Students who demonstrate understanding can: Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.
Clarification statement: Emphasis for the model is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students' school or state).
Assessment boundary: Assessment does not include Kepler's Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
Planets, moons, comets and even the hundreds of billions of stars in the Milky Way keep moving in organized paths for one reason: gravity. Students develop a model showing that every object with mass pulls on every other object, that the pull is stronger for bigger masses and weaker at greater distances, and that this attraction is what keeps orbiting bodies curving around a central mass instead of flying off in a straight line.
An orbit is a balance between an object's forward motion and the inward pull of gravity. Without gravity, a planet would travel off in a straight line; without forward motion, it would fall into the sun. The same idea scales up: the sun and its planets orbit the center of the Milky Way, held by the combined gravity of the galaxy. Models can be physical (a ball on a string swung in a circle, a scale walk across a football field) or conceptual, such as comparing solar system distances with the size of a school or state.
Kepler's laws and retrograde motion are outside this expectation; the goal is a clear qualitative picture of gravity as the organizer.
Astronauts float because they are falling around Earth together with their spacecraft, not because gravity is missing. Gravity at the space station is still about 90 percent of its value at the surface.
Every object with mass has gravity. The moon's gravity raises tides on Earth, and the sun's gravity holds every planet in orbit.
Students sometimes think something must keep pushing the planets. In space there is almost no friction, so a moving planet keeps moving while gravity bends its path.
Textbook diagrams squash distances. A scale walk shows that the planets are tiny specks separated by huge gaps, which surprises most classes.
Using a ball whirled on a string as a model, explain what Earth would do if the sun's gravity suddenly disappeared.
Answer: Earth would stop curving and travel off in a straight line, because gravity is the inward pull that turns forward motion into an orbit.
Pair a hands-on model with a scale model. The string model makes the inward pull visible, while a scale walk with the sun as a beach ball and Earth as a peppercorn shows how empty the solar system is. Simulations let students change a planet's speed or the sun's mass and watch orbits change shape or fail.
Assessment tasks typically ask students to identify what is missing from a model, or to explain what would change if a mass or a distance changed.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: A) Gravity
The sun's gravity pulls each planet toward it, bending its forward motion into an orbit.
Answer: D) It gets weaker
Gravity weakens with distance but never fully reaches zero.
Answer: C) They and the station are falling around Earth together
The station and astronauts are in continuous free fall around Earth, so they float relative to each other even though gravity is strong there.
Answer: Milky Way (also accepted: the Milky Way, Milky Way galaxy)
Our sun is one of the hundreds of billions of stars in the Milky Way, held together by gravity.
Answer: A) The pull of gravity toward the central body
The string pulls the ball inward, just as gravity pulls a planet toward the sun.
Answer: the sun (also accepted: sun)
The sun holds over 99 percent of the solar system's mass, so its gravity controls the planets' orbits.
Support an argument that the gravitational force exerted by Earth on objects is directed down.
Use mathematical or computational representations to predict the motion of orbiting objects in the solar system.
Use mathematical representations of Newton's Law of Gravitation and Coulomb's Law to describe and predict the gravitational and electrostatic forces between objects.
A full lesson with slides, activities and an exit ticket on gravity in the solar system and galaxies, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-ESS1-2 with an answer key, ready in about a minute.
Make a worksheet βTurn gravity in the solar system and galaxies into a quiz students answer online that marks itself, with a class summary for you.
Build a test βNo. The assessment boundary excludes Kepler's laws and apparent retrograde motion; the focus is gravity as the force that holds systems together.
Physical analogies, scale models along a field, or computer visualizations of orbits all fit, as long as the model shows gravity controlling the motion.