Physics Chapter 13

Spring tide

A high or low tide that occurs when the sun, Earth, and moon are all lined up. The tides due to the sun and the moon coincide, making the high tides higher than average and the low tides lower than average. Spring tides occur at the times of a new or full moon.

Other Types of Tides

Because much of the Earth's interior is deformable, we have Earth tides, though they are less pronounced than ocean tides. Twice each day the solid surface of Earth rises and falls as much as one-quarter meter.

Gravitational Field

Field lines can also represent the pattern of Earth's gravitational field. The field lines are closer together where the gravitational field is stronger. Any mass in the vicinity of Earth will be accelerated in the direction of the field lines at that location. Earth's gravitational field follows the inverse-square law. Earth's gravitational field is strongest near Earth's surface and weaker at greater distances from Earth The gravitational field of Earth at its center is zero! The gravitational field of Earth exists inside Earth as well as outside. In a cavity at the center of Earth, your weight would be zero, because you would be pulled equally by gravity in all directions.

Neap tide

Occurs when the moon is halfway between a new moon and a full moon, in either direction. The pulls of the moon and sun are perpendicular to each other. The solar and lunar tides do not overlap, so the high tides are not as high and low tides are not as low.

Ocean Tides

The ocean tides are caused by differences in the gravitational pull of the moon on opposite sides of Earth. The oceans bulge out about 1 meter on average, on opposite sides of Earth. Because Earth spins once per day, a fixed point on Earth passes beneath both of these bulges each day, producing two sets of ocean tides per day—two high tides and two low tides.

Weight and Weightlessness

The sensation of weight is equal to the force that you exert against the supporting floor.

Black Holes

The size of the sun is the result of a "tug of war" between two opposing processes: nuclear fusion and gravitational contraction. If the fusion rate increases, the sun will get hotter and bigger. If the fusion rate decreases, the sun will get cooler and smaller. When the sun runs out of fusion fuel (hydrogen), gravitation will dominate and the sun will start to collapse. The gravitational field strength near a giant star that collapses to become a black hole is the same before collapse (left) and after collapse (right).