9 Gravity Intensity Universal Law of Gravity The Moon Tidal Forces and Tides Gravity inside a Planet Einstein’s Theory of Gravity Black Holes

Intensity intensity is the power delivered per unit area SI Unit: watts/m2 Example: the brightness of this screen is determined by the intensity of visible light radiation falling on it.

Intensity Calculation intensity from a point source follows an inverse-square law dependence with distance

Gravity follows the Intensity Rule

Universal Law of Gravity all matter is weakly attracted the force is an inverse-square law where G = 6.67x10-11 N·m2/kg2 Example: Two 100kg persons stand 1.0m apart

The Earth-Moon gravitational attraction keeps the system together. The Moon is falling toward the Earth, but since it is moving it always misses.

From 4 million miles away on December 16, 1992, NASA's robot spacecraft Galileo took this picture of the Earth-moon system. The bright, sunlit half of the Earth contrasts strongly with the darker subdued colors of the moon. Our moon is one of the largest moons in the solar system. It is even larger than the planet Pluto. In this picture, the Earth-moon system actually appears to be a double planet.

Orbital Lock Moon with Earth caused by gravitational torque same side of moon always faces earth there is no “dark side” of the moon. all parts of moon are illuminated during a month. there is a “far side” of the moon. this side never faces earth.

Moon’s Orbit is inclined about 5 degrees to Earth’s Moon’s Orbit is inclined about 5 degrees to Earth’s. Eclipses occur when the moon crosses the Earth’s orbit in line with Sun and Earth.

Eclipses Solar Eclipse Total solar eclipses are rare events. Although they occur somewhere on Earth approximately every 18 months, it has been estimated that they recur at any given place only once every 370 years, on average (Stephenson, p.54). Lunar Eclipse occurs when moon is on opposite side of Earth.

Gravity causes differential (rather than constant) pull

Differential Pull Causes Tides

Spring Tides (largest)

Neap Tides (smallest)

Gravity inside and outside Earth.

Einstein modeled gravity as stretched space-time

Black Hole: caused by very small and massive object.

Wormhole, path to another universe?

Condensation of Matter

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