Earth rotation: What provides the energy?

Question

We all know the earth is rotating around its own axis. From a physics energy point of view, some thing has to provide energy for the rotation. What provides the energy to rotate?

Answer 1

The original energy that caused the rotation of the Earth when it was formed came from the gravitational collapse of the cloud of dust and gas that formed the solar system. Maintaining rotation at constant speed in a vacuum does not require any further input of energy. Conservation of angular momentum keeps the Earth rotating.

In fact, over long time scales (millions of years) the Earth’s rotation does slow down gradually due to energy lost in friction and tidal forces.

Answer 2

If there is no friction a rotation needs only initial energy. But earth has some friction caused by the tides, so the rotation gets slower with the time. But its only about 1ms less per day in 50 years.

Answer 3

Angular momentum is a conserved property in physics. Once something rotates, one needs some effort to make it stop.

In our usual environment, friction acts to stop the rotation of different object, no matter if we want it or not. That's why we are used to the need of energy to sustain rotation (or any other movement).

Earth, the other hand, does rotate (as far as we know) from before it was formed. The gas and dust that formed the Earth rotated and nothing stopped them as they coalesced into a planet.

Earth doesn't experience much of a friction or torque, compared to its own moment of inertia. There is near-perfect vacuum around. The major force slowing down the Earth is the tidal friction. The effect of tides is very small on human timescale, but measurable with modern tools. There is also geological evidence of shorter days (=faster rotation) in the distant past.

Answer 4

Your question is a bit unclear, so I'll give two answers:

1. Nothing needs to provide energy for the earth to keep spinning. In fact, the rotation is slowly slowing down due to angular momentum being lost to the moon, losing energy to the moon's orbit (it's pushed away from earth), and the tides (lots of friction).

2. However, the energy does have to have come from somewhere. And it did. Notice that I said that the moon is getting farther away from the earth, slowly. Where do we get to when we rewind time? The moon must have formed close to an insanely fast-spinning earth. Where did this fast-spinning come from?

   Well, the current theory on this is, that both earth and moon formed in a giant collision between the proto-earth and another planetoid called Theia (https://en.wikipedia.org/wiki/Theia_(planet) ). This impact was not head-on, Theia hit the earth with quite an offset. It is this offset that supplied the angular momentum to give the cloud of debris a fast twist. This is the energy and angular momentum that earth is still drawing upon when it accelerates the moon along its orbit and makes the tides rise and fall throughout the planet.