Question Video: Determining the Orbital Speed of Planets | Nagwa Question Video: Determining the Orbital Speed of Planets | Nagwa

# Question Video: Determining the Orbital Speed of Planets Physics • First Year of Secondary School

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Which planet in the solar system moves fastest along its orbit around the Sun?

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### Video Transcript

Which planet in the solar system moves fastest along its orbit around the Sun?

In our solar system, we have the Sun at the center and eight full-sized planets that orbit around the Sun. Starting closest to the Sun, these eight planets are Mercury, then Venus, then Earth, then Mars, then Jupiter, then Saturn, then Uranus, and then, finally, Neptune. We need to work out which of these planets moves the fastest around the Sun.

Normally, when we think about the speed of a planet, we would use the formula 𝑆 equals 2𝜋𝑟 divided by 𝑇, where 𝑆 is the orbital speed, 𝑟 is the orbital radius, and 𝑇 is the orbital period of the planet. However, we haven’t been given any information whatsoever about each planet’s orbital period. That means we can’t use this equation to help us with this problem.

Instead, 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, the Sun, we’ll only need to consider their distances from the Sun in order to compare their speeds.

We can say that these two quantities, the planet’s speed and the planet’s distance from the Sun vary inversely, because as one quantity increases, the other quantity must decrease and vice versa. Because of this, a planet that’s closer to the Sun moves faster around the star. And if a planet is further away from the Sun, it moves slower around the star.

If we look back at our drawing, we can see that Mercury is the closest planet to the Sun. This means that Mercury will be the fastest planet to orbit around the Sun. And this is our answer.

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