Gravity was mentioned. I can help out with that.

Gravity is one of the four forces, the other three are the strong nuclear force, the weak nuclear force and the electrostatic force. Gravity is the least well understood of these forces, it is a force that mysteriously arises as a result of a body having mass, although mass is pretty mysterious and there are a few theories floating around about that. Anyway, you probably don't want to know all this so lets jump into the physics of it.

Gravity is always an attractive force (although I've just created a thread in this very forum where it suggests to me that it isn't). To fit in with electrostatics we say that an attractive force is always negative. The reason for this is because protons and electrons attract, if you multiply the charges +1 * -1 you get -1, therefore the attractive force is negative.

If you have a 1000Kg car, the Earth pulls down on it with a force of 1000g N where g is the graviational field strength, this is just what the acceleration due to gravity would be if the ground suddenly weren't there. g can also be thought of as the force in Newtons that acts vertically downwards on a body of 1Kg.

There are some more complicated equations to deal with the forces of attraction between two masses for a certain separation of their centres and the change in gravitational potential when moving from one point in a gravitational filed to another. The equations for the two I've just mentioned are:

```
F = - G*m
```_{1}*m_{2}
--------
r^{2}
V = - G*m_{1}
------
r

Where G is the gravitational constant 6.67*10

^{-11}
If you are dealing with stuff near the Earth's surface then not much of this will have helped yet, I'm very sorry but there isn't really much to do about gravity near the Earth's surface other than constant acceleration and potential energy changes.

Since you did mention energy, I thought that I might show the equation for potential energy changes near the Earth:

It's considerably simpler that the other formulae and the mass of the Earth doesn't need to be included in the formula as it is assumed that g will stay constant.

My description of gravity and the laws thereof has probably been a little bit patchy because I don't have anything to bo specific about, so like chroot said, if you want better explanations then you're gonna need to post an example of the kind of question that you're having problems with.