Answer the following question given this paragraph: 

In harmonic motion there is always a restorative force , which acts in the opposite direction of the velocity. The restorative force changes during oscillation and depends on the position of the object. In a spring the force is the spring force; in a pendulum it is the component of gravity along the path. In both cases, the force on the oscillating object is directly opposite that of the direction of velocity.


Q: In harmonic motion there is always what force, which acts in the opposite direction of the velocity?


A:
restorative force