Question

Is retarding force always negative?

Answer 1

yes retarding force is always negative

Answer 2

Look at Newton's Second Law:

" Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object."

A retarding force, is simply put, the force that causes the acceleration of an object to be negative.

In F = ma , where F is the resultant force, the force that is against the direction of the object's current velocity is the retarding force.

For example, for a car, its forward force is F from motor. When the brake is applied, F resulting from brake  is the retarding force as it is against the car's velocity.