Human life is short, living on the earth for less than a hundred years, so the phenomena we see and the laws we grasp every day are actually intuitive daily life experiences on the surface of the earth.
However, the earth is so small in the vast universe that it has been difficult for us to directly perceive the laws of the universe outside the earth for a long time, and it is also doomed that our intuition and daily life experience are often unreliable and have great limitations.
So, what do we see on Earth every day?
The most obvious feeling is that everything falls towards the ground, and we have a sense of direction up and down, with the top of our head being above and our feet being below, and that's our intuition.
So, our intuition tells us that everything must fall downward. And the physics boss Newton also touched the great law of gravitation, letting us know that gravitational force exists, and there is a gravitational force between any two objects.
The existence of gravity is also the reason why we can stand firmly on the ground, and our intuition always tells us that objects will always be attracted to the earth and fall downward.
But the question is, with a mass of 60 trillion tons on Earth, why can such a heavy behemoth float in space for 4.5 billion years? Why doesn't the earth fall?
In fact, the emergence of this kind of problem is mostly caused by the limitations of our cognition, and to put it bluntly, it is a misunderstanding of the state of the earth.
In the small confines of the earth, we have a sense of direction up and down. However, once you come to the vastness of space, there is no such thing as a concept of up and down. In space, any object and space position is actually equal and equivalent. The so-called direction is only artificially defined by us, so the so-called "the earth is falling down or falling" has no meaning at all.
There is no concept of up and down in space, and the earth wants to fall down, but what about "down"?
But if the answer ends abruptly here, it is obviously not my goal of popularizing science, and we also need to understand other issues related to the problem.
For example, why is everything moving? This may seem like a simple question, but there are many fundamental laws of physics that are hidden.
In physics class, we all learned Newton's first law, also known as the law of inertia. Any object will always remain stationary or move in a straight line at a uniform speed when it is not subjected to an external force or the net force is zero. To put it bluntly, force is not the cause of the motion of the object, but the cause of changing the state of motion of the object.
We see objects falling downwards every day on Earth because of the gravitational pull of the Earth. In the same way, if the state of motion of the earth in the universe changes, it is also closely related to the force on the earth.
And if the Earth does not experience any force in space, or the net force experienced by it is zero, then the Earth will remain stationary or move in a straight line at a uniform speed. It doesn't matter which direction you are moving, because there is no concept of direction in space.
However, we know that the Earth's space environment is not empty, and that the more massive Sun is nearby, and that the mass of the Sun accounts for 99% of the mass of the entire solar system86%, there will be a strong gravitational effect on the earth, and it is this gravitational effect that has allowed the earth to orbit the sun for billions of years.
Since the Earth is gravitationally attracted by the Sun, the state of the Earth in the universe is certainly not stationary or moving in a straight line at a uniform speed.
So how does the earth move? Theoretically, due to the strong gravitational pull of the Sun, it is natural that the Earth should move towards the Sun and eventually fall to the surface of the Sun, which is the same reason that we see objects on the surface of the Earth falling to the ground every day.
But this is not the case, although the strong gravitational pull of the sun, the earth does not fall to the surface of the sun, but keeps moving in a circle around the sun, of course, not a perfect circle, but an ellipse. Why is that? Why doesn't the Earth fall to the surface of the Sun due to the gravitational pull of the Sun?
The answer is very simple, so although the earth is subject to the gravitational pull of the sun at all times, in the process of motion, the earth also has a tangent velocity perpendicular to the direction of gravity, which ensures that the earth will not collide with the sun when it falls towards the sun, and will always miss the sun.
So, the Earth is actually falling all the time, and the direction of the fall is indeed the center of the sun. However, since the Earth still has velocity in the direction of vertical gravity, the Earth's fall is not a complete free fall.
The movement and fall of the Earth is understood by example, the Newtonian Cannon, the famous thought experiment, and the theoretical basis for the launch of satellites around the Earth.
This thought experiment goes like this, if you put a cannon on a high mountain and fire it horizontally, we will find that the cannon will always fall to the ground, but as the cannon is fired faster and faster, the cannon will fly farther and farther.
Of course, we have reason to believe that when the speed reaches a critical point, the cannon will always miss the surface of the earth, and eventually fly around the earth, in fact, it will become a satellite of the earth. Now we know that this velocity is the first cosmic velocity, 7 per second9 km.
While the cannon does not fall to the ground, we cannot deny that the cannon actually falls towards the Earth all the time, only because the break is fast enough in the horizontal direction that it will always miss the Earth. Imagine that if the cannon hadn't fallen towards the ground, it would have definitely flown out of Earth in the end.
Of course, human beings will not be so simple when launching satellites, Newton's cannon is just a theoretical basis, in fact, human beings will first launch the satellite vertically, rise to a certain height, and then let the rocket and the vertical direction form an angle, so that the satellite can get a fast enough horizontal speed, so that the operation efficiency will be higher.
And the relationship between the cannon and the earth is equivalent to the relationship between the earth and the sun, and the earth is equivalent to a super cannon.
Not only the Earth, but any celestial body in the universe behaves like this, and our sun will also fall due to the gravitational pull of a larger celestial body, a massive black hole at the center of the Milky Way, with a mass equivalent to 4 million times the mass of the Sun.
However, the Sun is also moving at high speed like the Earth, so it does not fall into a supermassive black hole, but orbits around that black hole, which takes about 2500 million years.
A larger structure than the Milky Way is the Local Group of Galaxies, and above it is the Virgo Supercluster, the Laniakea Supercluster, all the way to the observable universe.
The ultimate question is, is our observable universe also moving around something, is it also falling? This question is obviously beyond the scope of our understanding, and with the continuous improvement of human technology, there may be an answer someday in the future.
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