Collisions in Two Dimensions

All objects with mass attract each other. The size of the force between two objects of mass m1 and m2 is
```
                  m1 * m2
      F  =   G * ---------
                    r^2
```
where r is the distance between the two objects, and G is the gravitational constant.
It turns out that gravity is very weak -- that means that the value of the gravitational constant G is very small:
```
       G  =  6.67 x 10^(-11)  N * kg^2 / m^2
```
In fact, the gravitational force is so small that it's very difficult to measure the force between any two ordinary, everyday objects (such as people or cars). Scientists have had to resort to ingenious and delicate equipment to measure such tiny forces.
On the other hand, gravity is the dominant force between objects on the very largest scales: planets, stars and galaxies. So, even though it's weak, it still plays a big part in the universe.

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