Err, two questions related to center of gravity and friction

[YenTe]

"since the distance of forces(the center of gravity forces) from the gravity are at a very large distance so they are just like parallel forces"

now what do parallel forces have to do with distance or vice versa?
Explain.

and then please explain the friction of sand.

anyone?!

 

[radou]

Since the distance to the center of the Earth is very big (around 6300 km), all gravitational forces can be considered acting in parallel directions. For example, imagine two objects on the surface of the Earth resting close to each other, let's say one meter. If you draw a sketch, and if you connect every object with a straight line to the center of the Earth, these lines will not be parallel. But since the distance to the center of the Earth is, as mentioned, very big, these two lines can considered to be parallel, which simplifies calculations, but still keeps them in limits of satisfying accuracy.