Newton's Third Law

Physics

1. Fundamental Concepts

Definition: Newton's Third Law states that for every action, there is an equal and opposite reaction.
Implication: Forces always occur in pairs and are equal in magnitude but opposite in direction.
Application: This law applies universally to all interactions between objects.

2. Key Concepts

Action-Reaction Pair: $${\text{{Action}}} \cdot {\text{{Force}}} = -{\text{{Reaction}}} \cdot {\text{{Force}}}$$

Vector Representation: $${\vec{F}}_{\text{{action}}} = -{\vec{F}}_{\text{{reaction}}}$$

Example Application: When you push on a wall, the wall pushes back with an equal force.

3. Examples

Example 1 (Basic)

Problem: A book rests on a table. Identify the action-reaction pairs.

Step-by-Step Solution:

The weight of the book (downward force) is the action force.
The normal force exerted by the table (upward force) is the reaction force.

Validation: The forces are equal in magnitude and opposite in direction, satisfying Newton's Third Law.

Example 2 (Intermediate)

Problem: A person standing on a skateboard pushes off a wall. Describe the action-reaction pairs.

Step-by-Step Solution:

The person pushes against the wall (action force).
The wall pushes back on the person (reaction force), causing the skateboard to move.

Validation: The forces are equal in magnitude and opposite in direction, demonstrating Newton's Third Law.

4. Problem-Solving Techniques

Identify Forces: Clearly identify all forces acting on the system and their directions.
Draw Diagrams: Use free-body diagrams to visualize the forces and their reactions.
Apply Newton's Laws: Ensure that for every action force, there is an equal and opposite reaction force.