Normal Force = Weight of Book: Agree or Disagree?

[Faiza]

Consider the following statement made by a student about a book at rest on a level table:

"The two forces exerted on the book are the normal force directed up and the weight of the book directed down. These are equal in magnitude and opposite to one another. By Newton's third law they are a third law force pair so the normal force is always equal to the weight of the book."
Do you agree with the student? Explain?

 

[Faiza]

No one ?

 

[Faiza]

i disagree maybe because that the normal force and weight are not acting on each other so it doesn't satisfy Newtons 3rd law action-reaction pair?

 

[e(ho0n3]

You are right to disagree but your reasoning is wrong. Newton's 3. law involves the forces on two objects, i.e. the force the table exerts on the book is equal in magnitude but opposite in direction to the force the book exerts on the table.

 

[e(ho0n3]

I disagree with your disagreeing :) The normal force and the weight act on the same body to give a resultant zero force. They are an action-reaction pair by Newton's third law because otherwise if the two forces did not cancel each other out then the book would be free to travel through the table.

No it is not an action-reaction pair because both forces are acting on the same body.

 

[arildno]

vsage:
You're simply wrong!
There's actually two action/reaction pairs here:
Weight of book/book's force on the Earth
Normal force on book from table/Normal force from book on table.

Weight of book/Normal force on book from table is not an action/reaction pair.

 

[arildno]

Air resistance and static friction has no relevance here.