Forces: Internal and External Forces in Physics

Forces in Physics

Introduction to Forces

Forces are fundamental interactions that cause objects to accelerate, change direction, or deform. Understanding the distinction between different types of forces is essential for analyzing physical systems in mechanics.

Key Vocabulary

• Force: A push or pull exerted on an object, measured in newtons (N).

• Internal Force: A force that acts between components within a system.

• External Force: A force that acts on a system from outside the system's boundary.

• Action-Reaction Force: Forces that occur in pairs, as described by Newton's Third Law.

• Reaction Force: The force that is equal in magnitude and opposite in direction to the action force.

• System: The object or group of objects being analyzed.

Types of Forces

• Internal Forces:

  • Act between parts of a system.

  • Do not change the total momentum of the system.

  • Examples: Tension within a rope, forces between molecules in a solid.

• External Forces:

  • Originate from outside the system's boundary.

  • Can change the motion of the system as a whole.

  • Examples: Gravity acting on a ball, friction from the ground.

Key Concepts

• Forces can be identified as "external" or "internal" depending on whether they cross the system boundary.

• When analyzing questions, always define the system clearly to distinguish between internal and external forces.

• Internal forces always come in equal and opposite pairs within the system and do not affect the system's overall motion.

• External forces are responsible for changes in the system's velocity or acceleration.

Newton's Third Law of Motion

For every action, there is an equal and opposite reaction. This means that forces always occur in pairs: if object A exerts a force on object B, then object B exerts an equal and opposite force on object A.

Equilibrium of Forces

• Forces can cancel each other out and have a net force of zero, resulting in equilibrium (no acceleration).

Example

• Example 1: If you pull on a rope attached to a box, the force you exert on the box is an external force. The tension within the rope is an internal force if the system includes both the box and the rope.

• Example 2: When two ice skaters push off from one another, the forces they exert are internal if the system is both skaters together; these forces do not change the total momentum of the system.

Key Equations

• Newton's Third Law: $F_{A \to B} = -F_{B \to A}$

• Net Force (Equilibrium): $\sum \vec{F} = 0$

Additional info: In physics problems, always define the system boundary before classifying forces. This helps in applying Newton's laws correctly and in understanding conservation principles.