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.
