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?