• Construct and present arguments using evidence to support the claim that gravitational interactions are attractive and depend on the masses of interacting objects. (MS-PS2-4)

• Use mathematical representations of Newton's Law of Gravitation and Coulomb's Law to describe and predict the gravitational and electrostatic forces between objects. (HS-PS2-4)

• 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. (6-8)

• 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. (6-8)
• 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. (9-12)

• When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object. (6-8)

• Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena. (9-12)

• Cause and effect relationships may be used to predict phenomena in natural systems. (6-8)

• Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. (6-8)

• Models can be used to represent systems and their interactions—such as inputs, processes and outputs—and energy and matter flows within systems. (6-8)
• Systems may interact with other systems; they may have sub-systems and be a part of larger complex systems. (6-8)
• When investigating or describing a system, the boundaries and initial conditions of the system need to be defined. (9-12)

• Modeling in 6–8 builds on K–5 and progresses to developing, using and revising models to describe, test, and predict more abstract phenomena and design systems. (6-8)
  • Develop a model to predict and/or describe phenomena. (6-8)
• Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. (9-12)
  • Use a model to predict the relationships between systems or between components of a system. (9-12)
  • Use a model to provide mechanistic accounts of phenomena. (9-12)

• 9-12: 1A/H1. Science is based on the assumption that the universe is a vast single system in which the basic rules are everywhere the same and that the things and events in the universe occur in consistent patterns that are comprehensible through careful, systematic study.

• 9-12: 4A/H4. Mathematical models and computer simulations are used in studying evidence from many sources in order to form a scientific account of the universe.

• 6-8: 4F/M3a. An unbalanced force acting on an object changes its speed or direction of motion, or both.
• 6-8: 4F/M3b. If a force acts towards a single center, the object's path may curve into an orbit around the center.
• 9-12: 4F/H2. All motion is relative to whatever frame of reference is chosen, for there is no motionless frame from which to judge all motion.
• 9-12: 4F/H8. Any object maintains a constant speed and direction of motion unless an unbalanced outside force acts on it.

• 6-8: 4G/M2. The sun's gravitational pull holds the earth and other planets in their orbits, just as the planets' gravitational pull keeps their moons in orbit around them.
• 9-12: 4G/H1. Gravitational force is an attraction between masses. The strength of the force is proportional to the masses and weakens rapidly with increasing distance between them.

• 6-8: 11B/M4. Simulations are often useful in modeling events and processes.
• 9-12: 11B/H1a. A mathematical model uses rules and relationships to describe and predict objects and events in the real world.
• 9-12: 11B/H2. Computers have greatly improved the power and use of mathematical models by performing computations that are very long, very complicated, or repetitive. Therefore, computers can reveal the consequences of applying complex rules or of changing the rules. The graphic capabilities of computers make them useful in the design and simulated testing of devices and structures and in the simulation of complicated processes.
• 9-12: 11B/H3. The usefulness of a model can be tested by comparing its predictions to actual observations in the real world. But a close match does not necessarily mean that other models would not work equally well or better.

This 40-minute lesson, created by a veteran high school teacher, gives kids explicit guidance in using the PhET simulation My Solar System to explore orbital motion and gravitational attraction. Great concept building activity. Note: Registration is required to access PhET teacher-contributed materials (it's free).

• Physical Sciences K-8 Planetary Motion Unit
• Physics First Planetary Motion Unit
• Conceptual Physics Planetary Motion Unit