How Orbits Are Influenced by Gravity & Energy

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  • 0:02 Kinetic and Potential Energy
  • 0:40 Orbits, Speed, and Energy
  • 2:05 A Case Example
  • 3:32 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explain how potential energy, kinetic energy, speed, and gravity are linked to the orbital motion of planets in our solar system. It will also define aphelion and perihelion.

Kinetic and Potential Energy

Have you learned about the famous kinetic and potential energy yet? If not, then very briefly put, kinetic energy is the energy of motion, and potential energy is stored energy.

A good example of this is a yo-yo. When you've got the yo-yo in your hand, before it starts to fall towards the ground, it's all potential energy. As the yo-yo falls towards the ground, the potential energy begins to change to kinetic energy since the yo-yo is now in motion. When the yo-yo gets all the way to the bottom, it has maximum kinetic energy. Kinetic energy and potential energy are really important in this lesson's discussion of orbits, speed, and energy.

Orbits, Speed, and Energy

In our solar system, planets move in a slightly elliptical orbit around the sun. The point at which a celestial body is farthest from the sun along its orbit is called its aphelion, and the point at which a celestial body is closest to the sun along its orbit is called its perihelion.

As an object's speed increases, its kinetic energy increases as well. This means that the kinetic energy of a planet is highest when it's at perihelion, since it moves faster when it's closer to the sun. On the flip side, a planet's kinetic energy will be smallest when it's at aphelion, which is where it's moving slowest along its orbit.

Now, let's go back to our yo-yo example for a sec. The farther the distance the yo-yo has to fall, the more potential energy it has prior to falling and the more kinetic energy it will pick up as it falls towards the ground.

The gravitational potential energy a celestial object has also increases with distance away from the sun. Gravitational potential energy is stored energy subject to gravitational attraction by another body.

As a planet nears the sun, its gravitational energy decreases, but its kinetic energy increases. However, the sum of the gravitational potential energy and kinetic energy is always constant along its orbit.

A Case Example

Let's try and understand this process through an example. On the screen is a picture of the sun and a planet moving in an elliptical orbit. All the way left, at the aphelion, is a planet moving very slowly. It's moving so slowly that the force of gravity pulls it inwards and the planet 'falls' towards the sun.

A planet moves slowest at the aphelion
Diagram of sun and orbital planet

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