Why do all objects move in straight lines?

In summary, all objects not affected by gravity keep moving in a straight line for infinite time, regardless of their mass or other properties.
  • #1
icantevenn
24
2
All objects not affected by gravity keep moving in a straight line for infinite...why?
 
Physics news on Phys.org
  • #2
icantevenn said:
All objects not affected by gravity keep moving in a straight line for infinite...why?
Are you certain of that?
 
  • #3
icantevenn said:
All objects not affected by gravity keep moving in a straight line for infinite...why?
As opposed to what? What would you expect them to do?
 
  • Like
Likes abdullahi abass
  • #4
icantevenn said:
All objects not affected by gravity keep moving in a straight line for infinite...why?
This isn't true, in general. For example, a charged object affected by an E field (but not gravity) will not move in a straight line.

The laws of physics are the same from place to place. Per Noether's theorem, that symmetry leads to conservation of momentum, which in turn leads to a free particle moving in a straight line. We don't know why the laws of physics have that symmetry, but such symmetries are considered to be "fundamental".
 
  • #5
Hi @icantevenn. I can share your frustration with this question - it seems completely counter-intuitive that something will keep moving forever without a force to keep it moving! But remember that your everyday experience is in a world where there are always forces acting on things, gravity, friction, air resistance, etc. I started a similar thread on a forum about 10 years ago trying to understand whether a moon orbiting a planet in a far outstrung area of space - unaffected by any other gravitational effect (impossible, but simpler to imagine) - would eventually fall into the planet, or orbit until the end of time. I must have asked the same question in half a dozen different ways because i was convinced the people answering the question must have misunderstood what I was asking as the answer seemed so counter-intuitive (the answer is that it will orbit until the end of time).

There's a short clip of Richard Fenyman being asked why magnets repel below. He does sort of get to the answer (well, maybe towards the answer), but it's great example of how difficult "why" questions are to answer. Don't lose hope though; in my experience, if you ask enough questions on a subject you eventually get to a point where you understand that the answer to your original question would be crazy if it was anything else... but it still might not explain the fundamental "why"...

 
  • Like
Likes icantevenn, Stephanus and Wuberdall
  • #6
How else would a moving object travel if not acted upon by an outside force?
If there is no force acting to slow or stop a moving object, the moving object won't slow down or stop! :)
A moving object will move infinitely and in a straight line unless acted upon by an unbalanced force. Without gravity or friction hindering its motion, there is no way a moving object could travel in a curved or zig-zagged line; such motion would be indicative of unbalanced forces pushing or pulling unevenly on the object.
To quote an old friend Isaac Newton: An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force.
 
  • #7
mgkii said:
Hi @icantevenn. I can share your frustration with this question - it seems completely counter-intuitive that something will keep moving forever without a force to keep it moving! But remember that your everyday experience is in a world where there are always forces acting on things, gravity, friction, air resistance, etc. I started a similar thread on a forum about 10 years ago trying to understand whether a moon orbiting a planet in a far outstrung area of space - unaffected by any other gravitational effect (impossible, but simpler to imagine) - would eventually fall into the planet, or orbit until the end of time. I must have asked the same question in half a dozen different ways because i was convinced the people answering the question must have misunderstood what I was asking as the answer seemed so counter-intuitive (the answer is that it will orbit until the end of time).

There's a short clip of Richard Fenyman being asked why magnets repel below. He does sort of get to the answer (well, maybe towards the answer), but it's great example of how difficult "why" questions are to answer. Don't lose hope though; in my experience, if you ask enough questions on a subject you eventually get to a point where you understand that the answer to your original question would be crazy if it was anything else... but it still might not explain the fundamental "why"...



Hi @mgkii
Thanks for your thoughtful and honest reply. "Why" is philosophical but now physics has enlarged its boundaries and it is safe to ask this. I think the question is perfectly legit. Those who misunderstand it do it intentionally for a lack of answer. Nothing wrong with counter intuitive; the thing is, there is yet no answer for this. So you often find diversions instead of a clear answer.
 
  • #8
Liza_semi-nerd said:
How else would a moving object travel if not acted upon by an outside force?
If there is no force acting to slow or stop a moving object, the moving object won't slow down or stop! :)
A moving object will move infinitely and in a straight line unless acted upon by an unbalanced force. Without gravity or friction hindering its motion, there is no way a moving object could travel in a curved or zig-zagged line; such motion would be indicative of unbalanced forces pushing or pulling unevenly on the object.
To quote an old friend Isaac Newton: An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force.
Hi, thanks for your note. What you have said is right. And I am aware of Newton's law. The question was not about how an object would behave with or without an outside force, but why it behaves the way it does.
 
  • #9
icantevenn said:
Hi @mgkii
Thanks for your thoughtful and honest reply. "Why" is philosophical but now physics has enlarged its boundaries and it is safe to ask this. I think the question is perfectly legit. Those who misunderstand it do it intentionally for a lack of answer. Nothing wrong with counter intuitive; the thing is, there is yet no answer for this. So you often find diversions instead of a clear answer.

There is an answer to this: it's simply not true. Gravity isn't the only force.
 
  • #10
icantevenn said:
All objects not affected by gravity keep moving in a straight line for infinite...why?

My question, of course, is misleading and was a result of being in a hurry. The essence of the question is: why do object keep moving in a straight line when not acted upon by a force? Is it an inherent quality of the fabric of space?
 
  • #11
Once moving the object has MOMENTUM.
An object which already has momentum will not change it's momentum unless some additional force is applied to it.
It keeps going on moving in the same direction it already is moving in.
Why would it do anything else unless friction or some other external forces was applied?
 
  • Like
Likes icantevenn
  • #12
icantevenn said:
Those who misunderstand it do it intentionally for a lack of answer.
I do not agree. Not all questions are legit and the "why" question is certainly one of those to which there is no answer. All an honest Scientist will / can tell you is how a number of Physical quantities relate to each other, using a model derived from from experiment or observation. There's no point in complaining that they don't know the answer to the 'why' question. They are not copping out. They are telling you as much as they can. The only aim can be to be able to predict what will happen in as many circumstances as possible.
"Ultimate truth" is, imo, cloud cuckoo land.
 
  • Like
Likes icantevenn
  • #13
icantevenn said:
there is yet no answer for this
icantevenn said:
why it behaves the way it does.
icantevenn said:
why do object keep moving in a straight line when not acted upon by a force
See post #4.
 
  • #14
mgkii said:
Aunt Many's in a hospital. Why? Because hydrogen atom form 1050 attached to oxygen atom. :oldlaugh: Surely Richard, can fool all of us, but nature can't be fooled.
RIP Richard...
 
  • #15
Stephanus said:
but nature can't be fooled
Who is 'nature' and why would it be fooled or otherwise? Do you believe there is an answer to any random question that can be assembled by a random selection of words? Why should you expect an answer to those 'why' questions?
 
  • #16
sophiecentaur said:
Who is 'nature' and why would it be fooled or otherwise? Do you believe there is an answer to any random question that can be assembled by a random selection of words? Why should you expect an answer to those 'why' questions?
Question? No, I didn't ask question. It's just my reaction to Feynman video, when he explained why Aunt Any is in the hospital and ended up in explaining the anomaly of water. And I remember his quote, from Challenger disaster 1986? that "Nature cannot be fooled". I like Richard, he was funny. I once read he cracked to a safe in Manhattan Project, he was very young at that time. 26 or something. Einstein was 65 at that time. Of course Einstein wasn't in Manhattan Project. He was a pacifist. Well, Richard cracked the safe by entering numbers that perhaps some scientist use.
Perhaps something like 314... pi, 1.618... golden ratio, 2.718..., Euler. I don't know. But the safe was cracked, he put some silly notes there. The intelligence were in panick.
 
  • #17
There I was - looking for an argument and all the time we were in agreement!
I read his autobiography a while ago. I seem to remember he read one combination scribbled on the back of the safe / cabinet. He was a smart cookie but I fear he wasn't aware that he was talking over the heads of most of his audience. His presentation was so good and such fun that they thought they understood him. I had a similar experience in a lecture by the late Herman Bondi.
 
  • #18
In our current understanding of physics, an object that is "moving" in a straight line, not experiencing any force, can be considered to be at rest. It is valid to consider any inertial reference frame as a stationary frame. So in our current understanding of physics, you are asking why an object at rest with no force applied will remain at rest forever. It is something you may just have to accept without proof.
 
  • #19
Questions like "Why?" were big back in the early days of physics. They called it "metaphysics". I have read some of their explanations... mostly seems like arguing in circles to me. The only real answer one can give is, "That's the way the universe is."
 
  • Like
Likes icantevenn
  • #20
icantevenn said:
My question, of course, is misleading and was a result of being in a hurry. The essence of the question is: why do object keep moving in a straight line when not acted upon by a force? Is it an inherent quality of the fabric of space?

I think you've just answered your own question there. Why do objects keep moving or stay at rest when no forces act upon them? It does indeed seem to be just an inherent property of the universe we live in. Now the question of why our universe has this (and other) inherent properties, that is a different and harder question.
It could be that the multiverse theory is right and that would solve the next why question; our universe has these inherent properties because it is part of an endless series of universes all with different laws and we just happen to live in the one with this set of laws and properties. (I believe this is the anthropic principle, can anyone confirm?). But again, then the why question can be extended to 'why do all universes have different sets of rules, and what rules govern the multiverse?'.

But this shows the difficulty, and often futility, of 'why' questions that has already been discussed here (and Richard Feynman so eloquently explains it).
 
  • Like
Likes icantevenn
  • #21
FactChecker said:
In our current understanding of physics, an object that is "moving" in a straight line, not experiencing any force, can be considered to be at rest. It is valid to consider any inertial reference frame as a stationary frame. So in our current understanding of physics, you are asking why an object at rest with no force applied will remain at rest forever. It is something you may just have to accept without proof.
Well yes. When all experiments point to this sort of thing happening, we can call it a Law. A problem arises when you get cosmological observations that require the addition of dark matter into the model because things don't behave as we might expect. Another layer of the onion is revealed.
 
  • Like
Likes FactChecker
  • #22
sophiecentaur said:
There I was - looking for an argument and all the time we were in agreement!
I read his autobiography a while ago. I seem to remember he read one combination scribbled on the back of the safe / cabinet. He was a smart cookie but I fear he wasn't aware that he was talking over the heads of most of his audience. His presentation was so good and such fun that they thought they understood him. I had a similar experience in a lecture by the late Herman Bondi.
You actually met Bondi?? The Relativity in common sense?? (if I remember correctly)Wow, I envy you. Whom else do you met, Oppenheimer?
And I forget where I read that. Richards' divorced, and his former wife said, not quote unquote, I didn't remember precisely the word. [He does calculus even on his bed :oldlaugh:]
Okay, let us be serious.
"Why" is important I think to some degree. Just consider how much does "why" cost, in Pathfinder, Curiousity, Sojourner. I hope they're not for millitary.
And the biggest 'failure' of all is Hubble :cool:. I read that, instead of answer, the scientists only found questions and more questions. But, it's the spirit of science.
 
  • #23
sophiecentaur said:
Well yes. When all experiments point to this sort of thing happening, we can call it a Law. A problem arises when you get cosmological observations that require the addition of dark matter into the model because things don't behave as we might expect. Another layer of the onion is revealed.
Your point about dark matter is interesting. I guess it would have been better if I said that it is something we have to accept unless a counterexample is found. Dark matter might provide that.
 
  • #24
Stephanus said:
You actually met Bondi??
He must have been a tad younger than Oppers, i think. He lectured (gave a guest lecture) at Reading Uni in 1966(ish). Needless to say, he was just 'some guy', as far as a snotty undergraduate was concerned. I mainly appreciate it in retrospect. But actually, that era was quite a dark ages when you think where we have come since then. Hoyle was the reasonable one in those days, with his steady state!
 
  • Like
Likes Stephanus
  • #25
Enclose said:
I think you've just answered your own question there. Why do objects keep moving or stay at rest when no forces act upon them? It does indeed seem to be just an inherent property of the universe we live in. Now the question of why our universe has this (and other) inherent properties, that is a different and harder question.
It could be that the multiverse theory is right and that would solve the next why question; our universe has these inherent properties because it is part of an endless series of universes all with different laws and we just happen to live in the one with this set of laws and properties. (I believe this is the anthropic principle, can anyone confirm?). But again, then the why question can be extended to 'why do all universes have different sets of rules, and what rules govern the multiverse?'.

But this shows the difficulty, and often futility, of 'why' questions that has already been discussed here (and Richard Feynman so eloquently explains it).
Yes, that is true. I think the weak anthropic principle explains it if multiverse theory is right, though I personally don't care much about that theory. Thanks.
 
  • #26
FactChecker said:
In our current understanding of physics, an object that is "moving" in a straight line, not experiencing any force, can be considered to be at rest. It is valid to consider any inertial reference frame as a stationary frame. So in our current understanding of physics, you are asking why an object at rest with no force applied will remain at rest forever. It is something you may just have to accept without proof.
"In our current understanding of physics, an object that is "moving" in a straight line, not experiencing any force, can be considered to be at rest.- Really? I didn't know this. Can you suggest me some papers or books related to this? Really appreciate it.

A followup to that: What about the distance it covers? Is it not considered traveling if it covers distance between two planets? Thanks.
 
  • #27
icantevenn said:
"In our current understanding of physics, an object that is "moving" in a straight line, not experiencing any force, can be considered to be at rest.- Really? I didn't know this. Can you suggest me some papers or books related to this? Really appreciate it.

A followup to that: What about the distance it covers? Is it not considered traveling if it covers distance between two planets? Thanks.
I should clarify my statement. I mean that without looking at external objects, you can not do a physics experiment that will detect the motion. In a closed room, the motion can not be detected. That means that any inertial reference frame has as much right to claim to be stationary as any other, from a physics point of view.
 
  • #28
icantevenn said:
What about the distance it covers? Is it not considered traveling if it covers distance between two planets?
There are two quantities involved here. The Distance traveled between two planets (or two towns) is not the same as the total Displacement between beginning and end of the journey. The Displacement would be the shortest possible route - which would be the path taken by a beam of light. But then, we have General Relativity stepping in and that tells us that light will follow a curved line (in terms of a cartesian map of the Solar System) when it goes close enough to a massive object. Space itself becomes distorted in the presence of mass.
 
  • Like
Likes icantevenn
  • #29
icantevenn said:
Can you suggest me some papers or books related to this? Really appreciate it.
https://en.wikipedia.org/wiki/Inertial_frame_of_reference , loaded with references.
icantevenn said:
Is it not considered traveling if it covers distance between two planets?
It's basically about the point of view you choose. You could just imagine the planets move relative to "it". That would make it extremely difficult to model but plausible. The fact that there is no preferred frame of reference is at the core of relativity.
 
  • Like
Likes icantevenn
  • #30
DaleSpam said:
The laws of physics are the same from place to place. Per Noether's theorem, that symmetry leads to conservation of momentum, which in turn leads to a free particle moving in a straight line.
Are you sure that Newton's first law can be derived from symmetry principles? I always thought it needs to be postulated separately in order to define the Lagrangian and use the least action principle in the first place. We need to define inertial reference frames first, right?
 

1. Why do objects move in straight lines?

Objects move in straight lines because of Newton's first law of motion, also known as the law of inertia. This law states that an object will continue to move in a straight line at a constant speed unless acted upon by an external force.

2. Is there any exception to the rule that objects move in straight lines?

Yes, there are exceptions to this rule. Objects can change direction or speed if they are acted upon by an external force. For example, a ball thrown in the air will follow a curved path due to the force of gravity.

3. What role does friction play in the movement of objects?

Friction can affect the movement of objects by slowing them down or changing their direction. When two surfaces rub against each other, friction is created, which can act as an external force and alter the object's original path.

4. Can objects move in a straight line forever?

In theory, yes, an object can continue to move in a straight line forever if there are no external forces acting upon it. However, in reality, there is always some form of resistance or friction that will eventually slow down or stop the object's movement.

5. How does the concept of momentum relate to objects moving in straight lines?

Momentum is a measure of an object's motion, taking into account both its mass and velocity. Objects in motion will continue to move in a straight line with the same momentum unless acted upon by an external force, which can change its direction or speed.

Similar threads

  • Other Physics Topics
Replies
15
Views
1K
Replies
4
Views
1K
  • Precalculus Mathematics Homework Help
Replies
17
Views
877
  • Introductory Physics Homework Help
Replies
25
Views
1K
  • Other Physics Topics
Replies
5
Views
726
  • Classical Physics
Replies
24
Views
5K
Replies
12
Views
777
  • Other Physics Topics
Replies
5
Views
2K
  • High Energy, Nuclear, Particle Physics
Replies
3
Views
946
Back
Top