# Lighting moving objects(Exposition)

### Illumination and circular movement

Now we bring together ideas about movement with ideas about illumination — so drawing on the work on seeing things. The evidence of our eyes — what we so carefully notice — turns out to need careful interpretation in order to make sense of the observations.

### Explaining changing patterns

To make anything move along a circular path it is essential to have a force that acts towards the centre of that path.

For example, to make a rubber bung travel around in a circular path on the end of a string, there must be a force acting towards the centre of the motion. The force acting towards the centre of the motion is called a centripetal force.

The word centripetal is a combination of two Latin words: centrum meaning centre and peto meaning to go in. The centripetal force goes in towards the centre.

It's necessary to have a centripetal force to maintain a circular motion because if there is no resultant force acting on an object (that is all forces acting on the object add to zero), then the object travels with uniform motion in a straight line, or stays at rest.