July 07, 2017

Tides of Ocean

Consider a situation of yourself at the beach, watching out for the tides in the ocean. Now consider this situation having yourself enjoy a cup of coffee. Look at your cup of coffee and ponder over a few minutes. Have you ever come across the fact that you don’t get to see those tides in lakes, ponds or even in your cup of coffee? Well if you haven’t and now that you would, here is a small correction on your assumption about how the moon’s gravity affects tides in our ocean body. Gravity in general affects motion of all objects identically, irrespective of their masses. Let us first set our assumption straight, the sun and the moon are both responsible for the tides in ocean and water. If the moon’s gravity is stronger at point A and weaker at point B where A and B are opposite points of the moon’s location, at the earth’s centre the net effect of this differential of the moon’s gravity across the earth is to stretch the oceans bulge out like a candy that stretches. At the earth-moon line, if the earth rotates underneath those bulges with no friction between the ocean and the earth’s crust, then at a given location at any point on the earth would experience 2 high tides per day and as you pass, 2 low tides per day at the spots normal to each other. Which is not exactly what we experience, so assuming Newtonian gravity and ignoring the sun’s effects for simplicity, considering the earth to be covered with water (ocean) as a whole and also by switching off the gravity on our planet, we get to place two points A’ and B’ along the earth-moon line, on earth’s surface at opposite sides. Relative to a frame out in the ambient space, point A’ closer to the moon gets attracted/accelerated towards the moon more than the earth’s centre and earth’s centre towards moon more than point B’. Thus A' and B’ are separated due to an invisible outward force. Let’s call this force as tidal force. The tidal force is a force that is the secondary effect of the force of gravity. It arises because the gravitational force exerted by one body on another is not constant across it: the nearest side is attracted more strongly than the farthest side, in simple words. Since the points and the earth’s centre are accelerating, they are not inertial, thus the acceleration relative to earth’s surface is anti-gravity. Tidal force involving acceleration of surface regardless of the mass of any object is known as object tidal acceleration of that object. Well to this point we get an idea about the tides, that no more work on our “stretching” assumption, but over a mechanism of “squeezing” (eg: pimple/zit).  It undergoes “cumulative sideways traction”, where the water molecules are pulled out towards the moon at the equator and are pulled towards the earth’s centre at the poles. Moon’s gravity being 10 million times weaker than the earth’s, the points on earth cannot be completely pulled, instead are being pushed towards the moon along the earth-moon line. Water molecules apply force on each other, on being pushed or pulled, increases water pressure. This global water pressure works against Earth's gravity to form two bulges: high-tide and low-tide bulges. These high-tide bulges stay put as Earth rotates underneath them every 24 hours, leading to a tide change every 6 hours. Low tides occur where the disruption caused by tidal force is weakest (and water pressure caused by Earth's gravity is strongest). This process of water being pushed, the ocean acts like a planet sized hydraulic pump. Dramatic tides can result where land and seafloor terrain funnel more seawater into one spot. The sun’s affects are only by 1/3rd, as it is far away from our planet. When the sun, the moon and the earth align in a same line then there in a high tidal force in the ocean. Now our small water bodies like lakes, ponds and even your cup of coffee wouldn’t undergo this process as they do not have enough area to be pushed out. They don't have noticeable tidal bulges because they lack enough liquid to create pressure that can visibly overcome the pull of Earth's gravity. These affects are highly visible in the form of waves in the ocean/sea-water, which may decrease and increase depending on the winds, non-uniformity of the surface that could help increase in water levels and the earth’s bulging surfaces at that particular point.

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