A virtual force required to apply Newton's Laws to rotating coordinates.
The animation below shows a laboratory experiment to demonstrate how a rotating coordinate system gives rise to a virtual force. A small marble rolls back and forth across a shallow rotating bowl under gravity (below left). When both the bowl and the marble are observed from outside (in an absolute coordinate system), the marble moves back and forth in a straight line, while the bowl rotates under it (upper left).
When the observer and the coordinate system rotate with the bowl, the marble moves on a circle (right). To explain the circular motion, the observer has to invoke a force that deflects the marble from a straight path. This virtual force is the Coriolis force.
Animation: © M. Tomczak, Creative Commons license