Internal Force And External Force

Understanding Internal and External Forces: A Deep Dive into Physics

Understanding the difference between internal and external forces is crucial for comprehending a wide range of physical phenomena, from the simple act of walking to the complex dynamics of celestial bodies. This article provides a comprehensive exploration of internal and external forces, clarifying their definitions, illustrating their applications with real-world examples, and delving into the scientific principles that govern their interactions. We'll also address frequently asked questions to ensure a complete understanding of this fundamental concept in physics.

Introduction: Defining Internal and External Forces

In physics, forces are interactions that can change the motion of an object. They are vector quantities, meaning they have both magnitude (strength) and direction. The distinction between internal and external forces hinges on the system being considered. A system is simply the object or group of objects we're focusing our analysis on.

An internal force is a force that acts within the system. It's an interaction between components inside the defined system. These forces do not affect the system's overall momentum. Think of it as forces that the system exerts on itself.

An external force is a force that acts on the system from outside. It's an interaction between the system and its surroundings. These forces can affect the system's overall momentum and cause acceleration.

The key difference is perspective: what's internal to one system might be external to another, depending on how the system is defined.

Examples of Internal and External Forces: Clarifying the Distinction

Let's illustrate this with some examples:

Scenario 1: A Car Accelerating

System: The car itself (engine, wheels, chassis, etc.)
External Force: The force of friction between the tires and the road (driving force), the force of air resistance, and the force of gravity. These forces act on the car from outside.
Internal Force: The forces within the engine (piston forces, combustion forces), forces between the car's parts (e.g., forces within the transmission). These forces are interactions within the car.

Scenario 2: A Person Jumping

System: The person.
External Force: The force of gravity pulling the person downwards, the reaction force from the ground pushing the person upwards.
Internal Force: The muscle forces within the person's legs generating the jump.

Scenario 3: A Rocket Launching

System: The rocket.
External Force: The force of gravity pulling the rocket downwards, the force of air resistance, and the thrust force from the rocket engines (this is external because the exhaust gases are considered outside the defined system of the rocket itself).
Internal Force: The forces within the rocket structure holding it together, the internal pressures within the fuel tanks.

Scenario 4: A Collision Between Two Billiard Balls

System: The two billiard balls.
External Force: The force of friction between the balls and the table, the force of gravity.
Internal Force: The force of impact between the two balls during the collision. This is considered internal because both balls are part of the system.

Newton's Laws and Internal/External Forces

Newton's Laws of Motion provide a framework for understanding how forces affect the motion of objects. Internal and external forces play distinct roles:

Newton's First Law (Inertia): An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an external force. Internal forces, on their own, cannot change the overall motion of a system.
Newton's Second Law (F=ma): The acceleration of an object is directly proportional to the net external force acting on it and inversely proportional to its mass. Internal forces cancel each other out and don't contribute to the overall acceleration of the system.
Newton's Third Law (Action-Reaction): For every action, there is an equal and opposite reaction. This law applies to both internal and external forces. However, the action-reaction pairs involved in internal forces are within the system, whereas those involved in external forces are between the system and its surroundings.

Conservation of Momentum and Internal Forces

A significant implication of internal forces is their inability to change the total momentum of a system. Momentum is a measure of an object's mass in motion (p = mv). The principle of conservation of momentum states that the total momentum of a closed system (one not subject to external forces) remains constant.

Internal forces, being interactions within the system, always come in action-reaction pairs. These pairs cancel each other out, meaning they have no net effect on the system's total momentum. Only external forces can alter the overall momentum of a system. This principle is crucial in understanding collisions, explosions, and other dynamic events.

Applications of Internal and External Forces: Real-World Examples

The concepts of internal and external forces have broad applications across numerous fields:

Engineering: Understanding internal forces is crucial for designing structures that can withstand stress and strain. Civil engineers analyze internal forces within bridges, buildings, and other structures to ensure their stability and safety.
Biomechanics: The study of human and animal movement relies heavily on analyzing internal and external forces. For example, biomechanists examine the internal forces within muscles and bones and the external forces acting on the body during activities like running or jumping.
Aerospace Engineering: The design and operation of aircraft and spacecraft involve careful consideration of internal and external forces. Internal forces within the structure must be managed to prevent failure, while external forces like aerodynamic drag and gravity must be accounted for in flight control.
Astrophysics: The dynamics of celestial bodies, such as stars and galaxies, are governed by gravitational forces (external and, in the case of interacting bodies, potentially internal). Internal pressures within stars counter the force of gravity, preventing collapse.
Vehicle Dynamics: The design of cars, trains, and other vehicles involves careful consideration of both internal and external forces. Engine power provides external forces that propel the vehicle, while internal forces within the vehicle's components must be managed for optimal performance and safety.

Advanced Concepts: Stress, Strain, and Internal Forces

Internal forces are not simply about the forces within an object; they're deeply intertwined with the concept of stress and strain.

Stress: Stress is the internal force per unit area within a material. It's a measure of how much force is being distributed across a specific cross-sectional area of a material. High stress can lead to structural failure.
Strain: Strain is the deformation of a material in response to stress. It represents the change in shape or size of a material due to applied forces.

The relationship between stress and strain is crucial in material science and engineering, allowing engineers to predict how materials will behave under various loading conditions. Understanding internal forces is essential for this analysis.

Frequently Asked Questions (FAQ)

Q: Can internal forces cause a change in the shape of an object?

A: Yes, internal forces can cause a change in the shape of an object (deformation). However, they cannot cause a change in the overall motion of the object (unless it's a system composed of multiple independent parts where these internal forces cause different parts to move differently). The overall linear momentum of the system remains unchanged.

Q: How do I determine whether a force is internal or external?

A: Define your system clearly. If the force acts between components within the defined system, it's internal. If the force acts on the system from outside, it's external.

Q: What about forces exerted by a magnetic field? Are they internal or external?

A: It depends on how you define your system. If your system includes the magnet generating the field, then the force exerted on your object by the magnetic field is external. However, if you choose to consider your object and the magnet as part of a single system, the magnetic interaction would be an internal force.

Q: Are gravitational forces internal or external?

A: Typically, gravitational forces are considered external. For example, the force of gravity acting on a ball is external to the ball itself. However, in the context of a larger system, such as a planet, the gravitational interactions between its constituent parts are considered internal forces.

Q: Can internal forces do work?

A: Yes, internal forces can do work, but they don't change the total kinetic energy of the entire system. For instance, internal friction within a rolling ball causes a conversion of kinetic energy to thermal energy, reducing its speed.

Conclusion: Mastering the Internal/External Force Distinction

Understanding the distinction between internal and external forces is fundamental to comprehending classical mechanics. This distinction is critical in applying Newton's Laws, understanding conservation of momentum, and analyzing various physical phenomena. By clarifying the definition of the system under consideration, one can accurately identify and classify forces, leading to a deeper understanding of how forces influence the motion and behavior of objects. The ability to differentiate between these types of forces is essential for problem-solving in physics and engineering, and provides the foundation for more advanced topics in mechanics and other related fields.