# Definition question for Newton's 3rd law

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#1
Would I get full marks for stating Newton's 3rd law as:

"every action has an equal and opposite reaction"

or would it be better to write:

"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A"
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1 year ago
#2
(Original post by luke78)
Would I get full marks for stating Newton's 3rd law as:

"every action has an equal and opposite reaction"

or would it be better to write:

"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A"
"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A"
is far better than

"every action has an equal and opposite reaction"
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1 year ago
#3
(Original post by luke78)
Would I get full marks for stating Newton's 3rd law as:

"every action has an equal and opposite reaction"

or would it be better to write:

"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A"
The latter - the former is vague.
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1 year ago
#4
I would go further than this - I think it should be made explicit what is equal and what is opposite.

If body A exerts a force on body B, then body B exerts a force on body A which is:

- equal in magnitude
- opposite in direction
- of the same type.

The last part is important and is often overlooked. Missing it out leads to common errors such as thinking that, for example, if a book is resting on a table then the weight of the book and the normal reaction force from the table on the book are a pair of Norton's third law forces, when they are not.

Another problem with the common short version of Norton's third law is the words "action" and "reaction". What are they supposed to mean? The law is about forces, so we should talk about forces. Action has a technical meaning in physics which is quite different to this (not that it comes up at A level), and reaction might make people think that a normal contact force has to be involved, which it doesn't.
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1 year ago
#5
(Original post by Pangol)
I would go further than this - I think it should be made explicit what is equal and what is opposite.

If body A exerts a force on body B, then body B exerts a force on body A which is:

- equal in magnitude
- opposite in direction
- of the same type.

The last part is important and is often overlooked. Missing it out leads to common errors such as thinking that, for example, if a book is resting on a table then the weight of the book and the normal reaction force from the table on the book are a pair of Norton's third law forces, when they are not.
....
I agree that the force of the same type is important. Do we really need to know it to say that weight of the book and the normal force (really hate the adding the word "reaction") acting on the book? I doubt so but knowing that Newton's 3rd law pair of forces is of the same nature/type may make the identification easier.

Not sure when Newton is changed to Norton in the history of science.
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#6
So if I wrote "If body A exerts a force on body B, then body B will exert an equal (in magnitude) and opposite (in direction) force (of the same type)on body A"

Is that as accurate as it needs to be for a level?
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1 year ago
#7
(Original post by luke78)
So if I wrote "If body A exerts a force on body B, then body B will exert an equal (in magnitude) and opposite (in direction) force (of the same type)on body A"

Is that as accurate as it needs to be for a level?
Yes, that would be perfect.
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1 year ago
#8
(Original post by Eimmanuel)
I agree that the force of the same type is important. Do we really need to know it to say that weight of the book and the normal force (really hate the adding the word "reaction") acting on the book? I doubt so but knowing that Newton's 3rd law pair of forces is of the same nature/type may make the identification easier.

Not sure when Newton is changed to Norton in the history of science.
Not sure if you are expressing any doubt about the book situation or not - so apologies if I'm telling you something you already know - but in that case, the N3L partner force for the weight of the book (i.e. the gravitational pull on the book by the Earth) is the gravitational pull on the Earth by the book. The N3L partner force of the normal force (i.e. contact force) from the table on the book is the contact force from the book on the table.

And as for Norton - that's autocorrect for you...
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1 year ago
#9
(Original post by Pangol)
Not sure if you are expressing any doubt about the book situation or not - so apologies if I'm telling you something you already know - but in that case, the N3L partner force for the weight of the book (i.e. the gravitational pull on the book by the Earth) is the gravitational pull on the Earth by the book. The N3L partner force of the normal force (i.e. contact force) from the table on the book is the contact force from the book on the table.

And as for Norton - that's autocorrect for you...

Again, you misinterpret what I am describing or questioning or doubting.

If we write N3L as
"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A”
the two forces are implicit of the same nature or type.

Students actually don't need to know the forces in N3L to be of the same type initially (if they don’t know initially but they need to be taught that the forces are of the same type in the end) and yet they should be able to know that the normal force acting on the object and weight of the object are NOT N3L pair of forces. This is usually a good exercise that I would do with my students before telling them the forces must be of the same type.

I don’t deny that your statement on N3L

If body A exerts a force on body B, then body B exerts a force on body A which is:
- equal in magnitude
- opposite in direction
- of the same type
makes N3L more explicit and can make the identification of the pair of forces in N3L for various situation easier.

IMO, the same nature or type of forces in N3L has more profound implication. Anyway, this would digress from what the OP is asking/seeking. I would leave it for another day.

I hope I have made things clearer now.
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1 year ago
#10
(Original post by Eimmanuel)
Again, you misinterpret what I am describing or questioning or doubting.

If we write N3L as
"If body A exerts a force on body B, then Body B will exert an equal and opposite force on body A”
the two forces are implicit of the same nature or type.

Students actually don't need to know the forces in N3L to be of the same type initially (if they don’t know initially but they need to be taught that the forces are of the same type in the end) and yet they should be able to know that the normal force acting on the object and weight of the object are NOT N3L pair of forces. This is usually a good exercise that I would do with my students before telling them the forces must be of the same type.

I don’t deny that your statement on N3L

makes N3L more explicit and can make the identification of the pair of forces in N3L for various situation easier.

IMO, the same nature or type of forces in N3L has more profound implication. Anyway, this would digress from what the OP is asking/seeking. I would leave it for another day.

I hope I have made things clearer now.
I think we've just been talking at cross-purposes and that we agree with each other.

The main reason I thought that the book/table thing was worth going into in detail is that I have seen this used as an example of Newton's third law, even by some teachers.
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1 year ago
#11
(Original post by Pangol)
I think we've just been talking at cross-purposes and that we agree with each other.

The main reason I thought that the book/table thing was worth going into in detail is that I have seen this used as an example of Newton's third law, even by some teachers.

To a certain, we are talking at cross purposes.

Just for fun and closure
A right and brilliant student named Newton came along and ask the following question:
The Moon orbit around the Earth and both the Moon and Earth orbit around the Sun. So in this case, all the forces are of gravitational force. How does the knowledge of the N3L pair of forces to be the same help in identifying the N3L pair of forces for the system: Sun, Earth and Moon?

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