Question

A rock moves in a straight path in space with constant speed. Is there any force acting on the rock? Is there any work done when the rock moves?

Answer 1

This is what Newton's First law states  " A body remains in a state of rest or uniform motion, unless it is acted upon by an external force. "
 You will get an idea from the following explanation ,
 Let us imagine a force F acts on a body of mass m ,  at rest. What will happen to the body ? 
It will have an acceleration a , such that a=Fma=Fm
it means that the body has an acceleration a, which is independent of time. It means it has the same acceleration at all times. 
so what about its velocity ?
It will have a velocity, v, such that  v=a∗tv=a∗twhere t is the time during which it has travelled after the application of the force F.
As  t tends to ∞∞, Velocity,v=a∗tv=a∗t    also tends to ∞∞, which is not possible but it has never been observed in real life. Because , in real life scenario, an accelerated body is constantly being acted upon by frictional  force from the surface below. This frictional force from the surface acting on the every object which are moving on the surface. The frictional force is responsible for duration of all motion on earth to be of finite time.  
Without break, if the acceleration is not given, the vehicle can be brought to rest after finite time because of frictional force from the surface ( without accident when there is no vehicle coming from the opposite side).
Now you would be clear on the uniform motion, the body will keep on moving in constant velocity if there is no frictional force from the surface.
This was actually first reasoned out by Galileo Galilei.  After his reasoning, only Newton put forth his first law of motion.

Answer 2

It is applied a Newton first law at the external force is applied on it