What always indicates the presence of forces acting on an object?

The presence of forces acting on an object is indicated by accelerations, which are changes in velocity. According to Newton's second law of motion, if a net force is acting on an object, it will result in an acceleration proportional to the net force and inversely proportional to the object's mass. This means that whenever there is a change in the speed or direction of an object's motion, it signifies that a net external force is acting upon it.

In contrast, if an object is experiencing zero net force, it will either be at rest or moving with constant velocity, meaning no acceleration is present. Changes in temperature do not directly indicate forces acting on an object, as temperature changes may occur without any net force. Lastly, an object moving with a constant velocity is not experiencing any acceleration, implying that the forces acting on it are balanced, and thus, no net force is at work. Thus, acceleration is the key indicator of forces influencing an object's motion.