# Negative gravitational force

1. Jan 22, 2006

### matt85

What does it mean if between an electron and a proton has negative gravitational force? Does this make the force attractive or repulsive?

2. Jan 22, 2006

### .:JimmY:.

Verbally, It means two masses (electron and proton -in your case) will be replelling each other.
But it is not possible b'coz Gravitational force depends on the product of masses and distance - and under any circumstance there is no way the product is coming out as "negative" to make a negative gravitational force.

3. Jan 22, 2006

### matt85

In the answer to a question last week from class, it was in fact negative. I know that for electrical force this would be attraction, but what would this mean for gravitational? Repulsion?

4. Jan 22, 2006

### .:JimmY:.

Well it couldn't be Gravitational force -you are talking about then. Either you have mistaken something else as Gravitational force or there is something wrong with your solution to the problem.

5. Jan 22, 2006

### rcgldr

The theories indicate that centripal force and the speed of an electron keep it orbit. The speed of electrons increases in fixed steps, each step representing a higher or lower kinetic energy state. I assume that the lowest step of engery is enough to prevent the electron from collapsing into the nucleus, else matter would be real unstable. When electrons step down in energy, they often release photons (I'm not sure of the exact requirements for photon release). Electrons can also absorb photons, but I'm don't remember if it's enough to jump them up to the next enery state.

It's pretty complicated, some orbital patterns are though not to even be elliptical, but figure 8 like. A collision between an electron and positron results in total conversion of the matter into energy. I don't know what happens with an electron / proton collision.

6. Jan 23, 2006

### pmb_phy

GR allows for negative gravitational force (aka antigravity) on cosmological scales. Since the universe is expanding at an accelerate the phenomena is said to result from antigravity which in term results from a non-zero cosmological constant.

Pete

7. Jan 23, 2006

### Staff: Mentor

I don't understand what you mean. Can you give us the exact question from your class?

"Negative" just indicates a direction; of course, for gravity the force is always attractive. Whether that's negative or positive, depends on how you have defined things.

Perhaps you talking about Newton's Law of gravity written this way:

$$\vec{F} = - G \frac{m_1 m_2}{r^2} \hat{r}$$

In this formula, F represents the gravitational force of $m_1$ on $m_2$, where $\vec{r}$ is the position of $m_2$ with respect to $m_1$. The negative sign just means that the force is opposite to the direction $\hat{r}$, which means that the force on $m_2$ points toward $m_1$.

8. Jan 23, 2006

### Robine

negative gravity means repulsion as posted above. But conventional matter does not exhibit repulsive gravity. It is represented in general relativity byu the cosmological constant. Einstein showed that gravity is based on mass, energy and pressure. Negative pressure means begative (repulsive) gravity. "The Fabric of the Cosmos" by Brian Greene discusses this in Chapter 10.