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Newton's Laws of Motion: Third Law

John Smith

John Smith

9 min read

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Study Guide Overview

This study guide covers Newton's Third Law, focusing on action-reaction pairs, their characteristics, and identification. It explores common force pair examples like friction and tension, and uses Atwood machines as a practical application of the law. The guide also provides a derivation of the acceleration formula for Atwood machines, common mistakes, and exam tips including free-body diagrams. Finally, it includes practice multiple-choice and free-response questions for exam preparation.

#AP Physics C: Mechanics - Newton's Third Law Study Guide 🚀

Hey there, future physics champ! Let's break down Newton's Third Law and make sure you're totally ready to ace that exam. This guide is designed to be your go-to resource, especially the night before the test. Let's get started!

#Newton's Third Law of Motion: Action-Reaction Pairs

For every action, there is an equal and opposite reaction.

It's more than just a catchy phrase; it's a fundamental principle! Let's make sure you understand it inside and out.

Key Concept

Newton's Third Law is all about force pairs. These forces are:

  • Always between two interacting objects.
  • Always the same type of force (e.g., both are tension, both are gravitational).
  • Always equal in magnitude but opposite in direction.

#Key Concepts:

  • Action-Reaction: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.

  • Simultaneous Forces: These forces occur at the same time; one doesn't cause the other.

  • Universal Application: This law applies to all forces: gravity, electromagnetism, etc.

Memory Aid

Think of it like a handshake: When you push on someone's hand, they push back on yours with the same force, but in the opposite direction.

#Visualizing Force Pairs

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A visual representation of Newton's Third Law. The force exerted by the hand on the wall is equal and opposite to the force exerted by the wall on the hand.

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Another example of action-reaction pairs. The force of the foot on the ball is paired with the force of the ball on the foot.

#Identifying Force Pairs

  • Identify the interacting objects: Who is pushing on whom?

  • Determine the force type: Is it tension, friction, gravity, etc.?

  • Visualize the direction: Are the forces acting in opposite directions?

Exam Tip

When drawing free-body diagrams, use the same color or circle force pairs to help visualize them. This can prevent errors when setting up equations!

#Common Force Pairs:

  • Friction between two surfaces
  • Tension in a rope or cable
  • Air resistance on a moving object
  • ...
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Question 1 of 11

A book is placed on a table. What is the reaction force to the force of the book on the table? 🤔

The weight of the book

The force of the table on the book

The normal force acting on the book

The force of gravity on the Earth