is there gravity at the centre of the earth?, page-32

At the risk of reigniting this topic: It should be noted that there is a gravitional force acting on us that is also due to the moon and everything else in the universe. Because the earth is very close by and the moon and everything else are distant these non earth forces are small, but they are there all the same. At the centre of the earth the distribution of mass is symetrical about the centre. So the net gravitational force is zero (ignoring the moon in this case) and the acceleration due to gravity is therefore zero. The acceleration due to gravity (the force that is pulling you towards the centre) will decrease as you fall towards the centre of the earth because the amount of matter that is behind (above) you is increasing and the amount of matter in front of you is decreasing. At the centre the gravitational forces are symetrical and therefore they balance. If the centre of the earth was hollow (which it isnt!) you would be weightless. If an object was falling down a tunnel from the surface of the earth to the centre of the earth and out the other side it would oscilate about the centre until frictional forces brought it to rest. This would of course be impossible to achieve in practice due to the rotation of the earth causing the object to hit the side of the hole (ignoring also the temperature and pressure of the interior of the earth). At the point of release, the object has a constant velocity parallel to the surface of the earth which is equal to the speed of the earth's rotation. The rotational velocity of any point on or inside the earth decreases towards the centre, so a falling object cannnot fall in a straight line to the centre of the earth unless it is dropped at the axis of rotation (pole).