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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=mXa
• 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.