: : The outer mass doesn't attract out. As we said earlier the net force on you in the hole at the center of the earth was zero.

: Zephram, this doesn't make sense to me. If, as you said, the net force at the center is zero, it must be because the surrounding mass is pulling outward.

The superposition of the field is zero, that why it does not pull outward. Your weightless everywhere in the hole in the center, not just at the very center. This is why there is no net pull outward.

:...At some point between the center of the inner, and the surface of the outer ball, is a point where the gravitational attraction is equal because the amount of mass above and below that point is equal. If that is true then it seems to me that this would prohibit a crushing of surface objects by gravitational force. I still believe gravity alone can never crush an object without first trapping it under some other object.

In the earth example its not directly crushed by gravity. What's in the center is crushed by pressure. The only time gravity alone can crush an object is when it has a large tidal force.

:Consider the delicate snowflake resting on its edge at the surface of the earth. By the way, I am truly enjoying this exchange and wish I had the skills to do the related math.

To do the math for the earth example just requires a little calculus and an understanding of superposition to work out the 3/4 I mentioned in the sliced earth case and Gauss's law in the hole in the center case.

• You're thinking about it in the wrong way - Wiley - November 7, 1999 - 23:00 UTC
  • Re: You're thinking about it in the wrong way - Wayne/">Wayne - November 8, 1999 - 04:00 UTC
    • No it doesn't - Wiler - November 9, 1999 - 23:51 UTC