Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

On my textbook is written that gravitational force is the force that attracts bodies with mass. But I've seen on a book that It actually attracts bodies with energy. I'm having a class tomorrow and I would like to know some argumments to use with - against my professor.

share|cite|improve this question
2  
How about you just listen to the professor instead of creating arguments for him? The gravitational force by definition is $Gm_1m_2/r^2$, i.e. it depends on mass. Now "energy is mass" is a statement that is tossed around without actually much thought involved, so this may be what your book referred to. Anyway, ignore it and stick with mass. – Chris Gerig Feb 22 '13 at 1:40
    
So afterall, my book is right. Is that what you`re saying? – beingthebe Feb 22 '13 at 1:47
1  
But considering general relativity It`s wrong? – beingthebe Feb 22 '13 at 1:56
2  
I wouldn't say it's "wrong." I'd say it's an approximation, and a very good one at that. – jld Feb 22 '13 at 1:58
1  
But if I agree with my teacher Ill be assuming that light isnt massless, which she is, right? Last question, I promise, last question. – beingthebe Feb 22 '13 at 2:06

In Newtonian gravity (what your textbook is talking about), the gravitational field couples to mass density, $\rho$. This is seen in the Poisson equation:

$$\nabla^2 \phi =4\pi G\rho$$

where $\phi$ is the gravitational potential. The argument you heard about energy comes from General Relativity, which is a more advanced (and more accurate/"correct") theory of gravitation. In GR, the gravitational field couples to the entire stress-energy tensor. The stress-energy tensor's components are energy density (which includes mass density by $E=mc^2$, kinetic energy, etc.), and momentum density. So gravity is really dependent on energy and momentum, not just mass.

share|cite|improve this answer

In relativity gravity is not a force anymore, instead it is geometry of spacetime. Therefore gravity (produced by energy which is equivalent to mass) would also attract massless particles without any force acting.

share|cite|improve this answer

protected by Qmechanic Mar 22 at 7:58

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.