Question

which force is needed to maintain the speed of a body constant?

Answer 1

A balanced force is needed to maintain constant speed because according to the newton's first law : an object will keep its state of motion as long as no net force is acting on the object .

Answer 2

according to newtons first law , a static object with no net force acting on it remains rest or if in movement it will maintain a constant speed .
 
                       hope it helps