My textbook explains Newton\'s Third Law like this: If an object A exterts a for
ID: 1375618 • Letter: M
Question
My textbook explains Newton's Third Law like this:
If an object A exterts a force on object B, then object B exerts an equal but opposite force on object A
It then says:
Newton's 3rd law applies in all situations and to all types of force. But the pair of forces are always the same type, eg both gravitational or both electrical.
And: If you have a book on a table the book is exerted a force on the table (weight due to gravity), and the table reacts with an equal and opposite force. But the force acting on the table is due to gravity (is this the same as a gravitational force?), and the forcing acting from the table to the book is a reaction force. So one is a gravitational, and the other is not. Therefore this is not Newton's Third Law as the forces must be of the same type.
Explanation / Answer
And: If you have a book on a table the book is exerted a force on the table (weight due to gravity),
That's where you went wrong. The force that the book 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 normal force. So the book exerts a (normal) force on the table, and the table exerts a (normal) force on the book.
But the force acting on the table is due to gravity (is this the same as a gravitational force?),
No, it's not, and in fact this force (the normal force) is only indirectly due to gravity. The only relevant gravitational force is the force exerted by the Earth on the book. And the book also exerts a gravitational force back on the Earth, but because the Earth is so heavy, that force has no noticeable effect. (The Earth also exerts a gravitational force on the table, and the table on the Earth, but those don't matter so much in this particular scenario.)
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