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Motion and Gravity in Space
• Each planet spins on its axis. The spinning of a
body, such a planet, on its axis is called rotation.
• The orbit is the path that a body follows as it travels
around another body in space.
•
• A revolution is one complete trip along an orbit.
Kepler’s Three Laws of Motion
Johannes Kepler• discovered three laws of planetary motion:
1. Orbits of the planets are elliptical.
• An ellipse is an oval-shaped path.
Kepler’s 2nd Law
2. planet moves fastest when it is near
perihelion and slowest when it is near
aphelion
Perihelion – moment closest to sun
Aphelion – moment furthest from sun
Kepler’s Third Law
3. planets more distant from the sun
take longer to orbit the sun..
For example, Mercury, the
closest planet to the Sun,
takes just 88 Earth days to
complete an orbit. But
Neptune, the furthest out,
takes 164 Earth years to
The graph above shows the distances of the
planets from the Sun and the speed at which they
travel. The shape of the curve shows that the
further out a planet is the longer it takes to
orbit the Sun.
Kepler Video
http://youtu.be/dRT3m2Wzyh4
• Kepler did not understand what causes the planets
farther from the sun to move slower than the closer
planets.
• Sir Isaac Newton’s description of gravity provides
an answer.
Sir Isaac Newton
• Although others had theorized the
existence of gravitational force, Newton
was the first to formulate and test the law
of universal gravitation.
Universal Gravitation
1. Gravitational force decreases with distance.
2. The greater the mass of an object, the
greater is its gravitational force.
The direction of the pull of gravity is always
towards the center of the mass
If gravity is pulling the Earth into the center of the Sun,
why does the Earth move in a circle?
Why doesn't the Earth crash into the centre of the Sun?
If the forces acting on the Earth were balanced,
then the Earth would move through space in a straight line
at a constant velocity.
The Earth is trying to move past the Sun,
the Sun's gravity is pulling the Earth sideways.
Bill Nye - gravity
http://youtu.be/6f6maa9xPDM
• Orbits Falling Down and Around - Inertia is an
object’s resistance to change in speed or direction
until an outside force acts on the object.
1. Gravitational attraction keeps the planets in their
orbits. Inertia keeps the planets moving along
their orbits.
2. There are no other forces acting on the Earth,
no friction because the Earth is moving through empty space.
The Earth moves in a direction between the two arrows.
3. The attraction of gravity between the two masses (Sun and
Earth) stays the same as long as the distance between them
stays the same.
4. The Earth must move in a circle to remain the same distance
from the Sun.
5. The force which keeps an object moving in a circle
is called the centripetal force.
Weightlessness Video
http://youtu.be/d57C2drB_wc
Mass is given relative to the Earth.
Diameter is given in thousands of kilometres.
Orbit time is given in years.
Distance from the planet to the Sun is given in millions of kilometres.
The distance is an average value for the slightly elliptical orbit.
Orbital speed is given (approximately) in kilometres per second.
Notice that the orbital speed decreases as the distance from the Sun
increases
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
Mass 0·05 0·8 1·0 0·1 318 95 14·5 17 0·002
Diameter 4·9 12·0 12·8 6·8 143 120 51 49·5 2.3
Orbit
Time0·24 0·62 1·0 1·9 12 29·5 84 165 248
Distance 58 108 150 228 780 1430 2870 4500 5900
Orbital
Speed48 35 30 24 13 10 7 5·5 4·8
Retrograde Motion6. Retrograde motion is the apparent westward motion
of the planets with respect to the stars.
retrograde motion usually means motion
which is contrary to the rotation of the
primary, that is, the object which forms the
system's hub.
SOHO was launched in December 1995 by an
Atlas Centaur rocket and became operational in
March 1996. SOHO weighs about two tons and with
its solar panels extended stands about 25 feet
across. It was launched in December, 1995. SOHO
will continue operating well past the next solar
maximum in 2001.
SOHO was designed to answer the
following three fundamental
scientific questions about the Sun:• What is the structure and dynamics of the
solar interior?
• Why does the solar corona exist and how
is it heated to the extremely high
temperature
of about 1 000 000°C?
• Where is the solar wind produced and
how is it accelerated?
The Solar & Heliospheric Observatory, is a project
of international collaboration between ESA and
NASA to study the Sun from its deep core to the
outer corona and the solar wind.
SOHO IMAGES and VIDEOSCLICK TO VIEW
Hubble Space Telescope