## What makes the Earth rotate? (Intermediate)

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.

#### Laura Spitler

Laura Spitler was a graduate student working with Prof. Jim Cordes. After graduating in 2013, she went on to a postdoctoral fellowship at the Max Planck Institute in Bonn, Germany. She works on a range of projects involving the time variability of radio sources, including pulsars, binary white dwarfs and ETI. In particular she is interested in building digital instruments and developing signal processing techniques that allow one to more easily identify and classify transient sources.