Topic 7A: Tides, Part II. Online Lecture: The Bulge Theory of Tides

Transcription

1 7A_2 Slide 1 Topic 7A: Tides, Part II Online Lecture: The Theory of Tides Overly Simplistic: no continents, ocean is the same depth everywhere

2 The Theory of Tides 7A_2 Slide 2 High Tide North Pole Low Tide High Tide = High Tide No = Low Tide Low Tide North Pole If water goes up in one place, it has to go down somewhere else: the extra water has to come from somewhere.

3 The Theory of Tides II 7A_2 Slide 3 #1 #3 North Pole High Tide High Tide #2 Low Tide #4 Low Tide What is moving, the or the Ocean?

4 Why 2 s? : How Gravity Works 7A_2 Slide 4 Every object in the universe is attracted to every other object in the universe. the more massive ( heavier ) the 2 objects are, the stronger the attraction the farther apart (the centers of ) the 2 objects are, the weaker the attraction the pull on the lighter object is the same as the pull on the heavier object Which pulls on you more strongly, the or the Sun? Why? Warning: The Sun s gravitational pull is stronger than the s, but it has a weaker effect on the tides. There is more to it than strength

5 Why 2 s? : s Pull on the Ocean 7A_2 Slide 5 The 's gravity pulls on the ocean, making a bulge in the ocean on the side of the facing towards the. How many high tides will there be each day if this is the only important factor? North Pole

6 Why 2 s? : s Pull on the I 7A_2 Slide 6 The 's gravity pulls on the, making it orbit a place called the barycenter. At First: Barycenter 2 weeks later

7 Why 2 s? : s Pull on the II 7A_2 Slide 7 When the moves in a circle (an orbit ), the water is flung away from the center of the circle (the location it orbits, the barycenter), creating a bulge in the ocean on the side of the facing away from the. North Pole Barycenter Circle or "Orbit" Bottle Center of the Circle Why doesn t the water get flung off the?

8 Why 2 s? : Putting Everything Together 7A_2 Slide 8 The s gravity pulls the ocean towards the, but the s small orbit around the barycenter (owing to the s gravity pulling on the ) flings the water away from the. caused by the 's orbit around the barycenter North Pole Which bulge is larger? Caused by the s gravitational pull on the caused by the 's gravitational pull on the ocean These 2 effects do not cancel, because gravity gets weaker with increasing distance. The s pull far side of the is too weak to keep the water from being flung outwards, whereas the s pull on the closer side of the overcomes this tendency.

9 Why Tides Get Later Each Day 7A_2 Slide 9 During a day the moves a little bit, so the ocean s bulges move a little as well. After 24 hours, each location on the 's surface will be back where it started, but the bulges will have moved on. The has to rotate a little farther before the location is in the bulge in the ocean beneath the. At First High Tide 24 hours later 25 Between High & Low: Rising Tide Tide How long does it take the to orbit the one time? hours later High Tide

10 Why There Are Spring & Neap Tides I Tidal Range 7A_2 Slide 10 Phase(s) Spring Tides Neap Tides Large Small Full & New Half The Sun s gravity tries to create 2 bulges in the ocean too, one towards the Sun and one away from the Sun. Sun s effect is weaker than the s, so the always determines where and when high and low tide will be. But the Sun can affect how high or low the tides get Remember: The Sun s gravity is stronger than the s. There is more to causing tides than strength

11 Why There Are Spring & Neap Tides II 7A_2 Slide 11 Full : Spring Tides New : Spring Tides E S E S #1 #3 Half : Neap Tides E S E S #2 #4 Half : Neap Tides How long does it take to go from spring tide conditions to neap tide conditions? How do the,, and Sun have to be positioned to produce Spring Tides? Neap?

12 Why There Are Spring & Neap Tides III 7A_2 Slide 12 Sometimes the & the Sun both want high & low tides to happen in the same places at the same time. makes high tides higher and low tides lower Sometimes the Sun wants it be high tide where the wants is to be low tide (and vice versa). Which will win? The & Sun work together, pulling water from some places to other places. But the Sun interferes, pulling up the low tide, and pulling down the high tide. & Sun fight about where high tide and low tide should be.