Everyone knows the principle of perpetual motion machine. Are all celestial bodies like this?

Because of the conservation of momentum and angular momentum, every celestial body in the universe is constantly rotating and moving. In a sense, they are indeed eternal.

But a perpetual motion machine is more than just being able to keep moving. A perpetual motion machine in the true sense needs to keep doing work and moving. If it just keeps moving, the whole world will be a perpetual motion machine, because according to the law of inertia, an object will keep its original state of motion without external force, so if it keeps moving from the beginning, it will keep moving.

However, virtually everything in nature is doing work. For example, all kinds of perpetual motion machines designed by civil engineers inevitably rub against fixed objects, that is to say, even if they are idling, they are actually doing work on fixed objects (such as the brackets of equipment).

Although it seems to be a fixed object, friction will generate heat, which will be dissipated as power consumption, so the so-called perpetual motion machine will stop quickly under the action of friction. If someone really invents a perpetual motion machine, it can at least keep moving under the action of friction even if it doesn't do extra work, which is basically impossible.

So are the planets in the universe moving forever? It doesn't seem to produce friction because there is a vacuum around it. However, this is not the case. Although there is a near vacuum around the planet, it is actually not a complete vacuum because there are thin gases.

But these gases are basically negligible, and we can ignore them. However, even if this direct friction is ignored, there is another effect that will slowly stop the rotation of the planet. You just need to look up at the moon in the sky to know what it is.

Yes, even the tidal effect. The celestial bodies will slowly slow down their rotation because of each other's tidal force until they stop, and the rotation of the moon is locked by the tidal force of the earth. And the earth will one day be locked by the tidal force of the sun, but it will be billions of years later. ...

So what happened to the celestial bodies during the tidal locking process? Let's take the earth and the moon as examples to illustrate. We know that under the action of tidal force, the celestial body will bulge in the direction of connection, and the surrounding will be sucked in, which is the reason for the tidal fluctuation of the earth.

When the earth's rotation is out of sync with the moon's revolution, the projected position of the earth's moon line on the earth's surface is always changing, that is, the protruding positions at both ends of the earth are always changing (actually, because the moon is revolving, this protruding position is not completely on the earth's moon line, which we assume is better understood), and this telescopic change will produce friction and slow down the earth's rotation.

How does tidal effect slow down rotation?

For the friction inside the earth, we may not be able to imagine how it will slow down the rotation of the earth. Think about whether it is a feeling of being pulled up by hair. But you can imagine the friction on the surface, the most obvious is the beating of the waves.

That is to say, the seawater lifted by the tidal force of the moon will beat the east coast twice a day (once to the moon and once to the back of the moon), which will have obvious deceleration effect. Obvious here does not mean obvious effect, but easier to understand.

You might think, didn't it slap the west coast, too Yes, because when the local moon ring passes through the mainland, theoretically, the sea water from the east and west coasts will rush to the mainland, causing the sea water from the west to rush to the east coast.

But there is still a difference between the two, because the position of this uplift is moving from east to west at a high speed. Because the earth rotates from west to east, its influence on the east coast of the mainland will be stronger than that on the west coast, so under the influence of the tidal force of the moon, the earth's rotation will eventually slow down. Therefore, celestial bodies are not perpetual motion machines.