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Why do all objects move in straight lines?

  1. Jul 18, 2015 #1
    All objects not affected by gravity keep moving in a straight line for infinite...why?
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  3. Jul 18, 2015 #2


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    Are you certain of that?
  4. Jul 18, 2015 #3


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    As opposed to what? What would you expect them to do?
  5. Jul 18, 2015 #4


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    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".
  6. Jul 19, 2015 #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"...

  7. Jul 19, 2015 #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.
  8. Aug 1, 2015 #7
    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.
  9. Aug 1, 2015 #8
    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.
  10. Aug 1, 2015 #9


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    There is an answer to this: it's simply not true. Gravity isn't the only force.
  11. Aug 1, 2015 #10
    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?
  12. Aug 1, 2015 #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?
  13. Aug 1, 2015 #12


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    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.
  14. Aug 1, 2015 #13


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    See post #4.
  15. Aug 2, 2015 #14
    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...
  16. Aug 2, 2015 #15


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    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?
  17. Aug 2, 2015 #16
    Question? No, I didn't ask question. It's just my reaction to Feynmann 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.
  18. Aug 2, 2015 #17


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    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.
  19. Aug 2, 2015 #18


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    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.
  20. Aug 2, 2015 #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."
  21. Aug 2, 2015 #20
    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).
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