Tidal force
Hearing the word tidal force, one can connect it to the tidal effect caused by the moon on the earth, leading to a rise in water level in the ocean. But what is tidal force? How does it work? does it only cause by the moon? and so many other questions arise. so, let’s find out the answer to every question.
What is tidal force?
The basic definition says that it’s the difference of the gravitational force per unit length on a body. The definition doesn’t make it easier to understand. so, let’s have an example. We all know that the earth is pulling us by gravitational force and it decreases as the distance between two bodies increases.
So, while you are standing on your feet the force will be greater on your feet than the force on your head as your feet are closer to the center of the earth than your head. Now, let’s understand it mathematically.[latexpage]
The gravitational force and The tidal force are given by-
\begin{align*}
F_g&=-\frac{GMm}{r^2}\\
F_t&=\frac{2GMm}{r^3}
\end{align*}
The tidal force of the earth on a 60kg person is approximately 0.000184 N/m which is so less that we can’t notice it. The tidal force between the moon and the earth is very high (1.03×10^12 N/m).
Effects of tidal force
Tidal locking
Have you ever noticed that we can see only one side of the moon and never the other one?
This relative motion is called tidal locking. The earth is exerting tidal force on the moon and this force is the same that the tidal force on earth but due to the very small mass of the moon the hemisphere closer to the moon has stopped. therefore, the revolution period of the moon around the earth and the rotational period of the moon are synced.
Formation of a ring system
The ring system around massive planets is also due to tidal force.
When a moon gets too much closer to its planet than the Roche limit, the tidal force tears it apart in pieces and then forms an elongated ring system around that planet. In our solar system, all the giant planets possess a ring system. however, among all of them, Saturn has the most extended ring.
Due to this ring system, when Galileo Galilei first saw the Saturn from his telescope, it looked like a pair of ears on it.
Spaghettification
The name seems to be funny, but the phenomena relate to its nomenclature. You might have come across the term black hole, either in a sci-fi movie or in a magazine. We won’t go into details of its properties.
but one thing to keep in mind is that they are very massive. The smallest of all is called a stellar-mass black hole, which possesses at least 1.5 solar mass(1 solar mass=(1.989±0.004) x 1030 kg), and they have a very small volume (less than 5km radius), which makes them the densest objects in the observable universe.
Have you ever wondered what will happen if you fall into one of them? The answer is quite scary, the tidal force around a black hole so large that before reaching the surface of it, you will be torn into atoms or maybe smaller. Your body will become a long, stretched line of atoms and that’s why the name of the spaghetti is used.
OCEAN TIDES
This effect is known to most people. the ocean tide. it happens twice a day, once when the moon is directly above the ocean surface and once when it’s on the opposite side.
when the moon is directly above the ocean, the water gets pulled by the moon with maximum force therefore sea level rises. But when it’s on the opposite side the water gets pulled by minimum force therefore it goes away from the moon and ultimately the sea level rises.
Although the sun applies more gravitational force on the earth than the moon (177 times), the moon’s tidal force on the earth is quite larger, 2.2 times to be exact. Therefore, the main cause of ocean tides is the moon, not the sun.
Types of ocean tides
Spring tides
When the moon, sun, and the earth are collinear, then the high tides are larger than usual and low tides are lower. This is because that the sun and the moon both add up to make a larger tidal force.
Spring tides happen only on a new moon and a full moon.
Neap tides
When the moon and the sun make an angle of 90 degrees to the earth. At this time the high tides are lower than usual and low tides are higher than usual. When both are perpendicular to each other, they lower their force effect as shown in the figure. Neap tides happen only on a first-quarter moon and a third-quarter moon.
Decrease in earth rotational speed
In ocean tides, the peak of the high tide is not directly below the moon. Because of the rotation of the earth, the momentum keeps the peak of the high tide slightly ahead of the moon.
However, the moon tries to pull that peak to its nearest position, leading to a decrease in orbital rotation. this situation will remain the same until the relative motion between the surface of earth closest to the moon and the moon itself becomes zero, i.e. the moon will always remain stationary at one place on the earth, and at that time only people, who will be in the closest hemisphere to the moon, will be able to see the moon in the sky. At that time the period of earth’s one rotation and the time period of the moon’s revolution will become equal.
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