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