Physics 1 - Chapter 6
Newton's Third Law
Terms & Objectives

Note: The following terms and objectives are based on the Indiana Standards 2000 for Physics 1. You may download the complete physics standards by clicking here, or view the standards relevant to Chapter 6 - Newton's Third Law here.

New Terms in Chapter 6:

action force	interaction	internal forces
mechanical system	Newton's Third Law	reaction force
external forces

Chapter 6 Objectives:

When you have completed Chapter 6, you should be able to:

1. ... state Newton's Third Law, and tell:
   • the relative magnitudes of the forces involved in an interaction.
   • the relative directions of the forces involved in an interaction.
   • the relative time of action of the forces involved in an interaction.
2. ... given the "action" force in an interaction, identify the "reaction" force.
3. ... recognize that two objects are always involved whenever a force acts, and explain why.
4. ... recognize when the action and reaction forces in an interaction do not cancel each other, and explain why this occurs.
5. ... tell what internal forces are, and explain why internal forces do cancel.
6. ... recognize that the accelerations of two objects involved in a Third-Law interaction do not have to be equal and explain why.
7. ... apply Newton's Third Law to a simple mechanical system (such as a horse and wagon or tension in a rope), by:
   1. describing each force acting as "a force on ____ due to ____",
   2. telling which forces in the system are action/reaction pairs,
   3. recognizing that these action/reaction forces are equal in magnitude and opposite in direction,
   4. determining the net force on each object in the system,
   5. and determining the acceleration of each object in the system.
8. ... recognize that, regardless of your strength, there is a limit to how much force can be applied to an object, and explain why.

Possible Misconceptions to Correct:

Do you believe that any of the following statements are true? They AREN'T! When you finish Chapter 6, you should understand that each of these statements is FALSE, and WHY it is false .

• WRONG: Pushes and pulls can be applied only by living things.
  
  
• WRONG: Things such as high-speed bullets contain force.
  
  
• WRONG: All equal-and-opposite forces cancel each other.
  
  
• WRONG: Reaction forces occur slightly after (in time) the action force has been applied.
  
  
• WRONG: Action and reaction forces are equal and opposite only under certain conditions.
  
  
• WRONG: Objects can push themselves. For example, the force that pushes a car is the force that the car exerts on the road.

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last update November 12, 2009 by JL Stanbrough