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Physics

Students examine the relationship between gravitational force and the motion of celestial objects.

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

- Explain that planets travel around their star in nearly circular paths called orbits
- Explain that the factors affecting a planet’s orbit are the planet’s mass, position, and velocity.
- Explain that a planet moving in a nearly circular orbit travels at a greater velocity than when it is traveling in a more elliptical orbit.
- • Explain that gravitational force is proportional to the masses of the objects and inversely proportional to the distance squared.
- • Define gravitational force as the attractive force between two masses that holds planets in their orbits

For centuries, people believed that all celestial objects orbited the Earth. During the 1500s, Nicolaus Copernicus proposed a sun-centered solar system, where he suggested that all the planets orbit the sun. It took decades for people to start supporting this idea. Many believed that if the Earth were in motion, we would feel as if we were moving. The work of Galileo, Brahe, and Kepler helped to convince people that a sun-centered solar system was the most accurate model to depict our solar system.

The work of Sir Isaac Newton led to even more ideas about how celestial objects move. His three laws about forces led to a deeper understanding about this motion. He knew that an object will continue its straight-line motion as long as there are no unbalanced forces acting on the object. When looking at the planets, he realized that in order for the planets to move in a nearly circular orbit, there must be a force acting on them. The force acting on the planets and allowing them to move in their orbit is the gravitational force.

The equation for calculating gravitational force is: F = G × (mass1) × (mass2) / (distance between them)2. G is the universal gravitational constant, and has a value of 6.67 × 10-11 Nm2/kg2. This equation can be used to calculate the masses of celestial objects without having to visit them. According to this equation, force is directly proportional to each mass and inversely proportional to the distance between them squared. The distance is measured from the center of one object to the center of the other object. The relationship between gravitational force and distance is known as the inverse-square law. This relationship applies to various situations, such as radiation, light, and electrostatic force.

Approximate Time | 20 Minutes |

Pre-requisite Concepts | Students should be familiar with these terms: gravitational force, 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 |