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The Question

(Submitted March 26, 1997)

I have been searching all over the place to find something about gravity wells of planets, stars and moons. I am in Grade 12 and would like some information accordingly.

The Answer

I assume that by "gravity wells" you are talking about the "gravitational potential" well associated with any object that possesses mass.

First, I need to make sure you understand what a "well" is in the world of physics. In the world of physics, a "potential" can be thought of as an energy field. There are 2 basic types of potentials, "barriers" and "wells". They get their names from what they look like when you plot them.

Now, applying this "barrier" and "well" idea to gravity, the shape is a well (assuming a constant gravitational field). So we talk about "gravitational potential wells". The equation in such circumstances is that the gravitational potential energy is equal to the one object's mass times the acceleration due to gravity of the 2nd object times the distance they are apart. In the case of two stars attracting each other or a particle being attracted by a neutron star, the gravitational field is not constant. It falls off as the square of the distance between the 2 objects. So the resulting equations get more complicated. Nevertheless, gravity is a wonderful thing...any two objects which possess mass will always exert a gravitational attraction on one another and this "potential" source of energy can be converted into other kinds of energy...such as heat, light, and sound. Try this: hold a book a couple of feet from the floor. Let go of it. Congratulations, you have just converted gravitational potential energy which the book possessed by being in the gravitational field of the Earth into sound! Conversion of gravitational potential energy into other forms of energy has some practical applications. When NASA launches an interplanetary satellite, it often takes advantage of a "sling-shot" effect to increase the satellite's speed. This is called a gravitational assist. In fact, the satellite Galileo actually came back to the Earth twice for gravitational assists on its way to Jupiter. You can see a diagram of this at

Hope this helps.

Andy Ptak and Laura Whitlock
for Ask an Astrophysicist

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