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All motion is relative

  1. Jun 19, 2015 #1


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    I spend a lot of time here helping newbies understand that it simply is not meaningful to talk about motion without any reference what it is that the motion is relative to.

    Today, I had THE most visceral reminder of that that I think I've ever had. My car's idle has somehow gone to being set way too high and I'm going to get in to the mechanic soon, but meanwhile, I'm very aware of the vigorous running of the engine when I park and before I turn the car off.

    In the grocery store parking lot I pulled through a space such that my car's front was facing the aisle and the cars on both sides of me had their trunks facing that aisle.

    I parked and RIGHT as I put the car into park, I was fairly sure, the car next to me started backing out and I was so convince that MY car was moving backwards, due to my having inadvertently put the car in reverse that my stomach lurched and I pressed the brake petal so hard I damned near broke my foot.

    It was a sheer panic reaction and of course would have been the right thing to do had I actually been moving. But I wasn't(*) o:)

    * relative to the ground, which is what counted.
  2. jcsd
  3. Jun 19, 2015 #2
    I don't want to imagine what would've happened if during that moment your foot had hit the accelerator instead :nb)
    Btw, shouldn't this go in the general discussion forum? (unless of course you want to discuss the scientific aspect of your experience)
  4. Jun 19, 2015 #3
    I've had this happen 3 times myself. It's really disconcerting. That disconnect between I'm slamming on the brake as hard as I can, but the car is still moving is freaky.
    Kinda glad to hear that I'm not the only one that this has happened to.
  5. Jun 19, 2015 #4


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    Similar confusions are a common occurrence when several people are moving small boats at the same time in a crowded anchorage. At anchorage speeds there's no tactile cue from the motion of wind and water.
  6. Jun 19, 2015 #5


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    Oh, yeah, now that you remind me I had a similar experience on a sailboat a LONG time ago, but that wasn't a panic situation because I wasn't driving and it wasn't my boat, so I just grabbed something and then looked sheepish.
  7. Jun 19, 2015 #6


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    GADS ... I hadn't even thought of that.
    That's where I started to put it but I don't seem to be navigating well, because all I saw were 5 or 6 specific sub forums and it clearly didn't seem to belong in any of them.
  8. Jun 19, 2015 #7


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    It's a classic with trains. It doesn't happen with the dinky commuter trains because the motors are right under your feet and you can hear them start. But train sets with separate electric power cars are different - I've had a few "this train shouldn't be going yet, am I on the wrong one" moments before realising either it's the train at the next platform pulling out or the station's pulling out with us.

    Side question, since phinds mentioned the concept: we typically say that there are no special or priviledged frames in SR, meaning that none is picked out by the laws of physics. But there are usually one or two frames picked out by the scenario - the local earth's surface rest frame being a case in point. Does anyone know if there is an "official" term for these?
  9. Jun 19, 2015 #8


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    Convenient frame!
  10. Jun 19, 2015 #9


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    That's probably the 'Me' frame and the 'My ex' frame. We ended up drifting apart at a considerable fraction of c. I couldn't stand her obsession with time dilation, and she saw me as too much length contracted.
  11. Jun 19, 2015 #10
    I think it would be great if you wrote an Insight article about this. Then, rather than having to answer each new thread with lots of dialogue, you can simply give a link to the Insight article.

    Last week, I was driving on a straight flat section of the Jersey Turnpike, and tried to imagine that the car was standing still and all the scenery was moving by backwards. It was quite a weird feeling.

  12. Jun 20, 2015 #11
    Along those same lines, I remember reading that something some astronauts have really enjoyed as part of being out on a spacewalk is this:

    No wind, no environmental sounds, nothing, just floating in their suits in an EVA orbiting the Earth, it's very easy for them to forget that they're orbiting and to start thinking of themselves as floating still with the Earth spinning silently underneath them.

    I can't help but imagine that must be a really amazing, profound feeling.

    Here's a beautiful video that I think captures it really well:

    Of course, what we're all talking about is how it's impossible to define absolute motion in a physical sense. Here, though, it's very difficult to decide whether the camera's orbiting the Earth or the Earth is spinning underneath in an (untrained) intuitive sense, too, though.
    Last edited: Jun 20, 2015
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