Force, fundamentally, is an interaction that, when unopposed, will change the motion of an object. These interactions are described in physics as a push or a pull, and they are responsible for every instance of movement and stability observed in the universe. To accurately analyze these actions, scientists must classify forces based on where they originate relative to the object being studied. This systematic classification is necessary because not all forces acting on an object actually contribute to a change in its overall movement. The distinction between an outside force and other kinds of forces becomes the first step in understanding and predicting how matter behaves.
The Essential Role of Defining the System
Before any force can be classified as external or internal, the scientist must first establish a boundary around the object or collection of objects under investigation. This defined object or group is known as the system, and it is the reference point against which all forces are measured. Everything outside of this boundary is considered the surroundings or the environment. The definition of the system is entirely determined by the person analyzing the problem, allowing for flexibility in the analysis of complex scenarios.
For example, if analyzing a car crash, the system could be defined as just one of the cars, or it could be defined as both cars combined. Once the system is chosen, forces are categorized based on whether they cross the imaginary boundary separating the system from its surroundings. This boundary is not physical but is a conceptual tool that determines the status of every force acting on the objects. If the defined system changes, the classification of the forces acting on it must also change.
External Force Versus Internal Force
An external force, often referred to as an outside force, is precisely defined as any force originating from the surroundings and acting upon the objects within the defined system. These forces cross the system boundary and have the ability to influence the system’s overall state of motion. The source of the external force is never included inside the boundary of the system itself.
In contrast, an internal force is an interaction that occurs exclusively between objects or components located within the system boundary. For instance, the gravitational pull between two astronauts in a defined system is an internal force. According to the principles of physics, these internal forces always occur in equal and opposite pairs between the interacting components.
Because they cancel each other out vectorially, internal forces cannot change the total motion of the entire system as a whole. Only external forces, such as a push from a person outside the system, can change the velocity or direction of the system’s center of mass. Internal forces may change the arrangement or shape of the objects inside the system, but they will not affect the system’s overall movement through space.
The Functional Impact on Motion
The primary importance of an external force lies in its ability to cause a change in the motion of the system it acts upon. A net external force, which is the vector sum of all outside forces, causes the system to accelerate. This acceleration is a change in the system’s velocity, which means speeding up, slowing down, or changing the direction of travel.
The magnitude of this acceleration is directly proportional to the size of the net external force applied. The acceleration is also inversely proportional to the total mass of the system. When the net external force on a system is zero, the system is in a state of equilibrium, and its state of motion remains unchanged. This means the system will maintain a constant velocity, which includes remaining perfectly at rest.
This relationship between force, mass, and acceleration explains why external forces are so important for understanding dynamics. If a force originates from outside the system and is unbalanced, it provides the necessary interaction to overcome the system’s inherent resistance to changes in motion. The study of external forces allows physicists to predict precisely how much an object’s speed or direction will change over time.
Identifying Common External Forces
Many everyday forces are classified as external because they originate from objects typically considered outside the system under analysis. These varied interactions, whether they require physical contact or act across a distance like gravity, all share the characteristic of originating outside the defined boundary and influencing the system’s movement.
Common External Forces
One universal example is gravitational force, often felt as the weight of an object, which is exerted by a large external body like the Earth. This force acts on any object system, pulling it toward the Earth’s center and crossing the system boundary.
Another common external force is the normal force, which is the perpendicular support force exerted by a surface, such as a floor or a table, on an object resting on it. The surface providing the support is generally outside the system, making the normal force external. Friction is also nearly always an external force, arising from the contact between the system and an external surface or medium, like air resistance or the drag from a road.
Tension is an external force when a rope or cable, which is outside the system, pulls on the object within the system. Any direct push or pull applied by an agent, such as a person or a machine, is categorized as an applied external force.