Why a book on a table is not an example of Newton's 3rd law?

[Karagoz]

In a physics test in a Norwegian website of a school book, it says following:

True or false?

(1) According to Newton's 3rd Law, the weight of a book that is on a table is just as big, but opposite the force on the book from the table (normal force). FALSE.

(2) According to Newton's 1st Law, the weight of a book that is on a table is just as big, but opposite the force on the book from the table (normal force). TRUE.

[Edited and corrected it, I think I should be more precise in translation].

Why is the first statement false?

According to the Newton's 3rd Law: For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.

So the size of the forces on the first object (the book) equals the size of the force on the second object (the table). Isn't the first statement just an example of the Newton's 3rd Law?

 

[Khashishi]

You are right. It looks like a bad test question.

 

[Nugatory]

There's some possibility that the translation into English has changed the meaning somewhere, but as I'm understandingbvg the two statements the first is true and the second is not.

 

[jbriggs444]

As I understand question 1, the "weight of the book" is intended to be the downward force of gravity on the book. The "power of book from the table" is intended to denote the upward force of table on book. Those two forces are not 3rd law partners.

They do add to zero and make the first law relevant, however.

 

[DrStupid]

Why is the first statement false?

Because the normal force of the table is not the counter force of the weight of the book according to lex 3. The counter force of the weigt is the gravitational force of the book acting on Earth and the normal force from the table is the counter force of the (electromagnetic) force from the book akting on the table. This might sound quite nitpichking but the difference becomes obvoius if you place a second book on the top of the first one.

 

[Dale]

(1) According to Newton's 3rd Law, the weight of a book that is on a table is just as big, but opposite the power of the book from the table (normal force). FALSE.

I think this is correctly identified as "FALSE". The one confusion is the use of the word "power" which should be "force". The weight of the book is the gravitational force. It does not form a 3rd law pair with the normal force. They are only coincidentally equal and opposite.

The gravitational force of the Earth on the book (the weight) forms a 3rd law pair with the gravitational force of the book on the earth. This gravitational force acts on the Earth and accelerates it upwards towards the book.

 

[TonyB_LLNL]

After some consternation - I tend to agree with Dale. This is subtle in terms of wording. The problem reduces to "a boulder lying on the ground". EACH is experiencing an equal and opposite gravitational force (due to mutual gravitational attraction) and EACH is experiencing an equal and opposite normal force (applied "electromagnetically" at their points of "contact". Both of these are "Third Law" observations. As we assume no acceleration is observed, the First Law demands there is no net force exerted upon either object. But (as the objects are in contact) this demands the "coincidence" of their gravitational and "normal" forces.

 

[CollinsArg]

Adding to what was already said, suppose I add a force on the book pressing on it with my hand against the table. Then the normal force of the table on the book becomes greater than before, but the weight of the book will stay the same, so, not always the normal force will be necessarily equal to the weight of the object.

 

[Mister T]

According to the Newton's 3rd Law: For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.

But the two forces mentioned are both on the book, so they cannot be a Third Law pair!

Earth pulls downward on book with a gravitational force, book pulls upward on Earth with a gravitational force. Note that the two forces act on different objects. They are a Third Law pair.

Also, book pushes down on table with a contact force, and table pushes up on book with a contact force called the normal force. Note that these two forces act on different objects. They are a Third Law pair.

 

[TonyB_LLNL]

@CollinsArg: If you press down on the book with your hand, you and the book become a "system" with respect to forces upon the earth. You must consider your total mass (you plus book) and your total normal force (your feet to the ground plus book surface). If you increase the normal force of the book to the table/ground by 20 units, you will have reduced the normal force of your feet to the ground by 20 units. All remains in balance.

 

[jbriggs444]

@CollinsArg: If you press down on the book with your hand, you and the book become a "system" with respect to forces upon the earth.

One is free to consider systems that are not closed. The force of book on table and of table on book do not depend on where one chooses to draw system boundaries.

Choosing boundaries carefully can make it more convenient to calculate some quantities and make it unnecessary to calculate others. As you point out, if one considers person plus book as a [motionless] system then it is quickly obvious that an increased external force of table on book can be accompanied by a correspondingly decreased force of floor on shoes without changing the weight of the book or of the person.

Drawing the boundary around the book alone yields the same result. An increased force of table on book can be accompanied by an increased external force of hand on book without changing the weight of the book.

Using the more restricted boundary removes the need to consider the person's weight.