Newton's Third Law for book on a table

AI Thread Summary
The discussion centers on the application of Newton's Third Law regarding a book resting on a table. Participants clarify that the normal force exerted by the table on the book and the weight of the book due to gravity are not Third Law pairs, as they act on the same object. Instead, the correct pairs are the gravitational force between the Earth and the book, and the contact forces between the book and the table. The normal force is a reaction to the weight but does not constitute a Third Law pair with it. Understanding these distinctions is crucial for grasping the principles of equilibrium and force interactions.
String theory guy
Messages
26
Reaction score
4
Homework Statement
Statement is below.
Relevant Equations
Newton's Third Law
This problem is from MIT's module again.
1670989944624.png

I have a doubt about Newton's Third Law. How is the normal and weight of the book not Newton's 3rd Law pairs?

When the book is placed on the table, I imagine that the book's weight exerts a force on the table and the table exerts and equal and opposite force of the same magnitude and opposite direction. This force is called the normal force.
 
Physics news on Phys.org
The Wikipedia entry for "Newton's Third Law" literally gives the answer to that specific question.
 
  • Like
Likes String theory guy and topsquark
String theory guy said:
When the book is placed on the table, I imagine that the book's weight exerts a force on the table and the table exerts and equal and opposite force of the same magnitude and opposite direction. This force is called the normal force.
I'd write it out this way:
The book's weight, the force of the Earth's gravity acting on the book ##w_{BE}##, means that the book exerts a force on the table, ##N_{TB}##. Likewise, the table exerts an equal and oppositely directed force on the book, ##N_{BT}##. Third force law pairs are the same type of force, so one can't be a normal force and the other a weight, and they act between the two objects, Book and Table. So, the Third Law pairs are ##N_{BT}## and ##N_{TB}##.

The weight is the cause of the normal forces but is not the Third Law pair of either of them.

-Dan
 
  • Like
  • Love
Likes PeroK and String theory guy
The third law pairs are:
- The gravitational force from the Earth on the book and the gravitational force from the book on the Earth.
- The contact force from the table on the book and the contact force from the book on the table.

Third law pairs never affect the same object. The weight and the contact force on the book are equal and opposite because the book is in equilibrium, not because they are a third law pair. For example, if you add a force pushing down on the book, tilt or accelerate the table - they will not be the same any more. Third law pairs are always the force of A on B and the force of B on A.
 
  • Like
  • Informative
  • Love
Likes member 731016, berkeman, PeroK and 2 others
topsquark said:
I'd write it out this way:
The book's weight, the force of the Earth's gravity acting on the book ##w_{BE}##, means that the book exerts a force on the table, ##N_{TB}##. Likewise, the table exerts an equal and oppositely directed force on the book, ##N_{BT}##. Third force law pairs are the same type of force, so one can't be a normal force and the other a weight, and they act between the two objects, Book and Table. So, the Third Law pairs are ##N_{BT}## and ##N_{TB}##.

The weight is the cause of the normal forces but is not the Third Law pair of either of them.

-Dan
Thanks @topsquark. However, how dose the book exert a normal force onto the table? I don't understand why one can't be the weight and the other can't be the normal force from the table. The weight is equal to the normal force from the book onto the table.
 
String theory guy said:
Thanks @topsquark. However, how dose the book exert a normal force onto the table? I don't understand why one can't be the weight and the other can't be the normal force from the table. The weight is equal to the normal force from the book onto the table.
Every poster so far has answered that question already.
 
  • Like
  • Love
Likes topsquark and String theory guy
String theory guy said:
However, how dose the book exert a normal force onto the table?
Because if the table acts on the book with a force ##\vec F##, then the book acts on the table with a force ##-\vec F##. That is what Newton's third law says.
 
  • Like
  • Love
Likes topsquark and String theory guy
hmmm27 said:
The Wikipedia entry for "Newton's Third Law" literally gives the answer to that specific question.
In case the OP or anyone else is having trouble finding it, there is no (English) Wikipedia page for "Newton's Third Law", I believe @hmmm27 is referring to https://en.wikipedia.org/wiki/Newton's_laws_of_motion#Third.
 
  • Like
  • Love
Likes String theory guy, berkeman, topsquark and 1 other person
@String theory guy : Check this video out, it will help you resolve your misconception:
 
  • Like
  • Love
Likes topsquark and String theory guy

Similar threads

Back
Top