Question Video: Describing the Speed of a Planet in an Elliptical Orbit When It Is Close to Its Host Star | Nagwa Question Video: Describing the Speed of a Planet in an Elliptical Orbit When It Is Close to Its Host Star | Nagwa

# Question Video: Describing the Speed of a Planet in an Elliptical Orbit When It Is Close to Its Host Star Physics • First Year of Secondary School

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Complete the following sentence: For a planet in an elliptical orbit, when the planet is closer to its host star, its orbital speed is ＿.

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### Video Transcript

Complete the following sentence. For a planet in an elliptical orbit, when the planet is closer to its host star, its orbital speed is blank. Is it (A) slower or (B) faster?

Let’s begin by going over some basic information about a planet in elliptical orbit around a star. The shape of the planet’s orbital path is an ellipse, and the star it orbits is located at one focus of the ellipse. Thus, as the planet orbits, its distance from the star is constantly changing, which in turn affects its orbital speed. To better understand why, we need to remember that the gravitational force between two objects depends on the distance between them. The force is strongest when the objects are closest together and weakest when they’re farthest apart.

So, imagine the planet at this point in its orbit, when it’s closest to the star. After moving a great distance towards the star, it’s now quickly changing direction to move back away from the star. Once it rounds this curve, the planet then has a big stretch of distance to cover over which it slows down as the gravitational force is acting against its direction of motion. And when the planet is farthest from the star, it’s moving at the slowest speed in its orbit. But it then continues on its path, now moving towards the star once again. Thus, the gravitational force is increasing and it’s working with, rather than against, the planet’s direction of motion, causing the planet to pick up speed. So once the planet reaches the closest point to the star, it’s moving at its fastest attainable speed, and we’re back to where we started.

So we’ve seen that the planet moves slowest when it’s farthest from the star and fastest when it’s closest to the star. We can therefore say that in an elliptical orbit, the planet’s orbital speed is faster the closer the planet is to the star. Answer choice (B) is correct then. Completing the sentence, we have that for a planet in an elliptical orbit, when the planet is closer to its host star, its orbital speed is faster.

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