According to the Newton’s law of inertia (Newton’s First Law of Motion) the body can move with a constant speed only if it is acted upon by the balanced forces.
In this question as the body of 10 kg is sliding with a constant velocity the gravitational pull of the earth on it is balanced by the opposing force (friction). So the force balancing the gravitational pull of the earth is equal to its weight = 10 ⨯9.8 N = 98 N
