Newton's 1st Law: Object at Constant Speed, No Reaction Force

[technician]

adjacent,

There are only two possibilities:

1) there is no force on the object: the object will move in a straight line at a constant speed (Newton's 1st).
2) there is a force on the object: the object will accelerate (Newton's 2nd) AND will exert a force on other objects (Newton's 2nd).

You seem to be thinking that there is some third possibility, there is not.

Newton's laws do net relate to there being NO FORCE...they relate to NO RESULTANT FORCE.
The word RESULTANT is important and is omitted too often, causing confusion...as here.

 

[technician]

An object is traveling at constant speed.There is no force on it(It means it won't also exert any force on any object)What will happen if I put something in its path?If it does not exert a force there will be no reaction force(Newtons 3rd law).Then what will happen to it?

Newton's laws refer to NO RESULTANT FORCE or NO UNBALANCED FORCE ...not NO FORCE.
There is a subtle but important difference

 

[Dale]

Newton's laws do net relate to there being NO FORCE...they relate to NO RESULTANT FORCE.
The word RESULTANT is important and is omitted too often, causing confusion...as here.

I think you are overstating it a bit. Newtons 2nd certainly does relate to resultant forces, and Newtons 3rd law certainly does not. I have seen Newtons 1st law expressed both ways:

A body moves in a straight line at a constant speed unless acted on by an unbalanced force
Or
A body moves in a straight line at a constant speed unless acted on by a force

I prefer the second, because experimentally it is generally easier to set up a no-force situation than a balanced-force situation. If there are no forces then there are automatically no unbalanced forces. Then the special case of balanced forces is taken care of in Newtons 2nd law.

So I usually think of only the 2nd as referring to resultant forces, and the others as referring to individual forces.

 

[technician]

Newtons laws relate to resultant forces...simple fact.

 

[Dale]

Newtons laws relate to resultant forces...simple fact.

No, they don't. I have never seen a single statement of Newton's laws where all three refer to resultant forces. None of the first 5 hits on Google relate all three laws to resultant forces, nor does my Serway textbook.

Can you provide even one example where all three laws relate to resultant forces? If not then your statement is not simple fact, it is simply false.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
"3.Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body."

http://teachertech.rice.edu/Participants/louviere/Newton/law3.html
"for every force there is a reaction force that is equal in size, but opposite in direction"

http://csep10.phys.utk.edu/astr161/lect/history/Newton3laws.html
"I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. "

http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt1
"an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force"

http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
"in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object"

 

[SammyS]

In my experience, textbooks generally refer to the NET force acting on a body: the net force being the resultant of ALL of the forces acting on a body.

Newton's 1st Law is sometimes written with reference to net force. Sometimes it's written without reference to net force, only referring to what happens to the body unless some force acts on the body.

Newton's 2nd Law, unless it's written in terms of impulse and momentum, is always written (in my experience) with reference to the net force acting on a body.

Net force has no place in Newton's 3rd Law.

 

[WannabeNewton]

There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

 

[ehild]

I have confusion on this Until it collided with something From where is the energy provided for it to produce a force on the object?

No need of energy to produce force. Just the opposite: The work of force adds energy to a body.

The force between the wall and ball does not come from nothing. You know that all materials are made of atoms. The atoms have the positive nucleus and the electron cloud around it, relative far from the nucleus. Imagine the ball gets very near to the wall. The electron clouds of the surface atoms of the wall and of the ball gets so close that the Coulomb force becomes appreciable between them. It is the force between like charges, repulsive. That force presses the surface atoms inward, but then the bonds between the atoms get shortened. It is like compressing springs, and a force opposite to the initial velocity of the ball is exerted to it.

ehild

 

[technician]

No, they don't. I have never seen a single statement of Newton's laws where all three refer to resultant forces. None of the first 5 hits on Google relate all three laws to resultant forces, nor does my Serway textbook.

Can you provide even one example where all three laws relate to resultant forces? If not then your statement is not simple fact, it is simply false.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
"3.Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body."

http://teachertech.rice.edu/Participants/louviere/Newton/law3.html
"for every force there is a reaction force that is equal in size, but opposite in direction"

http://csep10.phys.utk.edu/astr161/lect/history/Newton3laws.html
"I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. "

http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt1
"an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force"

http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
"in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object"

I am surprised by the hyperphysics wording. It is bad and I usually find this to be a great site.
It confirms to me that standard textbooks are the only reliable source of information.
Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

 

[WannabeNewton]

I am surprised by the hyperphysics wording. It is bad and I usually find this to be a great site.
It confirms to me that standard textbooks are the only reliable source of information.
Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

Neither of your statements capture the true implication of Newton's first law. What you have said is simply a result of Newton's second law as applied to vanishing net forces. Newton's first law has to do with inertial reference frames, as I have already said: There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

See Kleppner and Kolenkow "An Introduction to Mechanics", pages 55-56.

 

[technician]

Neither of your statements capture the true implication of Newton's first law. What you have said is simply a result of Newton's second law as applied to vanishing net forces. Newton's first law has to do with inertial reference frames, as I have already said: There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

See Kleppner and Kolenkow "An Introduction to Mechanics", pages 55-56.

Can we have a statement regarding Newton's first law that is appropriate for students in the first stages of studying physics...something they can cope with and something that will encourage them to want to find out more?
something along the lines of ...Nelkon&Parker, Duncan, Young&Freedman etc etc

 

[WannabeNewton]

Well 1. Kleppner is a first year text and 2. we can't downplay a law of physics and misrepresent its true meaning just for the sake of getting a newcomer interested. This is what leads to misconceptions about Newton's laws, misconceptions that fail to get removed unless he/she happens upon a good mechanics book like Kleppner (well Kleppner is IMO the best mechanics book out there, not merely good). Inertial reference frames are a pivotal part of Newtonian mechanics and the first law sets the stage for that.

 

[D H]

Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

Some texts say the first, others the latter. The distinction is irrelevant. By focusing on this minutia you are missing the point of Newton's first law.

Can we have a statement regarding Newton's first law that is appropriate for students in the first stages of studying physics...something they can cope with and something that will encourage them to want to find out more?
something along the lines of ...Nelkon&Parker, Duncan, Young&Freedman etc etc

Rhetorical question: Since Newton's second law implies the first, why do we need a first law at all?

The answer depends upon historical perspective. Newton was vying against the age-old tradition of Aristotelian physics. Aristotelian physics held that the natural state of an object was at rest and that some force was needed to keep an object in motion. Newton's first law was a flat-out rejection of that age-old tradition.

We aren't fighting Aristotelian physics anymore, but we still teach Newton's first law. Why? Any good introductory physics text will state that Newton's first law in one of the two forms stated in your first quote, but it will also state that Newton's first law ultimately is a statement about inertial frames of reference. This latter statement is key. Newton's first law provides a framework for testing whether a frame of reference is inertial, and hence whether Newton's second and third laws do apply.

Just as Newton rejected Aristotelian physics, modern classical physics reject some minor points in Newton's physics. One of those minor points is Newton's concept of an inertial frame. Newton had a concept of an absolute frame of reference, God's frame if you will. (Newton was a deeply religious man, even by the standards of his own time.) That concept of absolute motion is not needed and is not used in modern treatments of Newtonian mechanics. What is needed is some way to determine if a frame of reference is inertial. That is what Newton's first law is all about from a modern perspective.

 

[technician]

Rhetorical question: Since Newton's second law implies the first, why do we need a first law at all?

Probably the most sensible, meaningful quote in this whole thread...could not agree more.
first and second laws could be stated together...first statement describes what happens when there is no RESULTANT force...second statement therefore must describe what happens when there is a RESULTANT force...doesn't get much simpler.

The statements could be reversed and it makes no difference. (unless somebody knows otherwise ;))

 

[WannabeNewton]

No it doesn't! Obviously if you plug in a zero net force in the 2nd law, you will get what happens when there is no net force so why in the world would you think it only applies when there is a non-vanishing net force? Maybe you need to go back and read a proper mechanics text as well. You are missing the point of the first law. I love how you ignored the bulk of D H's post, which clearly elucidates the true point of the first law.

 

[sophiecentaur]

That doesn't make a sense to me.How can some thing make a force out of nothing?

Reading all your questions and comments makes me think of the early (pre-Newton) ideas about forces and motion. He had a hard time convincing people that "things don't naturally slow down", for instance. It was assumed that things slow down unless 'helped' to keep going and this lead to the false conclusion that the planets had to be driven round in their orbits. He reconciled the two phenomena of unchanging planetary motion and the way wheeled carts always slow down by introducing the idea of friction as a force.

Newton 1 and 2 imply that there is no force on an object if it is not changing velocity in some way which really says that forces are only there when a mass changes its motion. The force only 'appears' during a collision (or when some field starts to act) so, to put it your way, a change of velocity 'makes a force out of nothing'. (I know this seems to be stated the wrong way round but I think it's the best you can hope for .)

 

[D H]

Thanks. May be in the first question there could be inertial force or something that keeps the object moving in the same way.And exerting forces on objects?

Others have interpreted that "inertial force" as talking about non-inertial frames of reference.

I'm going to assume you might be thinking along rather different lines, that some force is needed to keep an object in motion. This is after all how the world appears to work. Stop stepping on the accelerator and a car will coast to a stop. Slide a hockey puck on a long, smooth sheet of ice and it will eventually come to rest. At rest with respect to the Earth is apparently the natural state of motion for terrestrial objects. Per this viewpoint, some force is needed to alter that natural state.

That Aristotelian point of view was precisely what Newton was arguing against with his first law of motion. Newton's first law says that the natural state of motion of some object is to continue moving however it was moving in the past. A force is not needed to keep some object in motion. Contrary to what Aristotelian physics says, a force is needed to bring an object in motion to at rest with respect to the Earth. That car coasts to a stop because of aerodynamic drag and rolling friction. The hockey puck slows down and comes to a stop because of dynamic friction. Whenever some object undergoes a change in velocity there is necessarily a non-zero net force acting on that object.

 

[Bhumble]

ehild, I don't agree with your assertion that no energy is needed to produce a force. There is some potential or kinetic energy required for work to be performed and while it seems that there could be an ideal problem where there is a force with absolutely no motion that does not seem to be practical.

Can you think of any case where there is no energy but a force is exerted?

Thanks to D H and DaleSpam as I am one of those that missed the point of the first law and always saw it as a wordier way of expressing Newton's second law.

 

[Doc Al]

Can you think of any case where there is no energy but a force is exerted?

Rest a book on a table. The book exerts a force on the table without expending any energy.

 

[Bhumble]

Rest a book on a table. The book exerts a force on the table without expending any energy.

I'm not saying work must be performed just that there must be potential or kinetic energy for a force to be present.

 

[WannabeNewton]

I'm not saying work must be performed just that there must be potential or kinetic energy for a force to be present.

A book at rest on a table has no potential nor kinetic energy if I pick my potential energy reference as the table the book rests on (assuming the book has negligible thickness).

 

[Bhumble]

A book at rest on a table has no potential nor kinetic energy if I pick my potential energy reference as the table the book rests on (assuming the book has negligible thickness).

Reference frame shouldn't matter because you still have potential energy from gravity between the book and presumably Earth or at least the table. Am I really missing something here? Seems to me like the only time this would ever be the case is in a closed system where we are ignoring potential energies and stating there is some outside force. But that isn't really practical.

I could argue oscillating particles for the book on a table but since we're already looking at an infinitely thin book it seems fair to concede this isn't a realistic scenario.

I suppose my problem lies in the idea that forces can come from nothing instantaneously but that doesn't sit we'll with me intuitively (which isn't to say I'm right).

 

[technician]

Newtons first and second laws relate to resultant (or Net) forces

This is posted to clear up any misunderstanding that my original post also included the third law. I made no comment about the third law!

 

[Dale]

This is posted to clear up any misunderstanding that my original post also included the third law. I made no comment about the third law!

You said "Newton's laws" in post 31 and 32. Newton's 3rd law is one of Newton's laws, so your statement could easily be understood incorrectly by a novice reader to have been intended to apply to Newton's 3rd law. This is why in post 33 I said merely that your comments were "overstated", not that it was "wrong". The statement went too far, as I explained in considerable detail.

It is easy to accidentally overstate something, I have done it many times myself. The proper response when someone points out a miscommunication is "oops, I didn't mean to say that". I have had to do that many times, it doesn't hurt very much.

 

[D H]

Thanks to D H and DaleSpam as I am one of those that missed the point of the first law and always saw it as a wordier way of expressing Newton's second law.

They say different things to me. The first says what happens when no forces are present. It's a null statement. The second says what happens when forces are present. It is consistent with the first in the special case of a null force. It has to be consistent with the first; any viable theory must necessarily be internally consistent or it is not viable.

That of course is viewing Newton's first two laws as laws of nature. Here's another rather different way of looking at them: They are not laws of nature. They are instead definitions. The first law defines the concept of "inertial frames". The second law defined the concept of "force".

From this more modern perspective, it's only the third law and Newton's first two corollaries that are laws of nature. Those first two corollaries, in modern parlance, state that forces are vectors and hence add as vectors. Another way to put it: Forces are subject to the superposition principle. Some instructors even teach the superposition principle as "Newton's fourth law".

That's a bit of a mis-statement of course; Newton's Principia only identifies three laws (plus a bunch of corollaries). Nobody in their right mind wants to teach those two corollaries as-is. They're worded archaically and geometrically. It's much easier to teach the algebraic superposition principle.

Reference frame shouldn't matter because you still have potential energy from gravity between the book and presumably Earth or at least the table. Am I really missing something here?

Yes. You are missing that potential energy contains an arbitrary constant. Alternatively, one can arbitrarily select a point at which potential energy is zero.

I suppose my problem lies in the idea that forces can come from nothing instantaneously but that doesn't sit we'll with me intuitively (which isn't to say I'm right).

One way to look at forces is not to look at forces at all. Look at energy instead. That's what you're going to see if you stay in physics beyond the freshman/sophomore intro to physics series of classes. Another way to look at forces (conservative forces) is that they are the gradient of some potential energy function. Forces don't arise from energy; they arise from how energy changes with respect to position. Notice that that arbitrary constant becomes irrelevant here as two smooth functions that differ only by an arbitrary constant have the same gradient everywhere.