Identifying Action Reaction Forces: A Newton's Third Law Analysis

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Homework Help Overview

The discussion revolves around identifying action-reaction force pairs as outlined by Newton's Third Law, based on a provided diagram. Participants are tasked with recognizing and articulating these pairs in a specific context involving a man, a monkey, a scale, and a tree.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants attempt to list pairs of action-reaction forces but question the validity of their selections, particularly regarding direct contact and the nature of forces involved. Some suggest focusing on objects that exert forces through direct contact, while others explore the implications of indirect forces.

Discussion Status

The discussion is active, with participants providing guidance on refining the understanding of action-reaction pairs. There is a focus on clarifying the criteria for valid pairs, particularly emphasizing direct contact between objects. Multiple interpretations of what constitutes an action-reaction pair are being explored.

Contextual Notes

Participants are encouraged to reconsider their initial answers based on the principle that only forces between objects in direct contact should be included. The discussion also touches on the distinction between contact forces and long-range forces like gravity.

storygeek
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Homework Statement


"Identify at least five pairs of action reaction forces in the following diagram"
u2l4a13.gif

Source: http://www.physicsclassroom.com/Class/newtlaws/U2L4b.html problem 2

Homework Equations


Newton's third law


The Attempt at a Solution


1) Man and monkey
2)monkey and scale
3)scale and tree
4)man and tree
5) monkey and tree
 
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You would be better off sticking to things that are in actual contact and thus can push/pull against each other. For example, the man is touching the rope, so man and rope can exert forces on each other. But the man is not directly touching the tree, so man and tree do not directly exert forces on each other. (Note: A long range force, like gravity, does not require direct contact.)
 
The best way to deal with things is to draw "bags" around things and see how the forces are acting on that single thing.

* Note *
You might want to think about tension =).
 
Are my attempts to the solutions valid? Man exerts force on Monkey and vice versa. Do indirect forces count in Newton's third law? Like the one between the tree and man.
 
storygeek said:
Are my attempts to the solutions valid? Man exerts force on Monkey and vice versa. Do indirect forces count in Newton's third law? Like the one between the tree and man.
Strictly speaking: No. Only include forces between objects that are in direct contact. (We ignore the teeny tiny gravitational force between man and tree!) The man doesn't really pull on the tree; he pulls on the rope! Redo your answers with this in mind and you'll have a deeper (and more correct) understanding of forces and Newton's 3rd law.
 

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