Re: How is gravity related to mass and does gravity always exist?

ALL, repeat, ALL, masses interact gravitationally. Conceptually the gravity that we experience that keeps us on the surface of the earth is exactly the same as the gravity between the two apples on the strings (in your example). The magnitude of the gravitational force between the two apples is, as you have said, smaller than that between either of the apples and the earth, but the force is the same type of force. Both the apples on the strings are also attracted by the earth, so the net force on either apple is the vector sum of the gravity from the earth and the gravity from the other apple and the tension of the string (there will also be gravitational forces from the other objects in the room!).

John Link, MadSci Physicist