Final answer:
A negative number in the net-force equation indicates the force is acting in the direction opposite to the predefined positive direction. It often means deceleration, opposing motion, or energy being removed from a system, as in the case of frictional forces acting against motion.
Step-by-step explanation:
In physics, a negative number in the net-force equation indicates that the force is acting in the opposite direction of the positive direction assumed in the problem. For example, if the positive direction is to the right (a common convention), then a negative force would act to the left. In dynamics, this could mean that the object is being decelerated if the force acting against the motion is greater than other forces propelling it. When an object is moving at a constant velocity, such as a man pushing a box across a floor, the net force is zero. This is represented by opposing forces that are equal in magnitude but opposite in sign, like a +50 N push counteracted by a -50 N frictional force.
The negative sign can also indicate the nature of work done. When the work done on an object is negative, it signifies that the energy is being removed from the object, as would be the case when friction does work against the motion of an object. Similarly, in situations where direction of motion is defined, a negative net force implies that the actual movement or the acceleration vector is in the direction opposite to what is designated as positive, such as downward or to the left in a standard coordinate system.