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

[storygeek]

Homework Statement

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

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

 

[Doc Al]

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.)

 

[AngeloG]

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 =).

 

[storygeek]

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.

 

[Doc Al]

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.