Straight lines

[Resha Caner]

Why do objects in motion travel in a straight line when force is absent?

 

[Cute Tink]

What would, otherwise, cause them to deviate from that straight line?

 

[Resha Caner]

What caused it to be on a straight line trajectory in the first place?

 

[sfs]

What caused it to be on a straight line trajectory in the first place?

Conservation of momentum.

 

[Hieronymus]

What caused it to be on a straight line trajectory in the first place?

An impulse with a direction.

 

[Cute Tink]

What caused it to be on a straight line trajectory in the first place?

Irrelevant. You stated the object was "in motion" and then indicated it was in a straight line.

 

[Hieronymus]

An impulse with a direction.

..or a force rather, which always has a direction...

Hmm... yeah i think that's correct..

 

[Hieronymus]

Conservation of momentum.

A spinning wheel though...

 

[Resha Caner]

Irrelevant. You stated the object was "in motion" and then indicated it was in a straight line.

My focus is on the straight line, so it is relevant. Let me elaborate further.

The fact that an unforced object will travel in a straight line was one of Newton's postulates. Other ideas had been proposed. For example, Aristotle favored the circle. Another aspect of Newton's postulates was that the unforced object's motion would continue - conservation of momentum as sfs mentioned. I'll accept that for now, putting aside those who proposed otherwise such as Jean Buridan.

However, I do need to note that both Newton and Buridan were proposing that when something is unforced, there is a parameter of the object that remains unchanged. The distinction is that Buridan chose position as the unchanging parameter of an unforced object, and Newton chose momentum as the unchanging parameter (which I'll simplify here to just a scalar speed).

Further, Newton's postulate was implicitly stipulating rectangular coordinates. We could easily choose polar coordinates instead to satisfy Aristotle's intuition. In a system with coordinates (r, theta, phi) where r is the radius and theta, phi are angles, we could pick constant values for r and phi, and a constant speed along the theta coordinate and claim that is how "unforced" should be interpreted.

So, why a straight line?

Honestly, it's not intuitive to me. My intuition would tell me that an unforced object should take a random walk - that it should have no particular direction at all. A straight line indicates to me something causing the object to follow a straight line ... and that leads into a QM question. Are neutrinos really appearing and disappearing or is it the same neutrino taking a random walk? If there is no substantive difference between the two, OK. But if it is important to assume the neutrino that appears is different than the one that just disappeared, why is that the conclusion?

 

[Cute Tink]

My focus is on the straight line, so it is relevant. Let me elaborate further.

The fact that an unforced object will travel in a straight line was one of Newton's postulates. Other ideas had been proposed. For example, Aristotle favored the circle. Another aspect of Newton's postulates was that the unforced object's motion would continue - conservation of momentum as sfs mentioned. I'll accept that for now, putting aside those who proposed otherwise such as Jean Buridan.

However, I do need to note that both Newton and Buridan were proposing that when something is unforced, there is a parameter of the object that remains unchanged. The distinction is that Buridan chose position as the unchanging parameter of an unforced object, and Newton chose momentum as the unchanging parameter (which I'll simplify here to just a scalar speed).

Further, Newton's postulate was implicitly stipulating rectangular coordinates. We could easily choose polar coordinates instead to satisfy Aristotle's intuition. In a system with coordinates (r, theta, phi) where r is the radius and theta, phi are angles, we could pick constant values for r and phi, and a constant speed along the theta coordinate and claim that is how "unforced" should be interpreted.

So, why a straight line?

Honestly, it's not intuitive to me. My intuition would tell me that an unforced object should take a random walk - that it should have no particular direction at all. A straight line indicates to me something causing the object to follow a straight line ... and that leads into a QM question. Are neutrinos really appearing and disappearing or is it the same neutrino taking a random walk? If there is no substantive difference between the two, OK. But if it is important to assume the neutrino that appears is different than the one that just disappeared, why is that the conclusion?

Based on your elaboration, I would simply repeat my question: what force is acting on something to cause motion other than a straight line?

There is no real world example available, given that gravity is a force and there is no convenient place where you can completely remove gravity from the equation.

If a force has acted on an object (causing it to be "in motion"), then how does that motion become random? Forces are, at the point of impact, a vector. You have to stop exerting force before your scenario can take hold, otherwise force is not absent.

Outside of subatomic particles, I'm really curious how you get to the intuitive position that something would move randomly without some force (including internal) acting upon it.

Moving the conversation to subatomic particles is a different matter from what I understand. Those are less understood and I'll let a physicist handle that part of the discussion.

 

[Resha Caner]

Moving the conversation to subatomic particles is a different matter from what I understand. Those are less understood and I'll let a physicist handle that part of the discussion.

OK ... though that's probably the more interesting bent to the conversation.

Based on your elaboration, I would simply repeat my question: what force is acting on something to cause motion other than a straight line?

Let me give you a little more history. In Cartesian physics, force only occurs when two substances are in contact. Therefore, for Descartes gravity was not a force because the two objects are not necessarily in contact. That was one of the reasons Newton hesitated to publish. He feared being accused of witchcraft to postulate "force at a distance" - being able to move objects without touching them.

So, if forces can move things without touching them, why can't straight line motion be motion caused by force at a distance? Why do we assume there is no force?

Put another way, how does one determine when force is present and when it is not?

 

[Cute Tink]

OK ... though that's probably the more interesting bent to the conversation.

Probably, but I don't have the inclination to educate myself that deeply into the subject.

Let me give you a little more history. In Cartesian physics, force only occurs when two substances are in contact. Therefore, for Descartes gravity was not a force because the two objects are not necessarily in contact. That was one of the reasons Newton hesitated to publish. He feared being accused of witchcraft to postulate "force at a distance" - being able to move objects without touching them.

So, if forces can move things without touching them, why can't straight line motion be motion caused by force at a distance? Why do we assume there is no force?

Put another way, how does one determine when force is present and when it is not?

A vacuum doesn't actually touch either, but the pulling force it exerts is similar in function. There is nothing physically touching the way my fingers hit the keys on my keyboard.

There is no reason why it cannot cause straight line motion at a distance. If you'll note from my posts above, the fact that gravity is exerting force at a distance is the problem with properly demonstrating an object moving without any other forces. From what I remember from my physics class, gravity is actually exerting force in a straight line, but the object will move in a curve if gravity isn't pulling at it from directly in front or behind, because it is an additional force, not necessarily a counter-acting force.

One could determine that force is acting upon an object by the effect it generates on that object (acceleration, deceleration or shift in direction for example).

 

[sfs]

A spinning wheel though...

A spinning wheel what?

 

[sfs]

So, why a straight line?

Because the behavior of stuff (i.e. the laws of nature) is invariant under translations. Which implies conservation of linear momentum, which implies straight lines.

and that leads into a QM question. Are neutrinos really appearing and disappearing or is it the same neutrino taking a random walk? If there is no substantive difference between the two, OK. But if it is important to assume the neutrino that appears is different than the one that just disappeared, why is that the conclusion?

It is not important to make that assumption. (On the contrary, there are many situations in which you're not allowed to distinguish between identical particles in QM, even in principle.)

 

[Resha Caner]

It is not important to make that assumption. (On the contrary, there are many situations in which you're not allowed to distinguish between identical particles in QM, even in principle.)

Good to know.

Because the behavior of stuff (i.e. the laws of nature) is invariant under translations. Which implies conservation of linear momentum, which implies straight lines.

I need to understand you better. I get the last step - that conserving linear momentum would imply motion in a straight line. But the first step seems almost tautological. Is there more to it than that?

Further, how is it established that material is invariant under translations? It feels like we're about to head into some circular reasoning - but maybe because I'm missing a piece of the puzzle.

 

[AirPo]

..or a force rather, which always has a direction...

Hmm... yeah i think that's correct..

Perhaps a careful re reading of the OP is needed.

 

[[serious]]

My focus is on the straight line, so it is relevant. Let me elaborate further.

The fact that an unforced object will travel in a straight line was one of Newton's postulates. Other ideas had been proposed. For example, Aristotle favored the circle. Another aspect of Newton's postulates was that the unforced object's motion would continue - conservation of momentum as sfs mentioned. I'll accept that for now, putting aside those who proposed otherwise such as Jean Buridan.

However, I do need to note that both Newton and Buridan were proposing that when something is unforced, there is a parameter of the object that remains unchanged. The distinction is that Buridan chose position as the unchanging parameter of an unforced object, and Newton chose momentum as the unchanging parameter (which I'll simplify here to just a scalar speed).

Further, Newton's postulate was implicitly stipulating rectangular coordinates. We could easily choose polar coordinates instead to satisfy Aristotle's intuition. In a system with coordinates (r, theta, phi) where r is the radius and theta, phi are angles, we could pick constant values for r and phi, and a constant speed along the theta coordinate and claim that is how "unforced" should be interpreted.

So, why a straight line?

Honestly, it's not intuitive to me. My intuition would tell me that an unforced object should take a random walk - that it should have no particular direction at all. A straight line indicates to me something causing the object to follow a straight line ... and that leads into a QM question. Are neutrinos really appearing and disappearing or is it the same neutrino taking a random walk? If there is no substantive difference between the two, OK. But if it is important to assume the neutrino that appears is different than the one that just disappeared, why is that the conclusion?

Think of it this way, an object at rest has no reason to stop being at rest, right? If there is a toaster sitting on a table, it will continue sitting on that table unless something knocks it off. It will not spontaneously fly out the window.

Now, imagine that table is on a train traveling down the tracks. You, a passenger on the train, would still not expect to see the toaster jump out the window. If you look to the window at a toaster outside the train, you expect it too to keep sitting on its table.

This is relativity. There is no real difference between an object in motion and an object at rest. An object in motion can be viewed as an object at rest in a different reference frame.

 

[timewerx]

Why do objects in motion travel in a straight line when force is absent?

How do you even know the object is moving at all?

If the observer is in the object itself, then the object would be completely still and everything else moving relative to it.

 

[Resha Caner]

Think of it this way, an object at rest has no reason to stop being at rest, right? If there is a toaster sitting on a table, it will continue sitting on that table unless something knocks it off. It will not spontaneously fly out the window.

You're appealing to intuition and/or experience. If you read some of my other posts in this thread, you'll see my intuition tells me otherwise. My experience is that unforced objects move in straight lines, yes. But why?

How reliable are intuition and experience in verifying the realities of science? I'll need more. And, FYI, I'm an engineer with much experience in mechanics. The question is a serious one, but it's not going to be easy to answer.

 

[essentialsaltes]

Noether's Theorem shows that symmetries of space imply certain conservation laws. Because physics *here* looks just the same as physics *there* (symmetry under a linear translation) linear momentum is conserved.