# Newton's first law and a body on an infinite and frictionless road

• I
Summary:
Do I need knowledge of thermodynamics to complete the picture of a body moving in constant velocity over an infinite and frictionless road?
Hi everyone! I have started to study physics this month and I got to the Newton's laws. According to Wikipedia, the first law is:
The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant

This definition made me wonder: If there was, let's say, a fictional road, infinite long and frictionless, and a body would move in constant velocity over it, could this be considered an object in perpetual motion? Would this body move infinitely along the road forever? This question led me to several topics on perpetual motion and I learnt this is a forbidden topic in the forum so I don't really want to get into that.

I know that when perpetual motion is discussed the topic goes to thermodynamics but I haven't got yet to that chapter so I'm wondering if just by knowing the Newton laws my picture of the situation is incomplete. I mean, do I need knowledge in thermodynamics to picture what would make the body stop in this situation of an infinite/frictionless road or it is true that theoretically it would keep moving forever?

russ_watters
Mentor
Summary: Do I need knowledge of thermodynamics to complete the picture of a body moving in constant velocity over an infinite and frictionless road?

Hi everyone! I have started to study physics this month and I got to the Newton's laws. According to Wikipedia, the first law is:

This definition made me wonder: If there was, let's say, a fictional road, infinite long and frictionless, and a body would move in constant velocity over it, could this be considered an object in perpetual motion? Would this body move infinitely along the road forever? This question led me to several topics on perpetual motion and I learnt this is a forbidden topic in the forum so I don't really want to get into that.

I know that when perpetual motion is discussed the topic goes to thermodynamics but I haven't got yet to that chapter so I'm wondering if just by knowing the Newton laws my picture of the situation is incomplete. I mean, do I need knowledge in thermodynamics to picture what would make the body stop in this situation of an infinite/frictionless road or it is true that theoretically it would keep moving forever?
The term "perpetual motion" has a meaning that is a little different from what the words themselves imply (see also "Big Bang"). What the term really means is a device that violates one or more of the laws of thermodynamics. Operating forever is neither a necessary nor sufficient criteria in the definition.

jbriggs444
Homework Helper
It would move forever. No need to invoke thermodynamics.

The statement of Newton's first law you have given starts "if the net force is zero". That is a big if. That if means that there is no friction. No air resistance. No thermal bath which could doppler shift into a net retarding force. All relevant thermodynamic interactions get hand-waved away by that if.

The term "perpetual motion" has a meaning that is a little different from what the words themselves imply (see also "Big Bang"). What the term really means is a device that violates one or more of the laws of thermodynamics. Operating forever is neither a necessary nor sufficient criteria in the definition.

Hi! I knew about the "big bang" (neither big nor a bang) but it's the first time I heard this kind of definition. Do you more sources about it? Not the topic of perpetual motion, but your suggested definition. Wikipedia points to the popular one.

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jbriggs444
Homework Helper
We normally take it for granted that the laws of physics are the same now as they will be in the future or have been in the past. If you have a device that could violate the laws of thermodynamics for a little while, violating them for as long as you please is a trivial exercise.

It would move forever. No need to invoke thermodynamics.

The statement of Newton's first law you have given starts "if the net force is zero". That is a big if. That if means that there is no friction. No air resistance. No thermal bath which could doppler shift into a net retarding force. All relevant thermodynamic interactions get hand-waved away by that if.

I understand that it's a huge "if", therefore my hypotetical road just to see if I was missing something in the picture that needs to be completed with thermodynamics.

You say that thermodynamics doesn't need to be involved. May I ask how does this situation differs from the typical topics about machines in perpetual movement that do require thermodynamics to explain the impossibility?

jbriggs444
Homework Helper
Newton's first law says what it says. Do not make it more complicated than it needs to be.

In practice, the "if" in that law is never exactly satisfied. But that's ok. Refer to Newton's second law when there is a non-zero net force.

russ_watters and hutchphd
russ_watters
Mentor
Hi! I knew about the "big bang" (neither big nor a bang) but it's the first time I heard this kind of definition. Do you more sources about it? Wikipedia points to the popular definition.
Keep reading past the first sentence (particularly, the "basic principles" and "classification" sections).

jonander
russ_watters
Mentor
You say that thermodynamics doesn't need to be involved. May I ask how does this situation differs from the typical topics about machines in perpetual movement that do require thermodynamics to explain the impossibility?
I think he means if this is about Newton's First law there is no need to bring in thermodynamics. An object in perpetual, constant speed motion isn't creating, absorbing or expending energy, so thermodynamics has nothing to say about it.

jonander
I think he means if this is about Newton's First law there is no need to bring in thermodynamics. An object in perpetual, constant speed motion isn't creating, absorbing or expending energy, so thermodynamics has nothing to say about it.

Thanks for the clarification!

Indeed, I think I have mixed the situation of a body in perpetual motion speaking only in the context of Newtonian mechanics with the full situation that brings up energy and thermodynamics arguments.

russ_watters
Dale
Mentor
2020 Award
your suggested definition. Wikipedia points to the popular one.
His suggested definition is essentially exactly out of that article:

“A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.”

This is what we don’t discuss here. Newton’s first law is fine.

anorlunda
Staff Emeritus
The closest you might get to satisfying that "if" is a rock drifing in inter-galactic space. If it never comes near a galaxy, it will maintain its velocity indefinitely.

russ_watters
Mister T
Gold Member
I mean, do I need knowledge in thermodynamics to picture what would make the body stop in this situation of an infinite/frictionless road or it is true that theoretically it would keep moving forever?

Saying that it's infinite/frictionless is just another way of saying it would keep moving forever. The point of Newton's 1st Law is that moving in a straight line at a steady speed is equivalent to being at rest. Have your car be at rest on the roadway and have an alien fly by in a straight line at a steady speed. The alien can claim that he's at rest and the car is moving in a straight line at a steady speed. There's no difference!

Now, if you want to have the car move along the roadway and coast to a stop then you need to understand something about thermodynamics to explain, for example, what happened to the car's energy of motion.

russ_watters, jonander and Ibix
ZapperZ
Staff Emeritus
Thanks for the clarification!

Indeed, I think I have mixed the situation of a body in perpetual motion speaking only in the context of Newtonian mechanics with the full situation that brings up energy and thermodynamics arguments.

Out of curiosity, are you "studying physics" exclusively using Wikipedia?

Zz

Out of curiosity, are you "studying physics" exclusively using Wikipedia?

Zz

Absolutely not. I'm enrolled in a distance learning Physics degree.

At the moment, I'm currently reading the "University Physics with Modern Physics" textbook.

ZapperZ
Staff Emeritus
Absolutely not. I'm enrolled in a distance learning Physics degree.

At the moment, I'm currently reading the "University Physics with Modern Physics" textbook.

So then why are you using "definitions" from Wikipedia? Is your text not sufficient?

Zz.

So then why are you using "definitions" from Wikipedia? Is your text not sufficient?

Zz.

The first Newton's law is defined in the same way both in Wikipedia and the textbook.

After Russ saying that the definition of a perpetual machine doesn't necessarily involve "operating forever" I got surprised and I just looked straight on the Internet (and in this case, Wikipedia for the definition)