Why does the Earth rotate?

This might come to you as a shock but there is no force that causes the planets (and, of course, our earth) to rotate. Most of the rotation comes about from the conservation of angular momentum.

2013-04-20 07_25_26-Why does the Earth rotate_ - Windows Live Writer

In this equation, m is the mass, w is the angular velocity in radians per second, and r is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant).

Explanation

All planetary and stellar systems are born from the collapse of dense interstellar clouds. The clouds may originally be very large (ever thousands of light-years across). Consider a portion of the cloud the collapses from a size of a light year or so to the size of the solar system. That is a huge change in the size of the system. So, the very slight rotation that the cloud has in the beginning is increased dramatically when the collapse takes place. In fact, this is one of the barriers, in star formation: there is excess angular momentum and there has to be a way of losing angular momentum before you can form a star.

Are there some laws also in the rotations?

Rotation results in a centrifugal acceleration that points radially from the centre of motion. So, there has to be some force that counteracts this acceleration; otherwise the body will literally fly away (in the case of orbital motion) or it will disintegrate (in the case of spinning). In the case of orbital motion, the counteracting force is gravity; I also explained this in my article How do astronauts float in space? Gravity causes the body to continually fall towards the centre, and this exactly counteracts the force resulting from the centripetal acceleration. In the case of a spinning object, it is the self-adhesion of the body itself that keeps it together. If it rotates too fast, the outwards acceleration felt by the elements in the body may be more than the force that keeps them bonded together, and if this happens, the body will break up.

Other than this, there is no real law concerning rotations.

If the Earth is spinning, why do we land on the same place when we jump or fall?

We’re all on the moving Earth, and we’re travelling at the same speed as Earth. So when we jump up, we keep travelling around at the same speed we were moving at before because there is no force to stop us. Now, if a huge force was applied to the solid Earth and caused it to stop spinning in a single instant, we would be in trouble because the Earth would have stopped moving, but since no force was applied to us, we would still be travelling at the same speed we were going before the impact. I guess if all the people were glued to the Earth then the force of impact would translate to us as well and we would slow down.

 

Imagine this as a car accident – if you’re travelling really fast on a road and you hit something and it made you suddenly stop, your body will fly forwards because you had a forward velocity and it will tend to stay in motion in that direction. This is known as inertia. Similarly, if Earth stopped really fast and we weren’t held down, we would fly pretty fast.

Why do some planets rotate the other way around?

Every planet in our solar system except for Venus and Uranus rotates counter-clockwise as seen from above the North Pole; so it is from west to east. This is the same direction in which all the planets orbit the sun. Uranus was likely to hit by a very large planetoid early in its history, causes it to rotate “on its side,” 90 degrees away from its orbital motion. Venus rotates backwards compared to other planets, also likely due to an early asteroid hit which disturbed its original rotation.

References & Resources

  1. Earth’s rotation – Wikipedia
  2. Angular momentum – Wikipedia
  3. Ask an Astronomer, Astronomy Department at Cornell University
  4. Patrick Pak-Cheuk WU. (2002). University of Calgary: Dynamics of the Postglacial Earth: A Modern PerspectiveEarth Rotation, Jn-dot, Nonlinear Rheology
  5. Patrick Maher. (Fall, 2009). Lecture 18: Copernicus on the Earth’s Daily Rotation
  6. Ann-Marie Pendril. (2008). Department of Physics, Göteborg University: How do we know that the Earth spins around its axis? 158-164. IOP Publishing Ltd.
  7. Slider image via Wikimedia Commons. Retrieved from:
    http://commons.wikimedia.org/wiki/File:Stars_Circle_over_the_Residencia_at_Cerro_Paranal.jpg
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