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# Newton's third law pair versus the first law pair watch

1. Hey guys, can anyone describe and explain to me the differences between the Newton's third Law pair and the First Law pair? Please provide examples if you can...
2. (Original post by Mehjabeen)
Hey guys, can anyone describe and explain to me the differences between the Newton's third Law pair and the First Law pair? Please provide examples if you can...
What, exactly, do you mean by a "First Law pair".
While Newton's 3rd does indeed refer to a pair of forces, Newton's 1st doesn't.
3. (Original post by Mehjabeen)
Hey guys, can anyone describe and explain to me the differences between the Newton's third Law pair and the First Law pair? Please provide examples if you can...
N1 stuff accelerates if there is an unbalanced force on it

N2 acceleration is equal to the rate of change of momentum

N3 Equal and opposite reaction force on an object from another object in contact.
4. (Original post by Mehjabeen)
Hey guys, can anyone describe and explain to me the differences between the Newton's third Law pair and the First Law pair? Please provide examples if you can...
With Newton's third law every force has an equal and opposite force. Both fores act on DIFFERENT bodies, act in the opposite directions, are of the same same type, have the same magnitude an have the same line and time of action. For example when a bird flies, the bird might exert a push force of say 5N downwards onto the air. The Newton's third law pair to this is the air exerting a push force upwards of 5N on the bird. Note how both forces are of the same type-both are push forces. Both forces act on different bodies- one acts on the bird and one acts on the air. This is the reason why the bird overall ends up gaining greater altiude. Both are of the same magnitude of 5N. Both forces act on opposite directions-one is upwards while the other is downwards.

With Newton's first law no resultant force=the object remaining at rest or continuing with constant velocity. So there's no acceleration. The fundamental idea is that a resultant force creates the acceleration of the object, so it decreases/increases velocity.

Understand
5. So can we say that the Newton's first law pair may or may not be equal, act on the same body, may or may not act in opposite directions and so on?
(Original post by krisshP)
With Newton's third law every force has an equal and opposite force. Both fores act on DIFFERENT bodies, act in the opposite directions, are of the same same type, have the same magnitude an have the same line and time of action. For example when a bird flies, the bird might exert a push force of say 5N downwards onto the air. The Newton's third law pair to this is the air exerting a push force upwards of 5N on the bird. Note how both forces are of the same type-both are push forces. Both forces act on different bodies- one acts on the bird and one acts on the air. This is the reason why the bird overall ends up gaining greater altiude. Both are of the same magnitude of 5N. Both forces act on opposite directions-one is upwards while the other is downwards.

With Newton's first law no resultant force=the object remaining at rest or continuing with constant velocity. So there's no acceleration. The fundamental idea is that a resultant force creates the acceleration of the object, so it decreases/increases velocity.

Understand
6. (Original post by Mehjabeen)
So can we say that the Newton's first law pair may or may not be equal, act on the same body, may or may not act in opposite directions and so on?
With Newton's third law only you talk about pairs of forces. This because a Newton's third law pair of forces share a number of similar things like same type of force, same magnitude, same line and time of action etc.

Like Stonebridge said, Newton's first law does NOT refer to forces as if they come in pairs. Newton's first law is essentially just a definition of a resultant force on a body and its effect. Also it's about things would just continue in their motion unless a resultant force acts on them (inertia).
7. (Original post by Mehjabeen)
So can we say that the Newton's first law pair may or may not be equal, act on the same body, may or may not act in opposite directions and so on?
Maybe you could tell us where, and in what context, you have found the expression "Newton's 1st Law Pair"?
Newton's 1st Law is about any number of forces acting on an object. It states that if the resultant is zero the object either remains at rest of moves with uniform velocity in a straight line.
If you just happen to have two forces acting on the object that would certainly be a pair. The fundamental difference between this and the 3rd Law is that in the case of the 1st Law the two forces would be acting on the same object. In the 3rd Law case they act on different objects.
But the expression "1st Law Pair" is not very helpful without the context.

I suspect this idea could have come from a question on equilibrium of forces and refers to an object resting on a table or suchlike, with its weight acting downwards and the normal reaction of the table acting upwards. You then have the typical case of two forces acting on the object in equilibrium. Questions like this often then ask you to identify the "Newton's 3rd Law" reactions to these forces.
If there is a specific question you are working on it would be better to post the whole question here if you want specific and meaningful answers.
8. The classic example would be a book resting on a table.

The weight of the book and the reaction force on the book from the table are equal IF the book is in equilibrium (first law says resultant is zero))

the reaction force of the book on the table and force of the table on the book are ALAWYS equal (Newtons 3rd law pair).

I'm guessing that its this kind of sitauation the OP is referring to.

I hate the term 'reaction force' becasue it leads to this kind of confusion. I refer to 'contact' forces.
9. The question was to state six differences between Newton's First and Third Law. I understand what you mean and maybe what you said later about the object being in equilibrium is to be assumed while answering.Thanks a lot for clearing that up though.
(Original post by Stonebridge)
Maybe you could tell us where, and in what context, you have found the expression "Newton's 1st Law Pair"?
Newton's 1st Law is about any number of forces acting on an object. It states that if the resultant is zero the object either remains at rest of moves with uniform velocity in a straight line.
If you just happen to have two forces acting on the object that would certainly be a pair. The fundamental difference between this and the 3rd Law is that in the case of the 1st Law the two forces would be acting on the same object. In the 3rd Law case they act on different objects.
But the expression "1st Law Pair" is not very helpful without the context.

I suspect this idea could have come from a question on equilibrium of forces and refers to an object resting on a table or suchlike, with its weight acting downwards and the normal reaction of the table acting upwards. You then have the typical case of two forces acting on the object in equilibrium. Questions like this often then ask you to identify the "Newton's 3rd Law" reactions to these forces.
If there is a specific question you are working on it would be better to post the whole question here if you want specific and meaningful answers.

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