Relative velocity is the velocity of an object in motion being observed from a frame of reference that is either in motion or stationary.

If the frame of reference is moving in the same direction as the object being observed, it will appear as though the object is moving slower than it is.

If the object being observed is on a moving surface, the velocity observed from that surface will be less than the velocity observed from a stationary surface looking onto the moving surface.

In Galileo’s example, the observed velocity of the fly, u, measured the velocity of the boat, v.

To find the velocity of the fly concerning the shore, S, he had to add the velocity of the boat to the observed velocity of the fly:\( s=u+v S=u+v\)