NATS 1745 Lecture 8 Review
1. According to Descartes, what is in between the planets, and what drives the planets'
Space is filled with an invisible fluid (ether) in which the planets are carried around the Sun by
whirling vortices. Thus, empty space does not exist.
Theory of matter is that matter is defined by the amount of space that it takes up
o If matter is defined by an amount of space, then all space is matter
Planetary motion is driven by the matter of space
2. According to Hooke, what causes the planets to move?
As Curator of Experiments for the Royal Society, he tested theories in many branches of
He argued, contrary to Descartes, that planetary motion is due to an invisible attractive
force that acts across empty space ("gravitational attraction")
3. What does it mean to say that gravity is universal?
It's the cause of motion both on Earth and is space.
Eg. Earth's gravity not only holds Earth together and causes free-fall, but also holds the
Moon in orbit
4. According to Hooke, why does a free-falling object increase in speed as it falls toward
the Earth? What would happen to the Moon's orbital speed if the Moon moved closer to
the Earth, and why?
Gravitational attraction increases with proximity to its source.
o Eg. The closer a body is to Earth, the stronger Earths pull on it
It would move around the Earth faster because the closer something is to the earth's
ground, the stronger pull it has on that object
5. What did Newton invent which made him particularly suited for deriving the
mathematical equations that describe the motion of the planets?
A polymath and introvert who, at 25, invented Calculus (the math that describes how quantities
change, and the only way to precisely describe motion)
6. What did Newton find when he passed sunlight through 2 prisms? What did he
conclude from this?
Prior to Newton, it was believed: when the sunlight passes through glass, the sunlight is
permanently changed into coloured light
Using 2 glass prisms, Newton proved: sunlight is composed of the colours of the rainbow
Concluded sunlight is composed of all the colours of the rainbow and sunlight refracts the
7. What is dispersion?
The splitting of light into its component colours when it passes through a medium, due to the
different refraction angles of each colour
8. What is chromatic aberration? Why did Newton construct a telescope with mirrors
instead of glass lenses? Chromatic Aberration is the failure of lens to focus all the colors in a light ray at the same
Newton constructed a telescope with mirror because it avoids chromatic aberration, since it is
not passing through any medium
9. Why are all large modern telescopes reflectors rather than refractors?
10. What does it mean to say that gravity obeys an inverse-square law? According to this
law, if one body doubles its distance from a source of gravity, by how much has the
gravity weakened? What if the distance is tripled?
All modern telescopes are reflectors (mirrors) rather than refractors (lens) because of the
difficulties in making large glass lens
11. What was the Royal Society's challenge in 1684, and who eventually solved it?
The Royal Society’s challenge in 1684 was for anyone to prove that gravity is defined by the
inverse square law. Newton had already solved the problem
12. What was the name of Newton's famous book in which he presents the definition of
gravity? Who urged Newton to publish this book?
Newton presents the definition of gravity in his book called the Principia. Edmund Halley urged
Newton to publish the book.
13. What did Newton prove about Kepler's 3rd Law?
Newton proved that Kepler’s 3 law (P2 = k x a3) applies to all orbital motion. K measure a
planet’s mass. K measures the mass of whatever its orbits around. (All orbital motion obeys
Kepler’s third law)
14. How was Newton able to derive the masses of Jupiter and Saturn? According to his
measurements, how do the masses of Jupiter and Saturn compare to the other visible
planets (no specific numbers necessary)? What do the masses and sizes of Jupiter and
Saturn tell us about their density and composition?
Newton got the masses of Jupiter and Saturn by applying Kepler’s 3 law to the orbits of their
moons. The mass of Saturn is nearly 80 times greater than earth’s mass and the mass of Jupiter is
more than 300 times greater than earth’s mass. Our solar system is dominated by Jup