Keplers Laws

Kepler's Laws

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Learning Objectives

• Describe the roles of gravity and tangential velocity in maintaining a planetary orbit.

• Explain Kepler's three laws of planetary motion.

• Define key orbital terms including ellipse, perihelion, and aphelion.

• Relate a planet's orbital speed and period to its distance from the Sun.

Key Vocabulary

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Gravity

The force that pulls objects together and keeps planets in their orbits around the Sun.

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Ellipse

An oval shape with two focal points, representing the true path of a planet's orbit around the Sun.

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Perihelion

The point in a planet's orbit where it is closest to the Sun, moving at its fastest speed.

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Aphelion

The point in a planet's orbit where it is farthest from the Sun, moving at its slowest speed.

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Orbital Period

The time it takes for a planet to complete one full orbit around the Sun, also known as its year.

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What Keeps a Planet in Orbit?

• Gravity acts as the centripetal force, pulling a planet toward the Sun.

• The planet's tangential velocity keeps it moving forward along its orbital path.

• Gravity's strength depends on objects' mass and the distance between them.

• A gravity well illustrates how massive objects have stronger gravitational pulls.

Planets orbit the sun in an elliptical path.

​First Law

Kepler’s Laws of Orbital Motion | How Things Fly

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Multiple Choice

What does Kepler's 1st Law state?

• all planets move in elliptical shaped orbits around the sun

• all ellipses have one foci

• the area that a planet revolves around the sun is equal

• the speed that a planet revolves around the sun changes

Planets move faster when they are closer to the Sun and slower when they are farther away.

​Second Law

Kepler’s Laws of Orbital Motion | How Things Fly

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Kepler's Second Law

This law provides a relationship between the distance of a planet from the Sun and the time it takes to complete one orbit. Planets that are farther from the Sun have longer orbital periods.

​Third Law

Kepler’s Laws of Orbital Motion | How Things Fly

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​Keplers Third Law

On your own, answer the remaining 6 Questions.

Multiple Choice

When do gravitational forces exist between two objects?

• They only exist between microscopic objects.

• They are always present between any two objects.

• Gravitational forces only act on objects falling on Earth.

• They only exist for objects in space.

Multiple Choice

What effect does increasing the distance between the two objects do to the gravitational force?

• The distance between two objects has no effect on the gravitational force

• It increases

• It stays the same

• It decreases

Multiple Choice

A planet moves __________ when it is farthest from the sun.

• faster
• slower

Multiple Choice

Where is the planet moving faster?

• Position A to B
• Position B to C
• Position H to I
• Position I to J

Multiple Choice

Based on the table, which planet in our solar system will have the fastest orbital speed?

• Venus

• Earth

• Saturn

• Mercury

Multiple Choice

You discover a new planet in our solar system. You measure it's orbital period to be 246 years.

Where is this planet likely to be located based on this information?

• Closer to the Sun than Mercury.

• Located between Earth and Jupiter.

• Located between Uranus and Neptune.

• Located beyond Neptune.