Download Keplers-Laws

Unit 3 Lesson 2
Kepler’s Laws of Planetary
Motion
Geocentric model fails to account
for phases of the inner planets
Heliocentric model easily accounts
for phases of the inner planets
Kepler’s Laws
• What are the shapes and important properties of
the planetary orbits?
• How does the speed of a planet vary as it orbits
the sun?
• How does the period of a planet's orbit depend
on its distance from the Sun?
Tycho Brahe (1546-1601)
• Collected vast
amounts of
astronomical data
(positions of different
bodies at certain
times)
Johannes Kepler (1571-1630)
Used Tycho Brahe's
precise data on apparent
planet motions and relative
distances.

Deduced three laws of
planetary motion.

Took him the last 30 years
of his life.

Kepler’s First Law
• The orbits of the planets
are elliptical (not
circular) with the Sun at
one focus of the ellipse.
• There is one point called
aphelion, where the
object is farthest from the
sun.
• The other point is called
perihelion, where the
object is closest to sun
Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas
in equal times.
slower
faster
Translation: planets move faster when closer to the Sun.
Kepler's Third Law
The square of a planet's orbital period is proportional to the
cube of the planet’s average distance from the sun.
P2 years is proportional to a3 AU
or
P2 (orbital period in years) = a3 (average distance from the sun in A.U.)
1 A.U. = 1.5 x 108 km
Translation: The further the planet is from the
sun, the longer the period.
Sir Isaac Newton (1642-1727)
Newton’s laws are fundamental principles
that govern the motions of all astronomical
bodies!
He was the first scientist to explain
Kepler’s laws mathematically how the force
of gravity behaved using
Newton's First Law of Motion
Every object continues in a state of rest or a state of uniform
motion in a straight line unless acted on by a force.
● Inertia - resistance to change in motion of object - is related
to its mass.
(Demo- air track)
●
Newton's Second Law of Motion
When a force, F, acts on an object with a mass, m, it produces
an acceleration (a) equal to the force divided by the mass.
a=
F
m
or F = ma
Acceleration is a change in velocity or a change in direction of
velocity.
Newton’s laws classify objects as accelerating or nonaccelerating, not as moving or stationary.
Newton's Third Law of Motion
To every action there is an equal and opposite reaction.
Or, when one object exerts a force on a second object,
the second exerts an equal and opposite force on first.
(DEMO – Air Track, Jet Cart)
What force governs the motions of astronomical
objects, and what factors determine how strong the
force is?
Newton's Law of Universal Gravitation
All objects in the universe attract each other
through gravitational force. This force depends
upon the masses of these objects.
Gravity between objects increase as the masses of
the objects increase
Gravity decreases when distance is increased
between two objects
Your "weight" is just the gravitational force between the
Earth and you. On the moon your “weight” would be about
1/7 what it is on Earth.
Gravitational Force
• The gravitational force is
always attractive
• The strength of the
attraction decreases with
increasing distance
How does gravity affect planetary
motion?
How does gravity affect planetary
motion?


The inward force that
causes an object to move
in a circular path is called
Centripetal force
Gravity is what is keeping
the planets in orbit around
the sun.