# Why is kinetic energy due to a planet's rotation treated as negative?

• 21joanna12
In summary, the conversation is discussing the calculation of the ratio of polar diameter to equatorial diameter for a typical neutron star. The question also asks about the understanding of why the gravitational potential energy and kinetic energy are of the same sign. The solution document provided explains that the rotation of the neutron star generates an effective potential and that the object at the equator has a higher gravitational potential compared to the object at the pole due to the rotation. Additionally, the document mentions an incorrect assumption about the gravitational potential for an oblate spheroid.

## Homework Statement

I am trying to answer the following question:
1. Among the animals that appear in the zoo of the Universe there are black holes and neutron stars. The mass of each of these is often the order of the mass of the Sun. The radius of a neutron star is about 10 km and a certain neutron star rotates at about 100π s-1.
Calculate the ratio of the polar diameter to the equatorial diameter for a typical neutron star. Assume that liquid or solid neutron material is incompressible. How do we know that it is rotating at this frequency?

## The Attempt at a Solution

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I was finding it ok, but there was just one part that I cannot understand. In the answers, when the surface of the neutron star was considered as an equipotential, the kinetic energy due to rotation of the star was treated as negative, in the same way as the gravitational potential energy. But surely it should be positive, because many space probes are launched from the equator because the kinetic energy due to rotation gives them some energy to start overcoming the gravitational potential energy, so less fuel is needed. So I don't understand why the gravitational potential energy and kinetic energy are of the same sign... My iPad is not allowing me to copy and paste the derivation here due to formatting, but I have attached the solutions document, and this is question 5 b)