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Acceleration - rate of change of velocity (speed or direction), occurs any time an unbalanced force is applied

Aphelion point of a planet’s orbit when it is at its greatest distance from the Sun

Astronomical Unit average distance from the Earth to the Sun

Center of Mass average position in space of a group of massive bodies

Centripetal Force force that pulls an object into a curved path

Eccentricity - measure of the flatness of an ellipse

Ellipse elongated circle shape of orbits

Escape Velocity speed required for an object to escape gravitational pull

Focus point an object orbits (elliptically)

Force a push or pull primarily gravity

Gravitational Constant - constant of proportionality in the law of Universal Gravitation

Gravitational Field field created by object with mass, determines its influence on other objects

Gravitational Force the attractive force a massive object has on all other massive objects

Gravitational Slingshot use of gravitational pull of massive object to increase velocity of a passing satellite

Heliocentric - solar system model with Sun at the center

Hyperbola - geometric shape of an unbound orbit

Inertia - tendency of objects to resist acceleration, Newton’s 1st Law

Inverse-Square Law strength of field decreases with square of the distance

Kepler’s Laws of Planetary Motion three laws which summarize the motion of the planets

Major axis - the long axis of an ellipse

Momentum - a measure of the inertia of a body, mass X velocity

Newtonian Mechanics - basic laws of motion, postulated by Newton

Parabola - shape of orbit with just enough energy to escape gravitational field

Perihelion - closest approach to Sun in an orbit

Radar - radio detection and ranging, waves are bounced off an object and timed to find distance

Renaissance - historical period with a rebirth in scientific inquiry

Semi-major Axis average distance of a planet from the Sun

Unbound - an orbit in which the satellite will never return to the object it orbits

Weight - a measure of the gravitational pull between two objects

1. Why did the Ptolemaic picture of the universe survive for so long? Traditionalists were reluctant to give up the belief that the Earth was the center of the universe.

2. What was the great contribution of Copernicus? Placing the Sun at the center of the solar system.

3. What was Copernicus’ major motivation for introducing the heliocentric model of the universe? Copernicus wanted to simplify the view of the solar system.

4. When were Copernicus ideas finally accepted? Late 1600s to early 1700s.

5. What is the Copernican principle? The Earth is not special in a cosmological sense.

6. What discoveries of Galileo helped confirm the views of Copernicus? He found moons that orbited Jupiter. He saw phases of the planet Venus.

7. What was Kepler’s contribution to astronomy? Kepler’s laws of planetary motion. 1. planets’ orbits are elliptical 2. planets sweep out equal areas of the ellipse in equal periods of time (move faster closer to the Sun) 3. square of a planet’s orbital period is proportional to the cube of its distance from the Sun

8. What did Kepler use as the basis for his ideas? The observations of Tycho Brahe.

9. Do Kepler’s laws let us specify the actual distances between orbits of the planets and thereby the scale of the solar system? Kepler’s laws tell relative distances, not actual distances.

10. How can radar be used to find the distance between Earth and Venus? INFERIOR CONJUNCTION SUPERIOR CONJUNCTION The sum of the two distances divided by two is one astronomical unit.

11. What is inertia? Give one example. Objects resist acceleration. Seat belts keep you from slamming through the windshield.

12. Consider the gravitational interaction between Earth and a baseball thrown into the air. If the force of gravity is acting on both of them, why does the baseball move toward Earth and not Earth toward the baseball?

F = m X a F on each object is equal, but m for the Earth is huge while m for the baseball is very small. That makes a for the Earth very small while a for the baseball is huge.

13. Why would a baseball go higher if it were thrown upward from the surface of the Moon? The mass of the Moon is less than Earth’s so the acceleration due to gravity is less. Therefore, it would take the ball longer to lose its velocity.

14. Why can the motion of a planet around the Sun be described as a tug-of-war? The Sun pulls on the planet and the planet pulls on the Sun. This makes them each orbit a common center of mass.

15. Do planets orbit the center of mass of the Sun? No, a planet and the Sun orbit a common center of mass.

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