Internal Forces And External Forces

Internal and External Forces: Shaping Our World, From Atoms to Ecosystems

Understanding forces is fundamental to comprehending how the world works, from the smallest subatomic particles to the largest galaxies. Forces are interactions that can change an object's motion or its shape. This article will delve into the crucial distinction between internal forces and external forces, exploring their definitions, examples, and the significant roles they play in various systems, including physics, engineering, and even biology. We'll examine how these forces interact to create the dynamic world we experience.

What are Internal Forces?

Internal forces are forces that act within a system. They are forces exerted by one part of a system on another part of the same system. Crucially, these forces do not cause a change in the system's overall momentum. While they might cause internal deformations or changes in the internal energy, the system as a whole remains unaffected in terms of its linear or angular momentum.

Think of it like this: Imagine a closed system, such as a box containing several billiard balls. When one ball collides with another, the force exerted by one ball on the other is an internal force. The momentum transferred between the balls is internal to the system; the total momentum of all balls within the box remains unchanged. This is a direct application of Newton's Third Law of Motion: For every action, there is an equal and opposite reaction. Internal forces always come in pairs, acting within the defined system boundaries.

Examples of Internal Forces:

• Forces within a solid object: When you bend a metal rod, the forces exerted by the different parts of the rod on each other are internal forces. These forces cause stress and strain within the material.
• Forces within a fluid: The pressure forces within a liquid or gas are internal forces. Consider the pressure exerted by the water molecules on each other inside a sealed bottle; this is an internal force.
• Forces between molecules: The electromagnetic forces holding atoms together in a molecule are internal forces. The bonds between atoms within a molecule are what define the molecule's structure and properties.
• Forces within a biological system: The forces exerted by muscles on bones within an organism are internal forces. These forces allow for movement and locomotion.
• Forces within a car engine: The forces between the pistons, crankshaft, and other parts of an engine are all internal forces. They result in the engine's operation, but do not directly affect the overall movement of the car (except indirectly through the transmission of force to the wheels).

What are External Forces?

External forces, in contrast, are forces that act on a system from outside the system. These forces can change the system's overall momentum, causing it to accelerate, decelerate, or change direction. They are interactions between the system and its surroundings.

Returning to the billiard ball example, if you hit the box with your hand, the force applied by your hand is an external force. This force will change the overall momentum of the system (the box and the balls). Similarly, gravity acting on the box is another external force.

Examples of External Forces:

• Gravity: The force of attraction between objects with mass is an external force acting on any object near a larger gravitational field, like Earth.
• Friction: The resistive force that opposes motion between two surfaces in contact is an external force. Friction acts on the system from outside, either due to the surface it’s resting on or the air surrounding it.
• Applied force: Any force directly applied to a system from an external source, like pushing a car or pulling a rope.
• Tension: The force transmitted through a rope, string, or cable when it is pulled tight is an external force on the object being pulled.
• Normal force: The support force exerted upon an object that is in contact with another stable object is an external force; for example, a book resting on a table.
• Magnetic force: The attractive or repulsive force exerted by a magnet is an external force.
• Electrostatic force: The force between electrically charged objects is an external force.

The Importance of Defining the System

The crucial aspect of distinguishing between internal and external forces lies in precisely defining the system under consideration. What constitutes "internal" or "external" depends entirely on the boundaries of the system.

For instance, consider a rocket launching into space. If we define the system as the rocket itself, the thrust from the engines is an internal force (though it's ultimately what propels the rocket). However, if we define the system as the rocket and the exhaust gases, then the thrust becomes an internal force between the rocket and the expelled gases. Gravity, however, remains an external force in both cases, acting on the system from outside.

Therefore, understanding the system's boundaries is paramount. A clearly defined system allows for accurate application of the principles of conservation of momentum and energy.

Internal Forces and Stress-Strain Relationships

Internal forces are central to understanding the behavior of materials under stress. When an external force is applied to an object, internal forces develop within the object to resist the deformation caused by the external force. These internal forces are responsible for the object’s response, whether it be elastic deformation (returning to its original shape after the force is removed) or plastic deformation (permanent change in shape). The relationship between the applied stress (force per unit area) and the resulting strain (deformation) is described by the material's constitutive equations, which are crucial in engineering design.

Internal Forces in Biological Systems

Internal forces are equally crucial in biological systems. Muscle contractions, for example, are the result of internal forces within muscle cells. These forces generate movement and enable various biological processes. The forces exerted by proteins within cells, as well as the interactions between cells, are all internal forces vital for cell structure, function, and organismal movement.

External Forces and Newton's Laws of Motion

External forces are directly related to Newton's Laws of Motion. Newton's First Law (Inertia) states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force. Newton's Second Law (F=ma) relates the net external force acting on an object to its acceleration. Newton's Third Law, while involving pairs of forces, emphasizes that these forces act on different objects, hence, external to each other within the relevant systems.

Applications in Engineering and Physics

Understanding the interplay between internal and external forces is paramount across various disciplines. In structural engineering, analyzing internal forces within a bridge or building under external loads (like wind or weight) is crucial for ensuring structural integrity and safety. In mechanical engineering, the design of machines and engines involves careful consideration of both internal and external forces to optimize efficiency and performance. In physics, internal forces play a role in analyzing systems ranging from atomic structures to the dynamics of galaxies.

Frequently Asked Questions (FAQ)

• Q: Can internal forces cause acceleration of the system as a whole? A: No. Internal forces cannot change the overall momentum of a system. Only external forces can cause the system as a whole to accelerate.
• Q: How do I determine whether a force is internal or external? A: Carefully define your system. Any force exerted by one part of the system on another is internal. Forces originating outside the system are external.
• Q: Can internal forces do work? A: Yes. Internal forces can do work within the system, changing the system's internal energy (e.g., increasing temperature through friction). However, they do not change the system's total mechanical energy.
• Q: What is the role of friction in internal vs. external forces? A: Friction can be both internal and external. Internal friction arises from the interaction of components within a system (like in a machine). External friction is the resistance from external surfaces or mediums (air resistance).

Conclusion

The distinction between internal and external forces is crucial for understanding the behavior of physical systems at all scales. While internal forces govern the internal dynamics and energy transformations within a system, external forces dictate the system's overall motion and interaction with its surroundings. A clear understanding of this distinction, combined with a careful definition of the system, is fundamental to applying the principles of classical mechanics and understanding complex phenomena in various fields of study, from engineering and physics to biology and beyond. By mastering the concepts of internal and external forces, we gain a deeper appreciation of the intricate interplay of forces that shapes our universe.