If an item A exterts a pressure on object B, climate object B exerts an equal yet opposite force on object A

It then says:

Newton"s 3rd law uses in all situations and to all varieties of force. Yet the pair of forces are constantly the same type, zb both gravitational or both electrical.

You are watching: What forces are acting on a book sitting on a table

And:If you have actually a publication on a table the book is exerted a pressure on the table (weight as result of gravity), and also the table reacts through an equal and also opposite force. Yet the force acting on the table is due to gravity (is this the exact same as a gravitational force?), and also the forcing exhilaration from the table come the publication is a reaction force. So one is a gravitational, and also the various other is not. As such this is not Newton"s third Law together the pressures must it is in of the same type.


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edited Mar 29 "16 at 10:45
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Qmechanic♦
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asked might 24 "12 in ~ 19:44
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Jonathan.Jonathan.
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And: If you have actually a publication on a table the book is exerted a force on the table (weight as result of gravity),

That"s where you go wrong. The force that the publication exerts on the table is not a gravitational force, it"s a normal force.

and the table reacts with an equal and opposite force.

That"s also a regular force. For this reason the publication exerts a (normal) pressure on the table, and also the table exerts a (normal) force on the book.

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But the pressure acting on the table is due to gravity (is this the same as a gravitational force?),

No, it"s not, and also in fact this pressure (the typical force) is only indirectly as result of gravity. The just relevant gravitational pressure is the pressure exerted through the planet on the book. And the book additionally exerts a gravitational force back on the Earth, but since the earth is for this reason heavy, that force has no remarkable effect. (The Earth additionally exerts a gravitational force on the table, and the table ~ above the Earth, however those don"t issue so lot in this details scenario.)


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answered might 24 "12 in ~ 20:38
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David ZDavid Z
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This is typical misconception with my student too, and the only way to understand it friend must draw all forces that plot on both objects (in full five forces)!

In order come make points clearer, I will label the pressure with i m sorry table acts upon book as $F_12$ and also not $F_ extN$! also suppose that $z$ axis is vertically up, for this reason positive pressures push increase and an adverse forces press downward.

There space two forces acting ~ above book, that is gravitational force $-F_ extg,book$ (downward) and also the force of table ~ above the publication $F_12$ (upward). Follow to an initial Newton law for the publication they are equal by magnitude

$$F_12 - F_ extg,book = 0.$$

According to the third Newton law publication must be exhilaration on table v the force $-F_12$ (downward). So there space three forces acting top top table: that is gravitational force $-F_ extg,table$, pressure of the publication $-F_12$ (both downward) and also the pressure of the floor $F_ extN$ (upward)!

Now let"s compose the an initial Newton"s legislation for the table

$$F_ extN - F_12 - F_ extg,table = 0.$$

Consequently

$$F_ extN = F_12 + F_ extg,table = F_ extg,book + F_ extg,table$$

The ground force must support both book and also table! Isn"t the obvious?

Conclusion: So 3rd Newton"s regulation is perfect valid for this case as well!

If girlfriend still do not understand, create on the record book, table, and also all five forces (two exhilaration on the book and three exhilaration on the table).