nicolaus Copernicus publish his sun-centered model of the universe?
Where did Galileo drop a cannonball and a musket ball?
Where did René Descartes propose three laws of nature?
When was Newton’s first Law published?
What was published in 1687?
What is the formula of the second law?
12 points
2) True or false: tick the right answer
a) Aristotle thought that falling objects were affected by weight
b) Copernicus thought that the planets revolved around the Earth
c) Galileo’s and Aristotle’s ideas were different
d) Descartes stated that when an object is moved it goes on moving
T
T
T
T
F
F
F
F
8 points
3) Scanning exercise: read the text very carefully and answer the questions:
a) What did Aristotle think about weight and falling objects?
b) Did he accept the notion of inertia?
c) What was Aristotle’s theory?
d) And Copernicus’s?
e) What is the difference between Aristotle’s and Galileo’s theories?
f) What does Descartes’s second law state?
g) How could Newton’s laws be used?
14 points
Newton’s First and Second Laws of Motion
The motion of an aircraft through the air can be explained and described by physical principals discovered
over 300 years ago by Sir Isaac Newton. Newton worked in many areas of mathematics and physics. He
developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in
1686, he presented his three laws of motion in the "Principia Mathematica Philosophiae Naturalis." Newton's
first law states that every object will remain at rest or in_______________in a straight line unless compelled
to change its state by the action of an external force. This is normally taken as the definition of inertia. The
key point here is that if there is no net force acting on an object (if all the external forces cancel each other
out) then the object will maintain a constant velocity. If that velocity is zero, then the object
remains__________________. If an external force is applied, the velocity________________because of the
force.
The second law explains how __________of an object changes when it is subjected to________________.
The law defines a force to be equal to change in momentum per change in time. Newton also developed
the calculus of mathematics, and the "changes" expressed in the second law are most accurately defined in
differential forms. (Calculus can also be used to determine the velocity and location variations experienced
by an object subjected to an external force.) For an object with a constant mass m, the second law states
that the force F is the product of________________and its acceleration a:
F=m*a
For an external applied force, the change in velocity depends on the mass of the object. A force will cause
a______________ ;and likewise, a change in velocity will generate____________ . The equation works both ways.
http://www.grc.nasa.gov
4) Find the highlighted words in the text and match them to their definitions :
a) the influence that changes movement =
b) the rate at which the speed of an object increases =
c) not doing anything active or not moving =
d) mass times velocity =
e) lack of activity =
f) the amount of matter in any solid object or in any volume of liquid or gas =
g) the speed at which an object is travelling =
h) the process of moving =
i) staying the same =
j) the way something moves or works, or the effect it has on something else =
10 points
5)
Read the text
again. The following nouns and phrases have been removed from the text.
Choose from them the one that fits each gap:
a)
b)
c)
d)
e)
f)
g)
h)
an object’s mass
a force
a change in velocity
uniform motion
the velocity
an external force
at rest
will change
16 points
Total score 60 points