Abeka Physics Section 8.4 Flashcards
Describe Ptomley’s ideas of the solar system
> that the heavenly bodies are fixed in enormous hollow crystalline sphere, each moving in daily rotation about the earth
that each of the innermost spheres carried a single body of the solar system
that beyond the solar system there was first a single sphere carrying all the stars, then a series of invisible spheres, and last, a final sphere called the Prime Mover
his model of the solar system was geocentric (meaning everything orbited around the earth)
Describe Nicolaus Copernicus’s idea of the solar system
> proposed that the sun, not the earth, is the center of the solar system
his heliocentric model has become the foundation of modern astronomy
Describe the work of Tycho Brahe, a Danish astronomer
> he decided that progress in astronomical science required more careful observation of planetary motion
he built an observatory on the island of Huen near Copenhagen where he worked for many years making detailed observations without the aid of a telescope
Describe Johannes Kepler
> studied Tycho Brahe’s voluminous data for twenty years
>concluded that his three empirical laws (laws of planetary motion) were obeyed by the planetary motions
Name and describe Kepler’s first law of planetary motion
> first law of planetary motion is the law of orbits
the orbit of every planet has the shape of an ellipse (has two foci [plural of focus]
the two foci of each planetary orbit are relatively close together; moreover, one focus is positioned exactly at the sun, and the other is an empty point in space near the sun
the planetary orbits are nearly circular about a solar center
Name and describe the second law of planetary motion
> the second law of planetary motion is the law of areas
an imaginary line connecting a planet to the sun will sweep out equal areas in equal times during the planet’s orbit
when a planet is at its aphelion (the farthest position from the sun in the planet’s orbit), it slows down to its minimum velocity
when the planet is at its perihelion (the closest position to the sun in the planet’s orbit), the planet speeds up to its maximum velocity
“the closer the planet is to the sun, the faster its orbit”
Name and describe the third law of planetary motion
> the third law of planetary motion is the law of periods
the square of the period (the time required to complete one orbit about the sun) of revolution is proportional to the cube of the average distance from the sun
equation from the law of periods:
T^2 = kr^3 where k is a proportionality constant, T is the planet’s period of revolution, and r is the average distance between the planet’s center and the sun’s center
“a planet with an orbit close to the sun has a shorter period than a planet farther away from the sun”
The science of mechanics is divided into two categories. Name and define both categories.
The science of mechanics is divided into kinematics and dynamics. Kinematics seeks merely to describe motion. Dynamics seeks to explain motion by describing its causes.
Kepler’s three laws of planetary motion was a ground breaking achievement in kinematics, but he did not show “why” these motions occur. Who picked up this task of explaining the reason for the three laws of planetary motion? Describe what he found.
> Newton had already developed his three laws of motion so he knew that an object like a planet deviates from a linear course only under the influence of a force
he was able to show that a planet would obey Kepler’s law of areas if the cause of its orbiting movement was a continuous force directed toward the sun
With the aid of Kepler’s first and third law, Newton determined that the force on a planet was proportional to the planet’s mass, and inversely proportional to r^2: F is proportional to m/r^2
his conclusion: “the force responsible for planetary motions must diminish as the square of the distance from the sun increases”
because the force drawing a planet to the sun is proportional to the mass of the planet, the sun attracts a larger planet more strongly than the sun attracts a smaller planet
because of Newton’s third law of motion, he reasoned that in addition to the sun’s force on the planet, there must be a reciprocal force from the planet on the sun
if the first force depends on the mass of the planet, m, the second force must depend on the mass of the sun, Ms
Describe the universal law of gravitation
> Newton suggested that the same kind of force exists between any two masses, regardless of size; specifically, that they are mutually attracted by a force F governed by the proportionality: F is proportional to m1m2/r^2 where m1 and m2 are the two masses and r is the distance between their centers
this relationship described above is the universal law of gravitation
its equation: F = G (m1m2/r^2) where the constant of proportionality, G, is called the constant of gravitation.
G = 6.673 x 10^-11 Nxm^2/kg^2
Define gravitation
Gravitation refers to the attractive force that exists between any two masses
Define gravity
Gravity refers to the attractive force exerted by a planet, a star, or other celestial body upon objects near their surfaces
Why is Newton’s law of gravitation said to be universal
Because it describes an attraction that exists between any two masses in the universe
Who came up with the accurate value of G, the constant of gravitation? Explain how.
> Henry Cavendish used a Cavendish balance with two small spheres connected to the ends of a thin rod suspended on a quartz fiber
light shining on a mirror attached to the fiber is reflected to a scale of measurement
two large lead spheres placed in fixed positions near the rod attract the small spheres, causing the rod to twist
the angle of twist, which is determined by how far the light moves along the scale, is proportional to G
since gravitational attraction between the spheres is extremely slight, accurate measurement of G requires that the balance be isolated from outside forces like air currents and electric charge
