This clip demonstrates the effects of the Coriolis and Centrifugal forces, by viewing various scenes from both rotating and stationary cameras. (The Coriolis force is also known as the Coriolis effect).
The first example shows a cannon fixed to a rotating disc. The cannonballs fly in straight lines since once shot no force acts on them. When this scene is viewed from the disc's frame of reference (i.e., as would be seen by a viewer that stands on the disc) the cannonballs seem to fly in a curved path. This demonstrates that in a rotating frame of reference one must take into account the Coriolis and Centrifugal forces. (Read more about them in Wikipedia: http://en.wikipedia.org/wiki/Centrifu..., http://en.wikipedia.org/wiki/Coriolis...)
The second example shows a pendulum swinging over a rotating disc. A pendulum swinging through a small angle approximates what is called "harmonic motion" in which the ball is pulled to the center by a force proportional to its distance to the center. In the pendulum, the string exerts a force whose vertical component balances gravity and the horizontal component (shown in the clip) causes the harmonic motion (approximately). The disc and the pendulum has the same period, meaning both complete a cycle at the same time.
When viewed from the disc's frame of reference the centrifugal and Coriolis forces appear yet again. This time the centrifugal force balances the string's horizontal component. This is because the centrifugal force is also proportional to the distance from the center but pushing outward instead of inward. The equal periods make the factor of proportion the same for both. This leaves the Coriolis force alone to act on the ball. Since it is always perpendicular to the object's path, this creates a perfectly circular path.
Finally, we decrease the pendulum's period to be 2/3 of the disc's period. Now the centrifugal force no longer balances that of the string and the motion becomes more complicated.
Note that there's a small scaling error in the visualization: the coriolis force shown should actually be double in length.