Gravity Inside the Earth

If we bring a mass from far away toward the Earth, the gravitational force increases...the closer it gets, the bigger the force. What happens in a tunnel to the center of the earth Earth?

If you're inside the Earth, as you get closer to the center of the Earth does the force of gravity increase, decrease, or stay the same?

  1. increase
  2. decrease
  3. stay the same

A parallel question: what is the force of gravity on an object at the center of the Earth?

  1. zero
  2. infinite

The net force is zero - there's as much mass pulling you one direction as there is pulling the opposite way. The force of gravity decreases as you move from the surface to the center.

This is a consequence of Gauss' Law for gravity: inside a spherical shell of mass M, the force of gravity due to the shell is zero. Outside the shell, the force is the same as that from a point mass of mass M, placed at the center of the shell.