Newton's Third Law for book on a table

In summary, the book's weight exerts a force on the table, which in turn exerts an equal and opposite force on the book. These forces are called the normal force.
  • #1
String theory guy
26
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
  • #2
The Wikipedia entry for "Newton's Third Law" literally gives the answer to that specific question.
 
  • Like
Likes String theory guy and topsquark
  • #3
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
  • #4
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
  • #5
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.
 
  • Sad
Likes PeroK
  • #6
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
  • #7
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
  • #8
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
  • #9
@String theory guy : Check this video out, it will help you resolve your misconception:
 
  • Like
  • Love
Likes topsquark and String theory guy

FAQ: Newton's Third Law for book on a table

What is Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert an equal and opposite force back on the first object.

How does Newton's Third Law apply to a book on a table?

When a book is placed on a table, the book exerts a downward force on the table due to gravity. According to Newton's Third Law, the table will exert an equal and opposite force back on the book, known as the normal force. This normal force keeps the book from falling through the table.

Can you give an example of Newton's Third Law in action with a book on a table?

Yes, when you push down on a book on a table, you are exerting a downward force on the book. In response, the book exerts an equal and opposite force on your hand, pushing it back up. This is an example of Newton's Third Law in action.

Does Newton's Third Law only apply to objects on a table?

No, Newton's Third Law applies to all objects and interactions, not just those involving a book on a table. It can be seen in action on a larger scale, such as with a rocket launching into space, or on a smaller scale, such as with two magnets repelling each other.

How does understanding Newton's Third Law help us in everyday life?

Understanding Newton's Third Law can help us predict and explain the motion of objects in everyday life. It can also help us design and improve technologies, such as cars and airplanes, by considering the equal and opposite forces at play. Additionally, it can help us understand and appreciate the interconnectedness of the physical world.

Similar threads

Replies
25
Views
779
Replies
12
Views
3K
Replies
13
Views
601
Replies
2
Views
14K
Replies
2
Views
1K
Back
Top