If a spring is stretched by a small displacement (say #x#), there is a force the spring exerts in the opposite direction tending the restore it's initial configuration of the not stretched state.
The force (within limits) is found to be proportional to the displacement,
#F = -kx# where negative sign signifies that the force is directed oppositely to the displacement.
The constant of proportionality #k# is called the spring constant and by magnitude is defined as, #k = F/x# thus if the value of spring constant is #a#, then for a unit displacement the restoring force exerted by the spring is #a# units.
