Lesson 1: Newton's First Law of Motion

Newton's First Law

Inertia and Mass

State of Motion

Balanced and Unbalanced Forces

Lesson 2: Force and Its Representation

The Meaning of Force

Types of Forces

Free-Body Diagrams

Determining the Net Force

Lesson 3 : Newton's Second Law of Motion

Newton's Second Law

The Big Misconception

Finding Acceleration

Finding Individual Forces

Free Fall and Air Resistance

Lesson 4 : Newton's Third Law of Motion

Newton's Third Law

Action and Reaction Force Pairs

Lesson 4: Newton's Third Law of Motion

Identifying Action and Reaction Force Pairs

According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs - known as "action-reaction force pairs." Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements describing who is pushing on who and in what direction. For example, consider the interaction between a baseball bat and a baseball.

The baseball forces the bat to the right (an action); the bat forces the ball to the left (the reaction). Note that the nouns in the sentence describing the action force switch places when describing the reaction force.

Consider the following three examples. The action force is stated; determine the reaction force. Use the "pop-up menu" to view the answer.

Athlete pushes bar upwards.


Bowling ball pushes pin rightwards.



Balloon wall pushes compressed air inwards.



Check Your Understanding

1. Consider the interaction depicted below between foot A, ball B, and foot C. The three objects interact simultaneously (at the same time). Identify the two pairs of action-reaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements. Depress the mouse on the "pop-up menu" to view the answer.




2. Identify at least six pairs of action-reaction force pairs in the following diagram.




Lesson 4: Newton's Third Law of Motion


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