At the centre of a very massive object, the gravitational force is zero. Is that net zero or is it gross zero? What I mean is: If two people are pulling on my limbs with equal force in opposite directions, my net movement is zero, but I am under great stress. The net forces on me sum to zero, but the gross forces are that of two people pulling on me. Is it the same with gravity? If I were at the center of the Earth, is there a test I could do to measure the absolute gravitational pull on me?