Gravity and the Orbits of Planets
This unit introduces gravity as a force and the nature of the orbits of planets. For content related to the wobbling of stars due to the orbits of planets, see the unit on Wobbling Stars and Barycenters.

Workshop Presentations

PowerPoint. Click to download the MS Powerpoint file (7.9 Mbytes).

HTML Click to view the presentation in html format.

PDF. Click to view or download the presentation in PDF (3.3 Mbytes).

Online Lecture Part 1. Click here to view a streaming lecture discussing our understanding of the nature of gravity. (~23 minutes).

Online Lecture Part 2. Click here to view a streaming lecture discussing Keplers's laws of planetary motion. (~19 minutes).

Classroom Activities

Gravity Meter Click to download this activity where students build a simple device that illustrates the direction of the Earth's gravitational pull. This activity may be done as a classroom activity with an embedded assessment of content knowledge.

Gravity Water Drop Click the link to view a simple activity that illustrates how the acceleration of water is the same for a stream of water and for an entire cup of water. This activity is from the Metro Family Magazine.

Falling for Gravity. Click to download this simple activity where students drop different objects to understand what causes them to fall at different rates. Many smart phones can record video in slow motion. Students can record their results using a video recorder for analysis. In addition, there are slow motion videos of objects being dropped for this activity below.

  • Click here to view a slow motion video of a golf ball and a baseball being dropped simultaneously for Part 1 of the activity. The two balls fall at the same rate even though they have very different weights.
  • Click here to view a slow motion video of two golf balls being dropped simutaneously. One is a regular golf ball and the other is the same size and shape as a golf ball but is hollow and has much less weight. Both balls dropped at nearly the same rate.
  • Click here to view a slow motion video of two golf balls being dropped simultaneously. One is a regular golf ball and the other is a wiffle-style golf ball with holes in its surface. The wiffle-style golf ball falls more slowly since the holes in its surface increase resistance with air.
  • Click here to view a slow motion video of a feather and a baseball being dropped simultaneously. The feather falls much more slowly due to air resistance.
  • Click here to view a slow motion video of two crumpled pieces of paper being dropped simultaneously for Part 2 of the activity. The paper that is crumpled tightly into a small ball falls more quickly than a piece of paper that is loosely crumpled into a larger ball. Although they have the same mass or weight, the larger ball of paper falls more slowly due to increased resistance with air.
  • Click here to view a slow motion video of a book and a sheet of paper being dropped separately in Part 3 of the activity. The paper drops much more slowly than the book due to resistance with air.
  • Click here to view a slow motion video of a book and a sheet of paper (laying on top) being dropped in Part 3 of the activity. The paper drops with the book since the book is pushing the air out of the way for the sheet of paper.


Comets: Very Eccentric Characters! This activity from the NASA Space Place teaches students about eliptical orbits. Using street chalk, students draw a large scale model of the orbits of the planets and comets. This was originally published in The Technology Teacher, April 1999, by the International Technology Education Association. PDF

Round and Round They Go! Click the link to view an activity online from NASA that models Kepler's third law where the orbital period of planets increases with the distance from the Sun. Note that this activity is an educational model and that the reason for the behavior of the model is not due to Kepler's third lawa but due to the conservation of angular momentum.

Why is the Neptunian Year So Long? In this activity, students calculate the orbital velocities of the planets to confirm Kepler's third law of planetary motion. Kepler's third law implies that the more distant a planet is from the Sun, the slower it moves in its orbit around the Sun.


Online Video and Media Resources (under construction)

What is Gravity? Minute Physics

What is Gravity? Veritasium

Misconceptions about Falling Objects. Veritasium

What Hits the Ground First? Veritasium

Best Film on Newton's Third Law. Ever. Veritasium

Why the Solar System Can Exist. Minute Physics

Is There Gravity in Space? Veritasium

The Difference Between Mass and Weigh. This short video explains the difference between the mass and the weight of an object. Veritasium.

Why are Astronauts Weightless? Veritasium

Gravity (Scientific Version of John Mayer's Gravity). Veritasium.

Why does the Moon Orbit Earth? Veritasium

Gravitational Wave Discovery! Evidence of Cosmic Inflation. Veritasium

Gravity Visualized.

Brian Cox visits the world's biggest vacuum chamber. BBC Two

Calculating Gravitational Attraction. Veritasium

Useful Websites

Your Weight on Other Planets.This online calculator determines your weight on other worlds in our solar system. Exploratorium.

Microgravity University. This page contains opportunities for undergraduate students to perform experiments during reduced gravity flights. NASA

NASA Grace Mission. This NASA mission uses satellites to measure variations in Earth's gravity to collect data about the Earth's environment.

NGSS Disciplinary Core Ideas

Grade Five
ESS1.B: Earth and the Solar System
The orbits of Earth around the sun and of the moon around Earth, together with the rotation of Earth about an axis between its North and South poles, cause observable patterns. These include day and night; daily changes in the length and direction of shadows; and different positions of the sun, moon, and stars at different times of the day, month, and year. (5-ESS1-2)

Middle School
ESS1.A: The Universe and Its Stars.
Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1)
ESS1.B: Earth and the Solar System.
The solar system consists of the sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the sun by its gravitational pull on them. (MS-ESS1-2), (MS-ESS1-3)
The solar system appears to have formed from a disk of dust and gas, drawn together by gravity. (MS-ESS1-2)
PS2.A: Forces and Motion
For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton's third law). (MS-PS2-1)
The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (MS-PS2-2)
PS2.B: Types of Interactions
Forces that act at a distance (electric, magnetic, and gravitational) can be explained by fields that extend through space and can be mapped by their effect on a test object (a charged object, or a ball, respectively). (MS-PS2-5)
Gravitational forces are always attractive. There is a gravitational force between any two masses, but it is very small except when one or both of the objects have large mass e.g., Earth and the sun. (MS-PS2-4)

High School
ESS1.B: Earth and the Solar System
Kepler's laws describe common features of the motions of orbiting objects, including their elliptical paths around the sun. Orbits may change due to the gravitational effects from, or collisions with, other objects in the solar system. (HS-ESS1-4)
PS2.A: Forces and Motion
Newton's second law accurately predicts changes in the motion of macroscopic objects. (HS-PS2-1)
Momentum is defined for a particular frame of reference; it is the mass times the velocity of the object. In any system, total momentum is always conserved. (HS-PS2-2)
If a system interacts with objects outside itself, the total momentum of the system can change; however, any such change is balanced by changes in the momentum of objects outside the system. (HS-PS2-2), (HS-PS2-3)
PS2.B: Types of Interactions
Newton's law of universal gravitation and Coulomb's law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects. (HS-PS2-4)
Forces at a distance are explained by fields (gravitational, electric, and magnetic) permeating space that can transfer energy through space. Magnets or electric currents cause magnetic fields; electric charges or changing magnetic fields cause electric fields. (HS-PS2-4), (HS-PS2-5)

Common Scientific Misconceptions

Gravity is related to magnetism.

Planets that are more distant from the Sun have less gravity.

An object's mass and weight are the same thing.

Heavier objects fall faster than lighter objects.

The orbits of planets around the Sun are circular.

The orbit of the Earth is a highly eccentric ellipse around the Sun.

The period of revolution for the outer planets is longer because they have further to orbit.

There is no gravity in outer space.

The Sun goes around the Earth.


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