Study guide: Gravity, Stars and the Solar System
Use models of gravity, brightness, day-night and scale.
Ideas to explain in your own words
- Earth’s gravitational pull acts toward Earth’s center. Down is local, so arrows from different surface positions point inward. Gravity does not require air or touching.
- Stars emit light. Apparent brightness depends on distance and intrinsic light output. The Sun appears very bright largely because it is much closer than other stars.
- Rotation is spinning about an axis. Revolution is traveling around another body. Earth’s daily rotation explains day-night; its annual revolution with tilted axis relates to seasons.
- Diagram length equals represented million kilometers divided by million kilometers per centimeter. Use the same scale for every compared distance.
Useful vocabulary
- inward
- direction toward Earth’s center
- intrinsic brightness
- light emitted by the source
- rotation
- spinning about an axis
- cm
- Centimeter is one hundredth of a meter.
- Diagram length
- Diagram length equals represented million kilometers divided by million kilometers per centimeter.
Watch your reasoning
- Check what inward and outward each mean. They describe different clues.
- Check what intrinsic brightness and apparent brightness each mean. They describe different clues.
- Check what rotation and revolution each mean. They describe different clues.
A worked review example
Worked reference-card problem: a card states “direction toward Earth’s center”; a second states “direction away from Earth’s center.” The first fits inward and the second fits outward. The reason is the described function or change, not the order of the cards or a favorite name. If their descriptions exchange places, their category labels exchange too. Keep the supplied descriptions attached to the correct case. Worked reference-card problem: a card states “light emitted by the source”; a second states “brightness measured from a location.” The first fits intrinsic brightness and the second fits apparent brightness. The reason is the described function or change, not the order of the cards or a favorite name. If their descriptions exchange places, their category labels exchange too. Keep the supplied descriptions attached to the correct case. Worked reference-card problem: a card states “spinning about an axis”; a second states “traveling around another object.” The first fits rotation and the second fits revolution. The reason is the described function or change, not the order of the cards or a favorite name. If their descriptions exchange places, their category labels exchange too. Keep the supplied descriptions attached to the correct case. Worked distinct example: A distinct model uses 10 million km/cm for 70 million km: 70/10=7 cm. Method: identify the quantities in the stated rule, substitute using compatible units, and perform the operations in the correct order. Divide real-model distance by scale distance per centimeter. This scale covers distance, not necessarily object size. Interpret the final unit and keep the model assumption visible; this calculation is not a measurement of a real learner or site.
Make a practice plan
- Use an example from Explain near-Earth gravity. Say what happens and point to the clue that helps explain it.
- Complete Distinguish star brightness and distance and correct the specific misconception in its feedback.
- Resolve a changed case from Connect rotation and revolution, then combine the four ideas in the transfer task.