The Applied Force: Fapp
- Any force could be considered to be an applied force, but we usally let Fapp stand for a force applied by a person or an action which directly pushs or pulls on a system.
- When we humans push or pull on an object we exert a force whose magnitude and direction are determined by us. This means that the magnitude and direction of such forces can never be calculated independently, like the weight of an object can be found from W = mg. Applied forces either have to be given or found using Newton's Laws. For example, if you pull down on a spring-scale and it reads 7.80 N, then you know by Newton's Third Law that the force you applied must be equal to that of the stretched spring.
- The applied force is just one of the many forces that could be acting on an object. It is not the big cheese, meaning Fapp is not equal to Fnet. If it is the only force acting on an object, then the applied force will be equal to the net force by default. It is extremely tempting to want to set Fapp equal to ma, but here again Fapp is not equal to ma unless it is the only force. This tendency may arise from the observation that when you personally change the magnitude of the force you are applying, the system's behavior will change. However, this is also true for any other force acting on the system--change it and the subsequent motion will change. An object's acceleration is a consequence of ALL the forces acting on a system, not just the applied force.