The Earth can be approximated as a sphere of uniform density, rotating on its axis once a day. The mass of the Earth is 5.97×1024 kg , the radius of the Earth is 6.38×106 m , and the period of rotation for the Earth is 24.0 hrs . A. What is the moment of inertia of the Earth? Use the uniform-sphere approximation described in the introduction.
I=?
B. Consider the following statements, all of which are actually true, and select the one that best explains why the moment of inertia of the Earth is actually smaller than the moment of inertia you calculated.
a- The Earth is an oblate spheroid rather than a perfect sphere. For an oblate spheroid, the distance from the center to the equator is a little larger than the distance from the center to the poles. This is a similar shape to a beach ball resting on the ground, being pushed on from above.
b- The Earth does not have uniform density. As the planet formed, the densest materials sank to the center of the Earth. This created a dense iron core. Meanwhile, the lighter elements floated to the surface. The crust of the Earth is considerably less dense than the core.
c- While the Earth currently has a period of 24 hours, it is in fact slowing down. Once it was rotating much faster, giving days that were closer to 20 hours than 24 hours. In the future, it is expected that days will become longer.
C. Where did the rotational kinetic energy of the Earth come from?
a- The solar system formed from a massive cloud of gas and dust, which was slowly rotating. As the cloud collapsed under its own gravitational pull, the cloud started to spin faster, just as an ice skater pulling his arms in will spin faster. Because all of the material that accreted to form the planet was rotating, the planet was rotating as well.
b- As the Earth formed, it experienced a series of collisions with asteroids and comets. These asteroids and comets hit the ball of rock that was forming into the planet off-center. Over time, the off-center collisions gradually caused the planet to rotate faster.
c- As the Moon orbits around the Earth, it creates tides on the Earth. Over time the tides have caused the Earth to rotate faster and faster.