I better read that book..
A book is resting on a flat table. Since its acceleration is zero, the total force acting on the book must be zero. Since according to Newton's third law of motion, for every action, there exists equal and opposite reaction, that should hold for our nicely resting book too! Suppose we think of the earth's pull on the book as the action force, what will be reaction force in this case?
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4 solutions
ya thats true but normal reaction from the table is the force which is acting in the opposite direction to earth's gravitational pull and it balances the gravity force becoz of which the book is in equilibrium.we know that action and reaction forces must be equal and opposite to each other and since gravitational attraction of the book exerted towards the earth is much smaller and negligible so the practical answer should be normal force exerted by table on book.
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First thing: how do you say that gravitational attraction is negligible? Thats your biggest misconecption as far as I see it..
If you remove the table, the book will certainly fall to the ground. Therefore, it is the table that is holding the book in place. The relevant answer in this case should have been the normal reaction exerted by the table.
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I constructed this problem exactly to remove the misconception about reaction force. Key point of Newton's third law is that only two bodies are involved. If one body exerts any kind of force on the other one, be it gravitational, electromagnetic, tension or any other, the other body exerts the same amount of force on the first body. There can be other bodies acting various forces on these two bodies but that doesn't affect the forces between these two bodies and while talking about action and reaction, you should completely ignore other bodies and their interactions. In the present case, there are two bodies acting two different forces on book: earth acts gravitational force on book while table acts normal reaction and even though these forces are equal and opposite, they are being acted on third body (the book) and not on each other! Hence these two forces do not form action - reaction pair. On the other hand, if you consider book and earth, then earth exerts some gravitational force on book and book also acts same amount of gravitational force on earth (by Newton's law of gravitation). Hence these two forces form action-reaction pair.
The question is to state the reaction force and NOT to state the force which keeps the book in equilibrium
The answer should be "normal reaction from the table"
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I constructed this problem exactly to remove the misconception about reaction force. Key point of Newton's third law is that only two bodies are involved. If one body exerts any kind of force on the other one, be it gravitational, electromagnetic, tension or any other, the other body exerts the same amount of force on the first body. There can be other bodies acting various forces on these two bodies but that doesn't affect the forces between these two bodies and while talking about action and reaction, you should completely ignore other bodies and their interactions. In the present case, there are two bodies acting two different forces on book: earth acts gravitational force on book while table acts normal reaction and even though these forces are equal and opposite, they are being acted on third body (the book) and not on each other! Hence these two forces do not form action - reaction pair. On the other hand, if you consider book and earth, then earth exerts some gravitational force on book and book also acts same amount of gravitational force on earth (by Newton's law of gravitation). Hence these two forces form action-reaction pair.
I fully agree
It has to be the normal force exerted by the table on book because gravitational force exerted by the book on earth will be very less compared to earth's gravitational pull.But N=mg here.
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Are you sure that force is very less? :) Actually it is exactly same, not any less..
The problem is poorly worded. There are different forces at work. The earth's pull on the book is countered by the book's pull on earth. However, the weight of the book (the earth's pull on the book) is also countered by the normal force from the table on the book. So the same force has two reactions depending on your point of interaction. Given the problem focused on the fact that the book is at rest, the normal force is the one responsible for that and should have been the correct answer.
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I constructed this problem exactly to remove the misconception about reaction force. Key point of Newton's third law is that only two bodies are involved. If one body exerts any kind of force on the other one, be it gravitational, electromagnetic, tension or any other, the other body exerts the same amount of force on the first body. There can be other bodies acting various forces on these two bodies but that doesn't affect the forces between these two bodies and while talking about action and reaction, you should completely ignore other bodies and their interactions. In the present case, there are two bodies acting two different forces on book: earth acts gravitational force on book while table acts normal reaction and even though these forces are equal and opposite, they are being acted on third body (the book) and not on each other! Hence these two forces do not form action - reaction pair. On the other hand, if you consider book and earth, then earth exerts some gravitational force on book and book also acts same amount of gravitational force on earth (by Newton's law of gravitation). Hence these two forces form action-reaction pair.
I agree with Koel Sharma's answer. The reaction force on the book is the the Normal force exerted by the table. Remember the reaction force has to be equal and opposite. The gravitational force exerted by the book on the earth is not equal to the gravitational force exerted by the earth on the book. But the Normal force exerted by the table on book will be equal to the gravitational force exerted by the earth on the book.
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"The gravitational force exerted by the book on the earth is not equal to the gravitational force exerted by the earth on the book". Please get your facts corrected before making such baseless statement with such high confidence.
I think the key is that the reaction forces are equal, opposite and of the same nature . In this case, gravitational force. The normal reaction would be classified as an electromagnetic force. Look up 4 Fundamental Forces . Correct me if I am wrong.
I agree with Koel Sharma. Had normal force been not there, the book could have gone through the table. Therefore, the reaction to the earth's pull on the book is the normal force exerted by the table on the book.
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I constructed this problem exactly to remove the misconception about reaction force. Key point of Newton's third law is that only two bodies are involved. If one body exerts any kind of force on the other one, be it gravitational, electromagnetic, tension or any other, the other body exerts the same amount of force on the first body. There can be other bodies acting various forces on these two bodies but that doesn't affect the forces between these two bodies and while talking about action and reaction, you should completely ignore other bodies and their interactions. In the present case, there are two bodies acting two different forces on book: earth acts gravitational force on book while table acts normal reaction and even though these forces are equal and opposite, they are being acted on third body (the book) and not on each other! Hence these two forces do not form action - reaction pair. On the other hand, if you consider book and earth, then earth exerts some gravitational force on book and book also acts same amount of gravitational force on earth (by Newton's law of gravitation). Hence these two forces form action-reaction pair.
The answer "Book's pull on the earth" is wrong. The answer should be "normal reaction from the table". That is why the book is static, no acceleration, because the book is stabilized by the table. Then the table is the one who has the reaction force in this case "table's pull on earth". Explain to me further your solution (in a true physicist way).
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I constructed this problem exactly to remove the misconception about reaction force. Key point of Newton's third law is that only two bodies are involved. If one body exerts any kind of force on the other one, be it gravitational, electromagnetic, tension or any other, the other body exerts the same amount of force on the first body. There can be other bodies acting various forces on these two bodies but that doesn't affect the forces between these two bodies and while talking about action and reaction, you should completely ignore other bodies and their interactions. In the present case, there are two bodies acting two different forces on book: earth acts gravitational force on book while table acts normal reaction and even though these forces are equal and opposite, they are being acted on third body (the book) and not on each other! Hence these two forces do not form action - reaction pair. On the other hand, if you consider book and earth, then earth exerts some gravitational force on book and book also acts same amount of gravitational force on earth (by Newton's law of gravitation). Hence these two forces form action-reaction pair.
Actually the answer is correct as it is the reaction to the gravitational force. Also note that action and reaction do not occur on the same object, whereas the normal force is a consequence of the weight of the book.
By newtons third law we know that for every action there is equal reaction.In this case if the action force is the earth pulling on the book then there reaction force must be the book pulling the earth.
Why "must" it be so? Why not normal reaction for example?
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aren't the book and the table in contact with each other , so we could consider the book and the table together as a single entity
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...no their relative motions are not restricted
nope we can't consider this situation in this way...
Action and reaction do not occur on the same object! The normal reaction is a consequence of the weight of the book, but not reaction to the gravitational force.
it has to be opposite to the earths pull, other wise the book falls on earths core, the books own gravitational pull in comparison to earth is negligible, the book pulled by earth pushes on the table, table reacts by pushing the book back with an equal and opposite re action, normal reaction, normal meaning normal to the plane the book pushes on, plane of the table top.
F= m1*m2/r^2..... so for every body attracting another body, it is attracted by the other body with same force..
Please note that the question is to state the reaction force to the action force exerted by the earth. Action and reaction forces , though equal and opposite, may not result in an equilibrium condition. We need to look at the following fact that
acceleration=Force/mass
Hence, though the forces are equal, acceleration need not be.
P.S.= This is not a solution to the problem as it has already been provided. This is my attempt to make a few fellow readers understand the question and also to understand that equilibrium condition is dependent on acc. and not forces.
Every object exerts gravitational force of its own which is directly proportional to its mass, since the mass of book is much smaller than earth so, it does not have any effect on earth and we don't come to know about it but it still exists and this pull exerted by book is the reaction in this case