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Kepler's laws quiz: how do planets move around the Sun?

Kepler's laws model planetary orbits as ellipses with the Sun at one focus and describe how orbital speed changes along the path. The quiz covers elliptical orbits, perihelion and aphelion, equal-area sweeping, period relationships, and the heliocentric model. See how you do on the questions.

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Question 1

What does apparent retrograde motion describe?

Retrograde motion is apparent motion on the sky.

Source excerpt
This apparent backward movement, which creates a loop in the sky, is called Retrograde Motion.

From the Knowverse course text.

Question 2

What is the central organizing body in a heliocentric model?

Heliocentric means Sun-centered.

Source excerpt
No, it took over 100 years for the Heliocentric model to be widely accepted. Many people found it hard to believe the Earth was moving, and it contradicted many long-held religious and philosophical beliefs of the time.

From the Knowverse course text.

Question 3

What shape does Kepler's first law give a planetary orbit?

The ideal Keplerian orbit is elliptical.

Source excerpt
Kepler's First Law specifically applies this geometry to the heavens, stating that planets move in elliptical orbits.

From the Knowverse course text.

Question 4

Where is the Sun in Kepler's elliptical-orbit model?

The Sun occupies one focus of the ellipse.

Source excerpt
Kepler used math to work backward from the observed positions of Mars. He found that the only way to match the planet's changing speed and position was to use an ellipse and place the Sun at one focus. The second focus is a mathematical requirement for the shape to exist, even if nothing is physically there.

From the Knowverse course text.

Question 5

What is perihelion?

Perihelion names the closest orbital distance from the Sun.

Source excerpt
The specific moment when a planet is at its minimum distance from the Sun is called Perihelion.

From the Knowverse course text.

Question 6

What does Kepler's second law say?

The law concerns swept area, allowing orbital speed to vary.

Source excerpt
This is also known as the Law of Equal Areas, and it describes how a planet 'sweeps' through space.

From the Knowverse course text.

Question 7

Where does a planet move fastest in a Keplerian elliptical orbit?

The planet moves faster near the Sun.

Source excerpt
Yes. According to Kepler's Second Law, a planet moves fastest when it is at Perihelion (closest) and slowest when it is at Aphelion (farthest). This will be explored in the next chapter.

From the Knowverse course text.

Question 8

For planets around the same Sun, period squared is proportional to what?

Kepler's third law connects period and orbital size.

Source excerpt
The law states that the square of a planet's period is proportional to the cube of its average distance from the Sun.

From the Knowverse course text.

Question 9

For fixed masses, what happens to gravitational force when separation doubles?

The force follows an inverse-square dependence on separation.

Source excerpt
This specific relationship is known as the Inverse Square Law.

From the Knowverse course text.

Question 10

How do mass and weight differ?

Changing gravitational conditions changes weight without changing the object's mass.

Source excerpt
This is because the truck has more mass, and more mass always means more inertia.

From the Knowverse course text.

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Orbits and changing speed

A planet can appear to reverse direction against the background stars even while continuing along its orbit. This apparent retrograde motion helped motivate competing planetary models. The heliocentric account describes planets orbiting the Sun, with their different orbital speeds changing how their motions appear from a moving Earth.

Kepler's first law models a planetary orbit as an ellipse with the Sun at one focus. Perihelion is the closest point to the Sun, and aphelion is the farthest. The second law says the line joining planet and Sun sweeps equal areas in equal times, so speed varies around an elliptical orbit.

Source excerpt
Chapter: Kepler's First Law

From the Knowverse course text.

The Planetary View — image from Wikimedia Commons
Solar system · NASA/JPL · Public domain

Periods and gravitational force

For planets orbiting the Sun, Kepler's third law relates the square of orbital period to the cube of the orbit's semimajor axis. More distant orbits have longer periods. The familiar solar-system form uses years and astronomical units. Comparing different central masses requires accounting for those masses as well.

Newton's law describes gravitational attraction between masses. Increasing either mass increases the force, while increasing their separation weakens it according to an inverse-square relationship. Weight is a gravitational force and can change with location. Mass is different. Orbital motion combines sideways velocity with gravitational acceleration toward the central body.

Source excerpt
This specific relationship is known as the Inverse Square Law.

From the Knowverse course text.

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Sources

Sources: The Laws of Motion. Each question or step includes an excerpt from the course.