Question Video: Understanding Orbital Speed Physics • 9th Grade

The diagram shows four planets orbiting a star. All of the planets have circular orbits. Which planet is moving the fastest? Which planet is moving the slowest? Is planet 3 moving faster than, slower than, or at the same speed as planet 2?

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Video Transcript

The diagram shows four planets orbiting a star. All of the planets have circular orbits. Which planet is moving the fastest? Which planet is moving the slowest? Is planet three moving faster than, slower than, or at the same speed as planet two?

Here we want to compare the orbital speeds of four planets in orbit around a star. Because they all follow nice circular orbits around the star, we’re assuming that the planets have negligible gravitational effects on one another and that each planet moves at a constant speed. Now we should recall that when a planet orbits a star, the planet’s speed is determined by its distance away from the star and the mass of the star.

Here, because all the planets are orbiting the same star, we’ll only need to consider their distances from the star in order to compare their speeds. We can say that these two quantities vary inversely because as one increases, the other must decrease and vice versa. Because of this, a planet that’s closer to a star moves faster around the star. And if a planet is farther away from a star, it moves slower around the star.

The first part of this question asks us which planet is moving the fastest. So, we can think about this as which planet is closest to the star. Looking at the diagram, planet one is closest to the star, so we know that planet one is moving the fastest. The next part of this question asks which planet is moving the slowest. The farther a planet is from a star, the slower it moves. And the diagram shows us that planet four is farthest. Thus, we know planet four is moving the slowest.

Finally, the last part of this question asks us to compare the speeds of planet three and planet two. We can see from the diagram that planet three is farther from the star than planet two is. And we’ve already established that the farther away from a star a planet is, the slower it moves. Therefore, because planet three is farther from the star than planet two, we know that planet three is moving slower than planet two.

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