Unit 05 Vocabulary

1. Newton's law of universal gravitation -- every body in the universe attracts every other body in the universe with a force that, for two bodies, is directly proportional to the product of their masses and inversely proportional to the square of the distances between their centers.
2. Universal gravitation constant -- a unit conversion factor, used to convert kg^2 / m^2 on the right side of Newton's law of universal gravitation into Newtons on the left side. It is symbolized with a capital G and is equal to 6.67 * 10^-11 N (m^2 / kg^2).
3. Inverse-square law -- any law that relates the intensity of an effect to the inverse-square of the distance from the cause. Gravitational attraction, electrical and magnetic attractions, light, sound and radiation intensities all follow this rule.
4. Weight -- the force that an object exerts on a supporting surface, which is often - but not always - due to the force of gravity.
5. Weightless -- being without a supporting force, as in free fall.
6. Spring tides -- high or low tides that occur when the Sun, Earth and Moon are all lined up. The high and low tides produced are higher and lower than average due to the added pull of both the Sun and Moon.
7. Neap tides -- high or low tides that occur when the Moon is at first or third quarter. Due to the perpendicular pulls of the gravities of the Sun and the Moon, the high and low tides that occur are less pronounced than average.
8. Gravitational field -- the influence that a massive body extends into space around itself, producing a force on another massive body. It is measured in Newtons per kilogram.
9. Einstein's theory of gravitation -- an alternate explanation for the observed effects of gravity in which there is no gravity - just the warping of spacetime due to the masses present.
10. Black hole -- A concentration of mass that results from gravitational collapse of a star, near which gravity is so intense that not even light could escape.
11. Dark matter -- Matter that makes up a significant amount of the universe, but that cannot be observed in any way other than by the gravitational effect it has on the observable matter around it.
12. Dark energy -- A theoretical energy that acts to push matter apart on very large scales. It has not been observed directly but is inferred from the acceleration of the expansion of the universe.
13. Projectile -- any object that moves with only the influence of the Earth's gravitation to affect it.
14. Parabola -- the curved path followed by a projectile under the influence of only constant gravity.
15. Satellite -- a projectile or small celestial body that orbits a larger celestial body.
16. Ellipse -- the oval-shaped path followed be a satellite. It is formed by finding all of the points for which the sum of the distances to the two foci remains constant.
17. Eccentricity -- the flatness of an ellipse, caused by the separation between its foci.
18. Kepler's first law of planetary motion -- the path of each planet around the Sun is an ellipse with the Sun at one focus.
19. Kepler's second law of planetary motion -- the line from the Sun to any planet sweeps out equal areas of space in equal times.
20. Kepler's third law of planetary motion -- the square of orbital period of a planet is directly proportional to the cube of the average distance of that planet from the Sun.
21. Escape speed -- the speed that a projectile must reach to escape the gravitational influence of the Earth or of another celestial body to which it is attracted.

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