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Objects in Orbit

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Lesson Focus

Space Objects: Gravity and Motion

Physics

Students examine the relationship between the gravitational force and the motion of planets, moons, and comets.

Now You Know

After completing this tutorial, you will be able to complete the following:

• Define gravitational force as the attractive force between two masses, and the force that governs the motion of celestial objects in the solar system.
• Identify the motions of comets and planets around the sun.
• Identify the motion of the moon around the Earth together with the motion of the Earth around the sun.
• Describe the relationship between gravitational force and the distance between two objects.
• Describe the relationship between gravitational force and the mass of the object.

Everything You'll Have Covered

For centuries, people believed that the Earth remained stationary while all the celestial objects orbited around usit. However, the motion of the planets in the sky was difficult to explain. At different points throughout the year, the planets appeared to reverse direction and move backwards. This retrograde motion presented a problem for those that believed in a geocentric, or Earth-centered, universe.

During the 16th sixteenth century, Nicholas Copernicus observed the motions of the planets and came up withdeveloped the theory of a heliocentric, or sun-centered, solar system. He believed that Mercury, Venus, Earth, Mars, Jupiter, and Saturn orbited the sSun. (At the time, they had not yet discovered the other planets in the solar system.) One of the flaws in his theory was his assumption that the planets traveled in perfectly circular orbits. It wasn't until many years later that Johannes Kepler discovered that the planets move in elliptical orbits. Copernicus's theory initially was initially not accepted by most people.

Through the efforts of such scientists as Galileo, Kepler, and Newton, people finally began to accept the idea of a heliocentric solar system and understand the motion of celestial objects in the universe. Newton's work involving forces led him to think about the motion of the planets and why they move in a nearly circular path. Since the planets do not move in a straight line, there must be some force pulling on them. The force that governs the motion of the planets, moons, and other celestial objects in our solar system is known as the gravitational force.

Tutorial Details

 Approximate Time 20 Minutes Pre-requisite Concepts Students should be familiar with these terms: comets, gravity, moon, motion, planets in the solar system, and velocity. Students should also understand Newton’s three laws about forces. Course Physics Type of Tutorial Concept Development Key Vocabulary celestial, celestial objects, comet