•We know that planets travel around the Sun.
Earth travels around the sun every 365 ¼
days.
•But what keeps the planets in this constant
motion around the Sun?
•Answer: Gravity
and Inertia!
What is Gravity?
Gravity is a force that attracts all
objects toward each other.
• Both the Sun and the planets have gravity
that pulls on one another.
Gravity
The strength of gravity between two objects
depends on two factors:
•1. masses of the objects
(If mass increases, force of gravity increases)
•2. distance between the objects
(If distance increases, force of gravity decreases)
Remember Newton’s First Law of Motion?
•The law of INERTIA
•An object in motion stays in
motion, with the same speed and
in the same direction, unless acted
upon by an unbalanced force.
Why do planets revolve around the Sun?
•Inertia and gravity combine to keep
planets revolving around the sun.
•If there were no gravity, inertia would
cause the planets to move in a straight
line.
•If only gravity existed, the planets
would be pulled into the sun.
Question:
What two factors act
together to keep the
Earth in orbit around the
sun?
Question: What two factors act together to keep
the Earth in orbit around the sun?
Answer:
•Inertia and gravity
Question: what planet would have the
fastest orbit around the Sun?
Question: what planet would have the
fastest orbit around the Sun?
Answer:
The
planet
closest to
the Sun,
Mercury
Question
•Describe what would happen to the
planets if the sun’s gravity:
•Increased
•Decreased
Draw a diagram to support your
answer.
Answer:
If the Sun’s gravity
increased, the planets
would begin to move
closer to the Sun.
If the Sun’s gravity
decreased, the planets
would begin to move
farther away from the
Sun.