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Understanding 9/11Business, government and public policy
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Business, government and public policy
- Publication date
- 1964
- Publisher
- Princeton, N.J., Van Nostrand
- Contributor
- Internet Archive
- Language
- English
Bibliography: p. 439-442
Search Educational Film Journals at Media History Project for references to this film
Summary
Uses animation to explain the causes of ocean tides, showing the effect of centrifugal force and the gravitational attraction of the sun and the moon on the waters. Discusses the uses of the ocean tides and points out that the activities of man must be planned to fit the rhythm of the tides.
- Access-restricted-item
- true
- Addeddate
- 2015-02-20 19:08:25
- Boxid
- IA1113015
- Identifier
- tidesoftheocean
- Lccn
- 64000047
- Oclc_id
- 4050724
- Ocr
- Elections meet the land men have observed ising and falling of the ocean surface and of call this change in ocean level the tides. People who live near the ocean sometimes see the beach and rocks uncovered. And speech completely under water at some places the difference between high tide and Lodi days six feet or less. At other places this difference maybe between twenty and forty feet. Here's a beach at low tide where the tide differences large. Here is the same beach a few hours later at high tide the water here has risen about tide is rising ocean what coast and into bays and inlets. In a few hours when the tide turns on water flows back to the ocean the tide falls. Extremely high tides are caused at the head of the Bay of Fundy by the funneling of the tides up the river channel the tide rises so rapidly that it appears to be a single moving wave with a foaming crest this sing wave is called a tidal bore the bore moves many miles upstream at a speed of more than eight miles an hour. Bores like this also occur in a few other rivers because title bars are high and move so fast they can be very dangerous to man and two ships there are rivals which occur later each day stood for all to see. At places where the tide rises very high. Ships can go far up river on the high tide to load or unload their cargoes special docs have been built the whole ship upright when the tide blows out. When the water is deep. The ship must be securely tied to the dock you know a few hours when there is scarcely any water left in the channel the ship breast safely on a wooden platform to protect its hall. Thus man has learned to use the extreme range of tides to help in shipping. Here at low tide. We can see a large fish trap that has been placed on the river bottom. It is built with the open end up stream so that fish are carried into it as the tide flows out. From a cliff we can just see the top of the fish trap at high tide. A few hours later at low tide the trap fish may easily be gathered. Local fish markets are supplied by the commercial fisherman who own the trap. In the same mudflats at low tide. Men dig for tasty clams in the few hours that the title flat so dry men work quickly to fill their buckets before the tide returns. At Sam Philippe Mexico near the head of the Gulf of California the great dominates the lives of the people. Boats can be brought far into shore on high tide and left on the sandy beach at low tide for painting all repair. But what causes this Rise and Fall of ocean water called tides for many centuries men have been puzzled and fascinated by this constant movement of the sea. Sir Isaac Newton's discovery of the laws of gravity. Led him to believe that the tides were caused by the gravitational on the waters of the earth. Let us first study the effect of the Sun. The Earth though attracted to the sun by gravity does not move toward it the reason is that the earth is circling around the sun like a weight on a string the faster you world such a wait the harder it pulls the outward Bowl is called centrifugal force the earth trust such a speed and at such a distance that this outward pull just balances the inward pull of gravity and the earth continues to travel around the sun instead of crashing into the sun or flying off into space. Here is a cross section through the center of the earth. This arrow stands for the force of the sun's gravitational pull on the earth the force points toward the sun. This arrow stands for the outward center the gold force on the earth as it orbits around the sun the force points away from the sun. These arrows must be the same length since they must balance each other exactly to keep the earth in its orbit around the sun. Newton found that the force of gravity is greater where two bodies are close together that it is where they are far apart so if we draw arrows representing the gravitational attraction of the sun at different places on earth the arrows closest to the sun will be longest and those files list away from the sun will be shortest. However the centrifugal force due to the Earth's motion around the sun is almost the same at every place on earth if we draw arrows representing the centrifugal force they are about the same length all over the earth. When we put all these arrows on the same picture we see that the gravity arrows and the centrifugal force arrows are not the same length and do not point in the same direction what is the total effect of these two forces which are different in direction and strength if we place the tail of the centrifugal force arrow on the head of the gravity arrow we can draw a shortcut arrow that represents the total force. If we combine each pair of arrows the results will look like this. The arrows on the sun side of Earth point toward the sun and those on the other side point away from the sun. These forces cannot squeeze our stretch the solid stiff earth very much. But these forces do change the shape of oceans whose waters are free to flow. The water moves outward at points just under the sun and just opposite the Sun the surface of the sea rises into bulges toward and away from the sun and falls between these two bulges these bulges would not cause tides if the Earth did not rotate. The ocean's surface would be higher at some places than others but it would not continually rise and fall. However the Earth does rotate and the sun is on different sides of Earth at different times of day. Tied bulges remain in line with the sun so that as the Earth rotates each part of the ocean passes through both bulges every day. If we place a flag on earth it will go all the way around once each day when the flag is between the bulges the water will be shallow when the flag is in either of the bulges the water is deep. For one complete turn of the earth we have high tide when the flag is toward the sun in daytime. Low tide when it is between bulges. High tide when it is opposite the sun at night and low tide when it is between bulges again. If we use a white line to trace the level of the ocean as it rises and falls along the flag pole we could make a tight curve like this. This away. Son died would look. There are two high tides and two low tides every day so far we have been considering only the effect of the sun on the tides. The moon causes the same sort of tied but the moon tide is nearly twice as high because the moon is much closer to Earth. The high sun tides are exactly twelve hours apart but the high moon tides are more than twelve hours apart about twelve hours and twenty five minutes. Therefore they cannot always occur at the same time as the high sun tides the orange areas show how the high moon tide lags further and further behind the high sun tied each day. When the sun moon and Earth are in line we have either a full moon or a new moon. And the two son tied bulges occur at the same place as the two Moon died bulges the two pairs of bulges add together to make larger bulges and these make larger tides. Whenever the sun and moon are pulling together the sun tied and the Moon died. Combined to make larger tides. These larger tides are called spring tides. One week later when the moon is at right angles to the sun and earth but to Sun bulges and two Moon bulges occur at different places and the two pairs of bulges partly cancel each other leaving only small bulges. Since the sun and moon are not pulling together the sun tied practically cancels the Moon died to make smaller tides. These small tides which occur half way between the spring tides are called Nip dives spring tides occur at every new and full moon. Therefore there are two sets of spring tides every month a set of leapt ides of periods between every two sets of spring tides at the first and third quarters of the moon. Another effect on the oceans occurs when the sun appears north of the quatre in summer months. Tied bulges remain in line with the sun. One ball July's north of the equator the other lies south as the earth turns it carries us around on a path like this. This path takes us through the thick part of one bulge but Mrs the thick part of the other bulge. If we cut across section through this path and look at it from the top it will look like this. The bulges we see are not equal as before we pass through both bulges when the earth goes around but one bulges larger than the other and one high tide is higher than the other. As the Earth goes around we will see a very high tide. Then a low tide. Then another high tide which is not as high as the first time I tied then still another low tide. The tied records will show high tides of an equal height. When the sun is directly over the equator in spring and fall the do Sun tides are equal but they are again. Unequal in winter when the sun is south of the equator. The moon and moon bulges move back and forth across the equator every month. The moon appears north of the equator part of the month. And south of the equator the other part. These north and south movements of the sun and moon and the continuous change from New Moon to full moon cause the tides to be different from week to week and season to season. The tides we have studied are those which would occur at the Earth were entirely covered by the ocean. But the real areas has continents dividing the ocean because of this the bulges cannot move directly or smoothly. Except where the ocean extends all around the earth as it does around the ad Arctic continent. We observe the effect of these tidal bulges only after they have travelled northward up the Atlantic Pacific and Indian Oceans. These oceans are vastly different in size shape and depth and affect the movement of tides in different ways in the Pacific Ocean which is the largest and deepest of the oceans the tides are about what we would expect there are two high tides each day but they are not of the same height. In the Atlantic Ocean However the effect of the north and south movements of the Sun and Moon is nearly cancelled by the shape and size of the ocean. The two high tides each day are nearly equal. Because title bulges are restricted by land masses and because they must travel great distances from the Antarctic Ocean they may not arrive exactly when the sound of moon is overhead. The high tides usually occurs several hours after the sun or moon has passed overhead. Tides behave differently in various places on the oceans. However if we measure the tides at any one place by reading the height every hour. Or by using a machine to make a continuous record we can learn enough about tides at that place to predict them very accurately by collecting tide information from many countries we can make tide tables for any place in the world anyone who needs to know the time and height of tides can find this information in the tide tables. Ships that go far up rivers on the high tides must know when high and low tides will occur. Large ships in New York Harbor preferred to come to the dock and leave when the tidal currents are weakest. This makes it easier for the tug boats to guide the ocean liners into the channel. Even very large ships must know when the tidal currents and the depth of water are safe for entering and leaving harbors shipping fishing and sailing must be planned to fit into the eternal rhythm of the tides.
- Run time
- 15:58
- Scanner
- Internet Archive HTML5 Uploader 1.6.1
- Scanningcenter
- sanfrancisco
- Source
- Lasergraphics ScanStation
- Worldcat (source edition)
- 4050724
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