Retrograde Motion, History, Gravity

Apparent Retrograde Motion

When planets move from East to West with respect to background stars, not planets normal motion.

Johannes Kepler

Kepler realized that all planetary orbits were not circular but elliptical. He helped make the Heliocentric model more accurate.

Geocentric Model of the Universe

Model of the universe, where the Sun, Moon, and other planets orbit around the Earth, which is at the center of the universe.

Momentum

Momentum = Mass x Velocity. The only way to change an object's momentum is to apply a force to it.

Sir Isaac Newton

Newton developed a theory of gravity, which explained the elliptical orbits of the planets, along with calculus and a new telescope. Gravity is the force that keeps the planets in their orbits.

Gravitational Encounter

A way in which objects can exchange orbital energy, this occurs when 2 objects pass near enough so that each can feel the effects of the other's gravity.

Acceleration

Acceleration is the rate of change of the velocity of an object. A = V 1 (Velocity at the end) - V 0 (Velocity at the start) / (divided by) T 1 (Time at the end) - T 0 (Time at the beginning)

Force

Any influence that forces an object to undergo a change in its movement, direction, or geometrical construction.

Nicholas Copernicus

Copernicus is the father of the heliocentric model, he was motivated by data and aesthetics to create a model that explained the inaccuracies of the Ptolemiac model

Tycho Brahe

Epitome of pre-telescopic naked eye observation, Kepler was his protégé

Newton's 3rd Law of Motion

Every force is opposed by an equal and opposite reactionary force.

Newton's Law of Universal Gravitation

Every mass attracts every other mass through the force called gravity. The strength of gravitational force attracting 2 objects is directly proportional to the product of their masses. The strength of gravity between 2 objects decreases with the square of the distance between their centers. Gravitational force follows an inverse square law, doubling the distance between 2 objects weakens the force of gravity by a factor of 2^2 (2 squared) or 4. F (g) = M1 (Mass of Object 1) x M2 (Mass of Object 2) / [Divided by] distance between the objects squared d^2

Equation for Newton's Universal Law of Gravitation

F (g) = G (Gravitational Constant) M1 (Mass of Object 1) x M2 (Mass of Object 2) / [Divided by] distance between the objects squared d^2 F (g) = G x M1 x M2 ÷ d^2

Free-Fall

Falling without any resistance to slow you down.

Newton's 2nd Law of Motion

Forces cause acceleration. The bigger the force, the bigger the acceleration. For a given force the bigger the mass, the smaller the acceleration. Force = Mass x Acceleration

Gravitational Constant (G)

G = 6.67 x 10 ^-11m^3/(kg x s^2)

Galileo

Galileo was the 1st telescopic astronomer, he refuted objections to the Heliocentric Model with Elliptical orbits, with Galileo's observations the Heliocentric Model became more widely accepted.

Bound Orbits

Orbits in which an object goes around another object over and over again

Unbound Orbits

Paths that bring an object closer to another object just once.

How the Geocentric Model accounted for Retrograde Motion

Ptolemy's model (Ptolemiac Model) of the Geocentric Universe, each planet moves on a small circle, an epicycle, whose center moves around the Earth on a larger circle, a deferent.

Escape Velocity

The Earth's escape velocity is 40,000 km/hr or 11 km/s, this is the minimum velocity required for a spacecraft to escape Earth's gravity for a spacecraft that starts near the surface.

Problems with the Geocentric Model

The Geocentric Model could not explain the planets' retrograde motion.

Heliocentric Model

The Heliocentric Model is the modern model for the Universe, in this model the Earth and the planets orbit around the Sun, which is at the center of the Universe.

Weightlessness

The absence of weight and the absence of stress and strain resulting from externally applied force.

Acceleration of Gravity

The acceleration of a falling object; g = acceleration of gravity (9.81 m/s^2)

Mass

The amount of matter in an object, that amount of matter in an object will not change, no matter where you are.

Weight

The force on an object due to gravity

Observations the Geocentric Model of the Universe accounted for

The geocentric model accounted for the rising and setting of the Sun, along with the movement of the Moon and Celestial bodies, also Aristotle observed things fall down-theorized that things are falling toward the center of the Universe.

Prograde Motion

The normal motion of planets, planets normally move from West to East with respect to background stars.

Net Force

The overall force acting on an object, the net force represents the combined effect of all the individual forces put together.

How the Heliocentric Model accounted for Apparent Retrograde Motion

The planets orbit the Sun at different speeds, retrograde motion is natural and it occurs when the Earth appears to overtake or pass another planet in its orbit. More distant planets move more slowly across the sky than planets that are closer.

Speed

The rate at which an object covers distance.

Velocity

The speed and direction of an object.

Inverse Square Law

The strength of gravity between 2 objects decreases with the square of the distance between their centers. Gravitational force follows an inverse square law, doubling the distance between 2 objects weakens the force of gravity by a factor of 2^2 (2 squared) or 4. F (g) = G (Gravitational Constant) M1 (Mass of Object 1) x M2 (Mass of Object 2) / [Divided by] distance between the objects squared d^2

Orbital Energy

The sum of a planet's kinetic and gravitational potential energies, which always stay the same

Tidal Force

The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted by one body on another is not constant across its diameter. The nearest side is attracted more strongly than the farthest side. Thus, the tidal force is differential.

Newton's 1st Law of Motion

Things in motion tend to stay in motion. Things at rest tend to stay at rest. An object moves at a constant velocity unless a net force acts to change its speed or direction.