Question

Can the FBD only completely describe the object's motion status?

Answer 1

No.

FBD does not describe the motion of the motion. However it does give us the complete information about all the forces acting on the object. This in turn gives the acceleration of the object.

But acceleration alone does not contain complete information of the motion of the object.. Velocity and acceleration together give us the complete information.

Eg.

A ball thrown vertically up with some velocity. If we draw the FBD we realize that the only acting force is gravity (neglecting the air resistance).

The direction of acceleration is downward but the velocity is upward and hence the object moves upward with decreasing velocity. Thus we see that FBD gives information of forces and hence acceleration only.