What Is Internal Force In Physics

What is Internal Force in Physics? A Deep Dive

Internal forces, a fundamental concept in physics, often seem deceptively simple. However, a thorough understanding requires delving into the intricacies of how they interact with external forces to govern the motion and behavior of physical systems. This comprehensive guide will explore the definition, characteristics, and implications of internal forces, clarifying their role in various physical scenarios. We'll explore examples, differentiate them from external forces, and examine their crucial role in concepts like momentum and energy conservation.

Defining Internal Forces: Forces Within a System

An internal force is a force that acts within a defined system. This means the force originates from within the system itself and acts on another part of the same system. Crucially, internal forces do not cause a change in the system's overall momentum. They may, however, cause changes in the system's internal configuration or the motion of its constituent parts.

Consider a simple example: a book resting on a table. The book is our system. The gravitational force pulling the book downwards is an external force (acting from outside the system). However, the forces between the molecules within the book, holding it together, are internal forces. These internal forces counteract each other, maintaining the book's structural integrity.

Key Characteristics of Internal Forces

Several key characteristics distinguish internal forces from their external counterparts:

• Action-Reaction Pairs: Internal forces always exist as Newton's third law action-reaction pairs. This means for every internal force acting on one part of the system, there's an equal and opposite force acting on another part. These forces cancel each other out when considering the system as a whole.

• No Net External Effect on Momentum: The crucial point about internal forces is their inability to change the total momentum of the system. While they might alter the motion of individual components within the system, the net effect on the overall momentum remains zero. This principle is directly linked to Newton's second law and the conservation of momentum.

• System Definition is Crucial: The classification of a force as internal or external entirely depends on how the system is defined. For instance, consider a car. If the car is our system, the forces between the engine and the wheels are internal. However, if we expand the system to include the road, the frictional force between the tires and the road becomes an external force.

Contrasting Internal and External Forces

The difference between internal and external forces is paramount in understanding how physical systems behave. Let's examine their contrasting effects:

Examples of Internal Forces in Various Systems

Let's explore specific examples to further solidify our understanding:

1. A Colliding Billiard Balls:

When two billiard balls collide, the forces they exert on each other during the collision are internal forces. These forces are equal and opposite (Newton's third law) and cause a change in the individual balls' velocities. However, the total momentum of the two-ball system remains constant (neglecting friction).

2. A Rotating Wheel:

Consider a spinning bicycle wheel. The forces between the spokes and the hub are internal forces. They maintain the wheel's structural integrity and contribute to its rotational inertia. The external forces acting on the wheel (like air resistance or friction on the axle) can change the wheel's overall rotation, but the internal forces only redistribute energy within the wheel structure.

3. A Rocket Launching:

In a rocket, the expulsion of hot gases provides the thrust. The forces between the gas molecules are internal. However, the interaction between the expelled gas and the surrounding air is an external force that influences the rocket's motion. The internal forces within the rocket don't affect its overall momentum directly, but the external force of the expelled gases against the environment provides thrust.

4. A Human Body:

Consider the human body as a system. The forces between muscles, bones, and organs are all internal. These forces allow for movement and maintain the body's structure. External forces, such as gravity or contact with the environment, influence the body's overall motion.

5. A Stressed Solid Object:

In a stressed solid object, like a stretched rubber band, the internal forces between the molecules are resisting the external stretching force. These internal forces, arising from intermolecular interactions, cause internal strain within the material. If the external stress surpasses the material's strength, the internal forces can no longer maintain the object's integrity.

Internal Forces and Conservation Laws

Internal forces play a critical role in the context of conservation laws, particularly the conservation of momentum and energy.

Conservation of Momentum:

The total momentum of a system remains constant in the absence of external forces. Internal forces, by definition, do not alter the system's overall momentum. This principle is crucial in analyzing collisions, explosions, and other dynamic events. Even if the internal forces cause significant changes within the system, the overall system's momentum is conserved, as long as no external forces are present.

Conservation of Energy:

While internal forces don't change the total momentum of a system, they can significantly alter the system's energy distribution. Internal forces can transform energy from one form to another (e.g., kinetic energy into potential energy). However, the total mechanical energy of an isolated system (without external forces or non-conservative forces like friction) remains constant. The internal forces facilitate the redistribution of energy within the system, ensuring that total mechanical energy is conserved.

Advanced Concepts and Applications

The concept of internal forces extends to more advanced areas of physics:

• Continuum Mechanics: This field extensively uses the concept of internal forces to model the behavior of solids, liquids, and gases under stress. Internal stresses and strains are crucial in describing the material's response to external loads.

• Molecular Dynamics: At the microscopic level, molecular dynamics simulations use internal forces (intermolecular potentials) to predict the collective behavior of atoms and molecules in various systems. These simulations are invaluable in materials science, biochemistry, and other fields.

• Celestial Mechanics: While gravity is an external force acting between celestial bodies, internal forces within these bodies (e.g., gravitational forces within planets) contribute to their overall behavior and stability.

• Fluid Mechanics: Internal forces within a fluid (viscous forces) are essential in understanding fluid flow, turbulence, and the behavior of fluids under various conditions.

Conclusion

Internal forces are a cornerstone of classical mechanics, impacting our understanding of how physical systems behave. While seemingly simple in definition, their subtleties and importance are vast. By understanding their key characteristics – their role as action-reaction pairs, their inability to change a system's overall momentum, and their crucial role in energy redistribution – we gain a powerful tool for analyzing a wide range of physical phenomena, from simple collisions to complex systems involving fluids, solids, and even celestial bodies. A thorough grasp of internal forces is fundamental for anyone seeking to develop a deep understanding of physics.