Chapter 6 Lesson 3:
Gravity and Orbit
What is gravity?
• Gravity is the force of attraction, or pull
between any two objects
• Each planet is drawn toward the sun by gravity
• Gravity is affected by the total mass, and
distance between the two objects
• An object’s mass always stays the same
• Your weight depends on mass, as well the
force of gravity wherever you are located
Weight and Gravity
Mass on Earth = 100
Weight on Earth = 100lbs
Earth’s gravity =1
Mass on Jupiter = 100
Weight on Jupiter = 236lbs
Jupiter’s gravity = 2.36
Mass on Moon = 100
Weight on Moon = 16.5lbs
Moon’s gravity = .165
What keeps objects in orbit?
• An orbit is a path an object takes around another
• Planets orbit the sun
• Moons orbit their planets
Inertia
•Inertia is the tendency of a moving
object to keep moving in a straight line.
•All objects have the property of inertia.
•Inertia alone would cause Earth to move in a straight line.
•Gravity and inertia together make Earth follow its path of orbiting
the sun.
•The effect of gravity and inertia make the planets move in a circular
orbit called an ellipse.
What causes the tides?
• Both the pull of gravity from the moon and
sun affect Earth
• Moon is closer to Earth than the sun
• Sun’s pull of gravity is less than the moon’s
pull of gravity
• Tide is the rise and fall of the ocean’s surface
• Most oceans have two high tides and two low
tides
• As the moon orbits around the earth, its gravity
pulls water on the side nearest to it away from
the land. This causes low tide
• About twice a month, the sun and the moon line
up and both pull in the same direction. This
causes higher tides and lower tides called “spring
tides”
• Neap tides are tides with the smallest range
which occur between spring tides, caused when
the sun and moon pull in different directions
Review Questions
• Jupiter is about 320 times larger than Earth, but its gravity is only 2.36
times larger than Earth’s gravity. What might cause this difference?
• Suppose you have a dog that weighs 50lbs. How much would it weigh on a
planet that had 3.45 times Earth’s gravity?
• Why dies food float in the space vehicle of the astronauts don’t hold onto
it?
• Why do stronger tides occur when the sun and moon are lined up?
• Why do spring and neap tides occur twice a month?
• How is the gravity between two objects affected by their masses?
• The rise and fall of the ocean’s surface is the _____________.
• How are mass and weight different?
• What force keeps earth in its orbit around the sun?
• What tide occurs twice a month when the sun and moon pull in different
directions?