Download LESSON 10

LESSON 10:
Gravity and Orbits
Modified from
http://www.psci.unt.edu/~roberts/Pics/Fall/Chapter%20
05.ppt#295,6,Isaac Newton (1643 - 1727)
http://www.csub.edu/Physics/phys110/Ch3.ppt#257,2,S
ection 3.1 Inertia
The Universal Law of Gravity
• Any two bodies are attracting each
other through gravitation, with a force
proportional to the product of their
masses and inversely proportional to
the square of their distance:
F=-G
Mm
r2
(G is the Universal constant of gravity.)
Understanding Orbital Motion
The universal law of gravity allows us to
understand orbital motion of planets and
moons:
Example:
• Earth and moon attract each other through gravitation.
• Since Earth is much more
massive than the moon, the moon’s
effect on Earth is small.
• Earth’s gravitational force
constantly accelerates the moon
towards Earth.
• This acceleration is constantly
changing the moon’s direction of
motion, holding it on its almost
circular orbit.
Moon
Earth
Orbital Motion and Gravity
• A force is any kind of push or pull exerted by
one object on another.
• Besides contact, friction, electric, magnetic,
elastic, pressure, etc. forces, Newton said
that objects also exert a gravitational force
on each other.
• The force of gravity causes all bodies to
attract all other bodies.
• Gravity, coupled with laws of motion, enabled
Newton to explain exactly how orbits work.
The Moon and the Earth
• the Earth will exert a
gravitational forces on
the Moon pulling the
Moon towards the
Earth.
• So, what holds the
Moon up?
• Why doesn’t it fall down
like if you drop a rock?
Moon
Gravitational
Force
Earth
The Moon and the Earth
• If the Moon was just sitting up
there, it would fall straight down
onto the Earth.
• But the Moon is moving,
“sideways” at a pretty high
speed.
• The Moon does fall down, but it
is moving sideways at the same
time.
• Just like if I throw a baseball, it
moves across the room while
falling downwards.
Moon’s
velocity
Moon
Gravitational
Force
Earth
The Moon and the Earth (3)
• Without gravity, the Moon would
move in a straight line, flying
away from the Earth.
• The orbit is a balance between
the natural straight-line motion
and the attractive pull towards
the Earth.
• The Moon is always falling
towards the Earth but it is also
always shooting away from the
Earth.
Path followed by the Moon
Moon’s
velocity
Moon
Gravitational
Force
Earth
The Sun and Planets
• Orbits of planets around the Sun work just
like the Moon’s orbit around the Earth.
• If the gravitational force and orbital speed are
exactly balanced, a planet will orbit in a
perfect circle.
• If the planet’s speed is a little faster or slower,
a non-circular orbit results.
• If the planet’s speed is much too fast or slow
it may escape the Sun altogether or fall into
the Sun.
Orbital Motion
In order to stay on a
closed orbit, an object
has to be within a
certain range of
velocities:
Too slow => Object falls
back down to Earth
Too fast => Object escapes
Earth’s gravity