Chapter 13: Gravitational Interactions

  • In this chaper you will see how gravity afects the earth's oceans and its atmosphere, and what gravity looks like at its extreme (such as black holes).

13.1 Gravitational Fields

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  • A gravitational field is a force field that exists in the space around every mass or group of masses, even you and me!
  • The earth's gravitational field is represented by imaginary field lines. The field lines are stronger when the lines are closer together. The direction of the field at any point is along the line the point lies on.
  • A particle, a school bus, Mr. Strong, or any mass in the vicinity of the earth for that matter will be accelerated in the direction of the field line at that location.
    • Gravitational Field is defined as: g=F/m. This shows that the strength of the earth's gravitational field is the force per unit mass exerted by the earth on any object.
    • Near the surface of the earth, the gravitational field strength is: g=F/m=9.8N/kg=9.8m/s^2. The value of g at the earth's surface depends on the mass of the earth and its radius.

13.2 Gravitational Field INside a Planet

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  • The gravitational field of the earth exists inside and outside of the earth.
  • If you were to drill a hole through the center of the earth, starting in the north and ending at the south, you would not gain speed the entire time. You would actually gain speed while moving towards the center, and lose speed moving away from the center (towards the south). During your fall, both the north and south are pulling on you at all times. While you are being pulled downward, you are also being pulled upward by the part of the earth that is above you. When you are in the middle, these forces cancel out, and you are pulled on from every side equally, making the net force equal zero. The gravitational field of the earth at its center is zero.

13.3 Weight and Weightlessness

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  • The force of gravity causes acceleration, and objects that are under the influence of gravity (everything on earth) are pulled toward one another.
  • The pressing against the earth is what we call weight. Gravity does not only press against us and keep us on earth, but it accelerates us too.
  • If you stand still while on a bathroom scale, the gravitational force between you and the earth pulls you against the scale, gving you a measuring. Newton's thrid says that whenever one body exerts a force on a secondy body, the second body exerts an equal and opposite force on the first. In this case, the floor and scale push upward on you as you push downward on them. You get your reading from the scale because inside it there are springs which that are compressed by the pair of forces put upon them.
    • If you tried to weigh yourself on an elevator, the reading would be much different. It would change during accelerated motion, not during steady motion. If the elevation where to accelerate upward, the scale and floor would push harder against your feet. The springs inside the scale would compress more, and your weight would appear to be increased.
    • If the elevator accelerated downward, your weight would be decreased. In fact, according to the scale, you would be weightless. It appears as though gravity is absent, and your body feels as though gravity is absent. However, gravity is not absent, and if we define weight as the force you exert against a supporting floor rather than the force of gravity that acts on you, we find that you are as heavy as you feel.

13.4 Ocean Tides

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  • Newton has shown that ocean tides are caused by differences in the gravitational pull of the moon on opposite sides of the earth. The moon's attraction is stronger on the earth's oceans closer to the moon, and weaker on the oceans farther from the moon. This is beacause the gravitational force is weaker with increased distance.
  • The ocean nearest to the moon is pulled upwards towards the moon. At the same time, the main body of the earth is being pulled toward to the moon (away from the ocean on the far side). The earth as a whole is closer to the moon than the far-side of the ocean is. The waters get slightly elongated at both ends. When this happens, the earth has what are known as "tidal bulges" on the inner and outer sides of the planet. The earth elongation is evident in the pair of ocean bulges on opposite sides of the earth.
    • There are two sets of ocean tides per day. There is "high tide" - When any part of the of the earth passes beneath one of the bluges, a high tide occurs.

    • Roughly six hours later, after the earth has made a quarter turn, the water level goes down 1 meter below the average sea level. This is known as low tide. Tides do not occur at the same time, the are constantly changing. While the earth spins, the moon moves in orbit and appears at the same position in the sky every 24 hours and 50 minutes. This means that the tide cycle runs on a 24 hours and 50 minute intervals.
  • The sun also has an effect on ocean tides, but not as much as the moon. In fact, the sun's pull on the earth is 180 times stronger than the moon's pull. The reason why the sun does not have as big of an influence as the moon is because of difference. Because the sun is so far away from the earth, there is not much differnce in pull on the one side of the earth to the other. There is a small difference in the gravitational pull between the sun and the ocean nearest to it. The small difference in solar pulls on opposite sides of the earth only slightly elongates the earth's shape and produces tidal bulges less than half those produced by the moon.
  • The size of the body of water also has an affect on tides. Lakes do not have tides because no part of the lake is any closer to the moon than any other part of the lake. There is no significant difference in the moon's pull.
      • When the sun, earth, and moon are all lined up, the tides are high than average tides and lower than average tides. These are known as spring tide (and no, they have nothing to do with the season of spring).
      • Depending on the alignment, an eclipse can form. Lunar eclipse is produced when the earth is directly between the sun and moon. A solar eclipse is produced when the moon is directly between the sun and the earth. The alignment is almost never perfect, so each month when the earth is between the sun and moon, we have a full moon, and when the moon is between the sun and the earth, we have a dark moon. Spring tides occur at the times of a new moon and a full moon.
      • Neap tides occur when the moon is halfway between a new moon and a full moon in either direction. The tides partly cancel each other due to the sun and moon. High tides are lower-than-average, and low tides are higher-than-average.
      • The tilt of the earth's axis also affects the tides, and causes two daily high tides in most parts of the ocean to be unequal most of the time.

13.5 Tides in the Earth and Atmosphere

  • The earth is molten liquid covered by a think solid crust. The moon and sun cause what are called earth tides as well as ocean tides. Twice a day the solid surface of the earth rises and falls by as much as 25cm.
  • There are also atmosphereic tides, but are smaller than earth and ocean tides.
  • Within the upper part of the atmosphere is what is known as the ionsphere, which contains ions. Tidal effects in the ionosphere produce electric currents that alter the magnetic field that surrounds the earth. This is known as magnetic tides. They regulate the degre to which cosmic rays penetrate into the lower atmosphere. The penetration affects the inoic composition of the atmosphere, which in turn is evident in subtle changes in the behaviors of living things.

13.6 Black Holes

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  • There are two important sequences going on in a star. One is the process of gravitation, which crunches all solar material toward the center. The other is thermonuclear fusion consisting of reactions similar to those in a hydrogen bomb. If fusion rate increase, the star becomes stronger and more intense. If the fusion rate decreases, the sun will become cooler and smaller in size.
  • When the star runs out of fusion fuel, gravitation becomes dominate, and the sun will start to collapse. When this happens to our star (the sun), the ignition of nuclear ashes will occur, and the sun will expand into a red giant. The sun will be so big that it will extend beyond the earth, swallowing it entirely. Once all of the helium is burned out, the red giant will die and collaspe. It will then become a black dwarf.
  • A star that is roughly 3 times the size of our sun has a different life. Once the flame of thermonuclear fusion starts, gravitational collaspe takes over, and does not stop. The material begins to cave in, and the density becomes infinite. Gravity is so enormous in these areas that not light cannot escape . This is known as a black hole.
  • Black holes are no more massive than the star that had collasped.
  • Black holes cannot be seen, but their effects can be.

All information was gathered from the text book known as Conceptual Physics.