# Can gravity increase on its own?

1. Jun 16, 2012

### schiz0ai

Forgive me, for i do not have any degrees in physics.:shy:
If 1 large object like a planet were to orbit near another large planet/moon.
It would in theory be possible for both objects to increase thier speed of rotation, as long as they end up rotating in opposite directions right? Of course it would require alot of energy.
But would that mean that the total ammount of gravity of both objects could increase

2. Jun 16, 2012

### Staff: Mentor

Sorry, your post looks a bit unclear in which order you want to have what.

You can remove some energy from the system, which reduces the radius between the objects (and therefore reduces the potential energy) and increases their velocity (and kinetic energy). The gravitational force between both increases, as their distance decreases.

What is a "total amount of gravity"?

3. Jun 17, 2012

### schiz0ai

the ammount of gravity a planet has depends on its mass and rotation speed right?

Of course its impossible for a single object to start rotating faster on its own.
But if it does this by makin another object rotate faster in the opposite direction, it should in theory be possible.

Would the gravity of both objects increase at the same time? because thier rotation speed increases?

4. Jun 17, 2012

### Staff: Mentor

Gravity does not depend on rotational speed. Just look at Newton's gravitational force equation.

5. Jun 17, 2012

### rbj

actually, Russ, if we get relativistic here, wouldn't the effective mass of the planet increase as the energy contained therein increases? and if its effective mass increases, wouldn't the amount of curvature of spacetime increase? perhaps you'ld have to spin the planet so fast that it would fly apart long before there would measurably be any increase of gravity that the planet would effectively "emit".

6. Jun 18, 2012

### Staff: Mentor

If you use non-standard terms, please define them. What is "the amount of gravity"? The gravitational force on other objects just depends on the mass.
The rotation can become important if the involved velocities are close to the speed of light (or the measurements are extremely precise), but that happens in neutron stars only.

7. Jun 18, 2012

### schiz0ai

im sorry, i thought gravity depended on rotation speed.
I figured it out after googling some.

8. Jun 20, 2012

### vociferous

In Newtonian mechanics, surface gravity does depend on rotational speed (some of the gravitational force of the Earth's mass is canceled out by the acceleration generated by the rotation of the Earth) but it does not affect the gravitational force between two particles, ie, the Earth and the Sun.