 A simple example is Newton's second law of motion — the relationship between the displacement "x" and the time "t" of an object under the force "F", is given by the differential equation

which constrains the motion of a particle of constant mass "m".In general, "F" is a function of the position "x"("t") of the particle at time "t".The unknown function "x"("t") appears on both sides of the differential equation, and is indicated in the notation "F"("x"("t")).