A central force is one that depends only on the distance r between the moving particle and a point fixed in space, the center.

Mark point 'B' at a distance 'r' from it, and join to form a line segment.

The attractive force on unit mass a distance r from a body of mass M is just.

As above, the distance r from the slit 1 is:

Find the electric field at a distance r from the centre of the cloud both for r<a and r>a.

Assuming that, at this large distance r, a is smaller than a, .

Mark point D in a third, orthogonal, dimension a distance r from all three, and join to form a regular tetrahedron.

This is because it provides a very simple relationship between distance traveled around the circle and the distance r from the centre.

For attractive forces, F(r) is negative, because it works to reduce the distance r to the center.

In acoustics one usually measures the sound pressure at a given distance r from the source using the 1/r law.