1

Gravity would cause two objects in a vacuum to move toward each other. I understand that gravity is a force that exists as a product of energy's original conversion into mass and the continuing change in form of energy and mass. I also understand that the potential energy in the two objects changes to kinetic energy as a result of gravity. My real question is: what causes the form of energy to transform? Why do the objects' masses cause them to move?

  • 2
    Do you have a definition of mass other than "that which quantifies how objects move under gravity"? (If you answer something with inertia, be aware that though there's no demonstrated difference between inertial and gravitational mass, there's also no reason to believe there should(n't) be) – ACuriousMind Oct 21 '14 at 19:58
  • possible duplicate of How exactly does curved space-time describe the force of gravity? – ProfRob Oct 21 '14 at 20:10
  • That gravity can be described in the Newtonian limit with high precision by a potential is an observational fact (it is, for instance, not true for magnetic forces). That potential energy becomes kinetic energy is a consequence of energy conservation AND the accounting fact that there are only two types of energy at play. Consider the case of falling in an atmosphere. Even though gravity acts with an (almost) constant force, bodies quickly attain their terminal velocity and all additional work done by gravity is converted into air movements and, ultimately, heat. – CuriousOne Oct 21 '14 at 22:53

1 Answers1

0

Generically, if I have a particle which has potential energy $\phi(x,y,z)$, then the force on that particle will be given by ${\vec F} = - {\vec \nabla}\phi$. So, generically, the motion of particles will "try" to minimze the potential energy. In particular, the only points where the particle will not move will be those points where $\nabla \phi = 0$, or, in simple terms, the potential energy doesn't change under small displacements.

Furthermore, these points will only be stable stationary points if they also represent a minimum of the potential.

Zo the Relativist
  • 41,373
  • 2
  • 74
  • 143