Why is oceanic crust sub-ducted under continental crust

The oceanic crust slides under the continental crust due to the differences in their densities. The continental rust is more felsic (contains more silica) which makes it lighter than the oceanic crust which is more mafic (contains more fe and mg)

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.

Awoodchuck would chuck all the wood he could chuck if a woodchuck could chuck wood.