why does the earth rotate on its axis

As a father and a science lover I was dumbfounded when my 7yr old son asked me "What makes our earth turn (rotate)? He then asked "Did it start a long time ago and it just keeps turning or is something pushing it to turn"? How does the Earth continue to rotate around its axis? Where does the energy to keep it moving come from? Our everyday experience teaches us that an object must be "pushed" by a force in order to keep it moving. Otherwise, it will slow down and eventually stop. But this intuition is absolutely wrong. If an object is moving, then a force is required *to slow it down or stop it*, not to keep it moving. (Hence, "Objects in motion tend to stay in motion. Objects at rest tend to stay at rest. ") In our everyday experience, it's the force of friction that tends to stop Earth-bound objects from moving forever. But for the Earth rotating on its axis, there is no force working to counteract the rotation (except the tidal effect of the Moon, but that's working very slowly), so you don't need to have any input energy to keep it spinning.

What started the earth rotating in the first place? The shortest answer is angular momentum. Angular momentum is simply the name we give for the fact that things tend to rotate. (Just like regular momentum is the tendency for things to move. ) The Earth formed out of a nebula that collapsed. As the nebula collapsed it began rotating, which may seem odd, but actually not rotating is far stranger than rotating. The Earth's rotation comes from the initial tendency to rotate that was imparted on it when it formed, only the relatively weak tidal forces from the Moon act to slow it down. This page was last updated on June 27, 2015.
Though we can't feel it, planet Earth is constantly spinning beneath our feet.

The Earth rotates on its axis, an imaginary line that runs through the center of the planet, through the North and South poles. The axis is the Earth's center of gravity, around which it rotates. Though spinning at 1,000 miles per hour, the Earth takes 24 hours to make a complete rotation. Scientists continue to work towards an understanding of why the Earth spins and continues to rotate on its axis. Most scientists speculate that a shock wave from a supernova went through a cloud of cold hydrogen, forming a solar nebula. The momentum caused the nebula to spin into a planetary disk. When the solar system was being formed, it is likely that collisions of these clouds contributed to the tilt and rotation of the Earth as we know it today. The laws of physics state that an object that is in motion will remain so until an outside force acts upon the object.

The Earth keeps spinning because there is nothing to stop it, as space is a vacuum. Not even earthquakes have been able to keep the earth from its rotation. While it is unlikely that any outside force will act on the Earth to stop its spin, the rotation of the planet is slowing. This is caused by tidal friction created by the oceans' movement. Tidal friction is caused by the of the moon. The result of tidal friction is that over the course of a century, the length of day can be extended by a few moments. The axis that the Earth is situated on is not a vertical line, but is at a 23. 5 degree tilt. This angle is what causes the different climates and seasons at varying times around the globe. In addition, human beings mark time by the Earth's rotation. One full spin encompasses the measure of a day.

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