Learning goal: to be able to find the centroid, center of mass, and center of gravity for lengths, areas, and volumes of simple, uniform objects. the centroid of an object is the object's geometric center. for objects of uniform composition, the centroid is also the object's center of mass.

a mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. this uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.a geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.