A gravity well is one of the most engaging educational tools I have ever used. They appeal to students of all ages. Gravity wells can be used to teach physics and Earth science topics like orbital motion, gravitational theory, and solar system concepts. Physics and physical science students can apply their knowledge about motion to what they observe on the gravity well. Gravity wells can be simple DIY versions, elaborate home-made constructions, or store-bought kits like the one sold by Arbor Scientific. In this blog I will discuss specific demonstrations and activities that can be done by teachers and students.
Setting the Stage: Mass Curves Spacetime
I start my gravity well demonstration with 2 large metal spheres, the Colliding Spheres sold by Arbor are ideal, bocce balls work too. I explain that the two-dimensional surface of the gravity well is an analogy with 4 dimensional spacetime. When mass occupies an area of spacetime, it causes it to curve. I place the sphere on the gravity well and the fabric curves. A two-dimensional surface curving into a third dimension is much easier to understand than the curving of 4-dimensional spacetime! Objects detect this curvature and respond to it. I hold the sphere and place the other sphere about 30 cm away. I ask the students to predict what will happen when I release both spheres. Some students are surprised when both spheres move toward each other and collide. They are used to the behavior of objects in the gravitational field of one very large object, the Earth. When they release an object, it falls to the Earth. The Earth "falls up" to meet a falling object, but this effect is negligible because of the large difference in mass.
Introducing a Central Mass
I remove the two spheres and place a large object in the center of the gravity well. I use a 2 kg mass, but experiment with your setup to see what works best. Point out that the larger mass causes the fabric to curve more and the curve is steeper closer to the mass. This is true for the curvature of spacetime by matter too. I hold a marble on the gravity well and ask students to predict what will happen when I release it from rest. Few are surprised when it falls toward the large mass, collides, and comes to rest in the center. I show that anywhere I place the marble on the gravity well, it will fall in. This means the gravitational force is present everywhere and does not go away far from the large mass.
Demonstrating Orbital Motion
I repeat giving the marble a sideways push. It still falls toward the large mass but doesn't collide. The marble "falls" back up, then down, tracing a roughly elliptical shape. The marble is orbiting the large mass. I give the marble a larger push so that it traces a roughly circular shape. The marble quickly loses energy and ends up in the center. This is caused by rolling resistance. The Earth doesn't slow down from rolling resistance in its orbit, but it does emit gravitational waves and spiral in closer to the Sun. Reassure students that this is a very small effect. The Sun will turn into a red giant and engulf the Earth before the Earth falls into it!
I then demonstrate the properties of orbits by repeatedly rolling marbles and asking students questions as they observe. These are the concepts that students can develop from their observations:
Circular Orbits
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Objects in a circular orbit have a constant speed but constantly changing velocity
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Objects in a circular orbit take more time to complete an orbit than one closer in
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The gravitational force is perpendicular to the velocity for objects in circular orbits
Elliptical Orbits
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Objects in elliptical orbits speed up when getting closer and slow down when getting further from the mass
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Objects in elliptical orbits have the greatest speed when closest to the mass and the least speed when farthest from the mass
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Objects in elliptical orbits have the greatest kinetic energy and linear momentum when closest to the mass and the least when farthest from the mass
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The angle between the gravitational force and the velocity is constantly changing for objects in elliptical orbits
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The gravitational force on an object in an elliptical orbit increases as it gets closer to the mass and decreases at it gets further away
General Principles
- The greater the curvature of the fabric (spacetime), the faster objects move.
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Gravitational force does not disappear if you move away from a planet or star.
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All objects bend spacetime; even small objects like the marbles can clump together as they roll
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An orbiting clump of marbles will stretch out as they approach the mass (tides) and eventually be separated (tidal disruption)
Favorite Gravity Well Activities
One of the best ways to deepen student understanding of gravity is to let them explore with a gravity well. Beyond demonstrations, structured activities give students a chance to discover patterns, ask their own questions, and connect abstract ideas to physical motion. The following are some of my favorite classroom-tested activities that use the gravity well to model some big ideas in astronomy and physics. Each activity includes a short description, procedures, and the key concepts it supports.
Activity 1: Order from Chaos in the Classroom
Grade Levels: MS 6–8, HS 9–12
Purpose: Have your students simulate the creation of the solar system with marbles. They will be amazed to see an orderly solar system form out of apparent chaos. Will a second trial have the same result? They will want to find out!
Procedure:
- Place a heavy steel ball (~1 kg) in the center of the gravity well.
- Students practice rolling 1–2 marbles in a circular path near the edge.
- Distribute 5–6 marbles per student, two colors (but only one color per student), with ~20% excess of one color.
- On a signal, students roll marbles in assigned directions (e.g. red clockwise, green counterclockwise).
- Observe collisions and interactions; over time, only marbles of one color settle into the same orbit direction.
Key Concepts:
- The solar system formed from collisions and gravitational interactions in the solar nebula. There were many collisions resulting in material combining, falling into the center and being ejected.
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Remaining material naturally settled into consistent orbital directions over time.
NGSS: MS-ESS1-2, HS-ESS1-4, HS-ESS1-6
Activity 2: Pulled Out of Shape
Grade Levels: MS 6–8, HS 9–12
Purpose: Tides are an important phenomenon but are hard to understand. Simulate tidal forces using marbles to show students how tides form on both sides of a planet due to the difference in gravitational force. Observe how strong tidal forces can disrupt the planet as it gets close to its star.
Procedure:
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Place a heavy steel ball (~1 kg) in the center of the gravity well.
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Hold 4–8 marbles as a clump near the outer edge and roll gently in a tangential direction to create near-circular motion. It is easier to keep the marbles together with an overturned cup or roll of tape to keep them clumped while rolling.
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Observe how the clump stretches as it approaches the central mass, then breaks apart.
Key Concepts:
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Tides result from differences in gravitational force across an object.
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Strong tidal forces can disrupt and even destroy objects (Roche limit).
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Historical relevance: Saturn’s rings, early Earth-Moon system.
NGSS: MS-ESS1-2, ESS1B
Activity 3: More than One Way to Go Around a Star
Grade Levels: MS 6–8, HS 9–12
Purpose: Students find objects in space fascinating, but how do you bring them into the classroom? The gravity well allows students to model a solar system and the orbital motion of planets. Orbital shapes and properties can be inferred by observing the motions of marble “planets” around a central “star”.
Procedure:
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Place a heavy steel ball (~1 kg) at the center.
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Release marbles from rest to observe straight-line attraction to the center.
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Give marbles slight tangential velocity to create elliptical orbits; increase tangential speed for near-circular orbits.
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Vary orbit radius and central mass to observe changes in speed and orbit period.
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Optional: Launch marbles at angles between tangential and radial to simulate parabolic/escape trajectories.
Key Concepts:
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Orbits form when a central force acts on a moving object.
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Circular orbits: constant speed; smaller radius = faster orbit, shorter period.
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Elliptical orbits: speed greatest at closest approach, slowest at farthest point.
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Greater central mass increases orbital speed at the same radius.
NGSS: MS-ESS1-2, HS-ESS1-4
Conclusion
A gravity well provides a simple, hands-on way to explore gravitational concepts and orbital motion. Demonstrations help students visualize how mass curves space and influences motion, while structured activities let them test predictions, observe patterns, and connect abstract ideas to physical behavior. Large gravity wells make these effects easier to see, but smaller tabletop setups can also support meaningful exploration. Pairing the activity with guiding questions or assignments helps keep students engaged and focused, making the gravity well a versatile tool for teaching physics and Earth science concepts.
About the Author
Dan Burns taught Physics, AP Physics, and Earth/Space Science at Los Gatos High School for 27 years. Prior to teaching, Dan was an aerospace engineer at Lockheed. After retiring from teaching, he worked as a curriculum and training developer at PASCO scientific. Dan has written curriculum for NASA, SETI, USGS, LLNL, and NMSI. Dan now leads the workshop program for new physics teachers, PTSOS.org and conducts Physics with Phones workshops at AAPT meetings around the country.
