According to Newton's first law, if an object is slowing down, what must be happening? A. No forces are acting on it.

B. A single force is acting on it.

C. Balanced forces are acting on it.

D. Unbalanced forces are acting on it.

According to Newton's first law of motion, if the net force acting on an object is zero, the object remains at rest, or if the object is already moving, continues to move in a straight line with constant speed. Galileo realized the motion of an object doesn't change until an unbalanced force acts on it.