Video Transcript
Complete the following
sentence. For a planet in an elliptical
orbit, when the planet is farther away from 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 farthest from the star. After moving a great distance away
from the star, it’s now gradually changing direction to move back towards the
star. Once it rounds this back curve, it
then has a big stretch of distance to pick up speed as it approaches the star and
the gravitational force on the planet increases. When the planet is closest to the
star, it’s moving at its fastest attainable speed. But then, it must continue on its
path, moving away from the star once again. The gravitational force is now
acting against its direction of motion, causing the planet to slow down. Thus, once the planet reaches the
farthest point from the star, it’s moving at its slowest speed in the orbit, and
we’re back to where we started.
So, we’ve seen that the planet
moves fastest when it’s closest to the star and slowest when it’s farthest from the
star. We can therefore say that in an
elliptical orbit, the planet’s orbital speed is slower the farther the planet is
from the star. Answer choice (A) is correct
then. Completing the sentence, we have
that for a planet in an elliptical orbit, when the planet is farther away from its
host star, its orbital speed is slower.
