Science project idea on what causes the water to rise.
What Causes the Water to Rise?
From the fact that the highest tides occur at the time of the full moon and the new moon, what will you suspect? But since the moon is about 240,000 miles from the earth, at first thought it seems hardly possible that it can exert a power sufficient to pull up such an enormous amount of water. This problem remained unsolved until Sir Isaac Newton, in the seventeenth century, showed that the force which causes an object to fall to the earth is the same force which causes this tidal wave approximately every twelve hours and twenty-five minutes. In order to understand the cause of tides it is necessary for us to consider this force. The essentials of Newton's discovery are, that every particle of matter has an attraction for every other particle of matter; and that the strength of this attraction is directly proportional to the mass or amount of material and inversely proportional to the square of the distance between their centers. This means that if the moon were twice as large, it would pull upon the earth with twice the force it does now and that if it were twice as far from the earth as now it would pull upon the earth with a force only one fourth as great as at present. In accordance with this law of gravitation, there is a pull between the center of the earth and every object we know. The measure of this pull constitutes the weight of a body. The reason that the earth does not seem to be pulled toward the ball that is dropping may be understood from the following experiment. Tide Science Project Experiment
Connect two marbles, A and B, of equal size, by a rubber band. Draw the marbles apart and allow the elasticity of the band to pull them together. Compare the  amount of movement of each marble. Now connect a very small marble with a very large one by a rubber band. As before, compare the amount of movement of each when they are pulled together by the elastic. If there is ten times as much material in the large marble as in the small one, the large marble will move one tenth as far as the small one. Explain, then, why the ball falls to the earth, and why the earth does not seem to rise to the ball. This force acts in solid bodies, through the center of mass of the body. It is because of this that a mason's plumb line points to the middle of the earth. In objects on the surface of the earth, this center of mass or center of gravity, as it is called, is the point of a body at which its weight may be counteracted by a single upward, vertical force. The location of the center of gravity is easily found. Suppose the center of gravity of a piece of cardboard is to be found. Suspend the cardboard by a thread. Draw a line on it continuous with the line of the supporting thread. Now suspend the cardboard in the same way from another point of attachment. The point of intersection of the two lines will be the location of the center of gravity. Explain. The fact that the center of gravity of a body tends to get as near the center of the earth as possible is illustrated by the tipping over of bodies. Why does a flat stone on the ground show no tendency to tip, while the same stone standing on its edge tips over very easily? A body is said to be in stable  equilibrium when it cannot be tipped without raising its center of gravity; a body is in unstable equilibrium when it cannot be tipped without lowering its center of gravity.
 Let us now go back to the tides and endeavor to understand how this force of gravitation causes them. The moon attracts the solid earth as if the entire mass of the earth were concentrated at its center. The water of the ocean, however, is 4000 miles from the center of the earth. What, therefore, is the relative pull of the moon upon the solid earth, and upon the ocean on the side nearest the moon?
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