Transcript Chapter 19.1 Part 2

Chapter 19.1 Part 2
Planetary Motion
• The solar system which is now 4.6 billion years old, is
not simply a collection of stationary planets and other
bodies around the sun.
• Each one moves according to strict physical laws. The
ways the Earth moves, for example, cause seasons and
even day and night.
Rotation and Revolution
• Rotation: The spinning motion of a body (Earth) on its
axis. By rotating on its axis ½ of the earth is facing the
sun at different times causing days and nights.
• In addition to rotating on its axis, the Earth also travels
around the sun in a path called an orbit.
• This motion around the sun along its orbit is called
revolution.
• The amount of time it takes for a single trip around the
sun is called a period of revolution.
• The period for the Earth to revolve around the sun is 365
days.
Planetary Orbits
Why do the planets continue to revolve around the sun?
Does something hold them in their orbit?
Why doesn’t gravity pull the planets pull toward the sun or
why they don’t fly off into space?
These questions were answered by scientist in the 1500s
and 1600s.
Kepler’s First Law of Motion
• Discovered that planets
did not move in a circle
around the sun, but in an
elongated circle called an
ellipse.
• An ellipse is a closed
curved in which the sum
of the distances from the
edge of the curve to two
points (Called foci) inside
the ellipse is always the
same
• Major axis: the maximum
length of an ellipse.
• Semimajor axis: half of the
distance and is used to give
the size of an ellipse.
• The semimajor axis of Earth’s
orbit is 150 million kilometers.
• It represents the average
distance between the Earth
and the sun and is called one
astronomical unit, or one AU.
Keplers Second Law of motion
• Kepler also discovered
that the planets seem to
move faster when they
are close to the sun amd
slower when they are
farther away.
• To keep the area of A,
equal to the area of B, the
planet must move farther
around its orbit in the
same amount of time.
Keplers Third Law of Motion
• Compares the period of a planet’s revolution with its
semimajor axis.
• By doing some mathematical calculations kepler was
able to calculate a planets distance from the sun from
knowing a planets period of revolution
• The data he used to make these laws came from his
former boss Tyco Brahe data.
Newton’s Law of Universal Gravitation
• Kepler wondered what caused the planets closest to the
sun to move faster than the planets farther away, but
never got an answer.
• This didn’t happen until Sir Isaac Newton was able to do
it with his ideas of gravity.
• Today we do not fully understand gravity. But Newton
was able to combine the work of earlier scientist to
explain how the force of attraction between matter works
• Law of universal gravitation states that the force of
gravity depends on the product of the masses of the
objects divided by the square of the distance between
them.
• If two objects are moved twice as far apart, the
gravitational attraction between them will decrease by a
factor of 2 x 2 = 4. If the objects are moved 10 times as
far apart, the gravitational attraction will decrease by a
factor of 10x10 = 100
Falling Down and Around
How Newton explained the orbit of the moon
• Newton explained that the pull of gravity causes the moon to fall
toward the Earth while its forward motion tries to have the moon fly
off into space. With the two forces somewhat balanced causes the
moons resulting path to be curved.
• This same principle holds true for all bodies in our solar system.