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Understanding the concept of relative motion

  1. Jan 23, 2012 #1
    Imagine a car moving at fixed speed relative to the pavement of a street. I understand that from the point of view of the observer on the pavement, the car is moving. But from the point of view of the observer in the car, the pavement id moving. But how could this be? How could the pavement be moving?

    Am I to believe that when the car engine is moving the earth rather than the wheel?
  2. jcsd
  3. Jan 23, 2012 #2
    There is no absolute [correct] frame of reference...all frames are equally valid.
    Of course you are free to 'believe' whatever you want.

    It's like asking, 'which is a correct graph plot: y vertical, x hozizontal, or the opposite'?
    either works. Actually these are coordinates, rather than 'frames' but you get the idea.

    If you do the math correctly, either frame will work. But often one is easier than

    Getting used to this idea becomes especially important in relativity
  4. Jan 23, 2012 #3
    The whole point is - you really can't tell who is moving and who is not moving but only that one object is moving with respect to another. The earth and cars and planes are great examples. Think of the fact that the earth is revolving about its own axis. Now put an airplane above the earth flying so that it is always daylight. The plane stays so the sun is in the same position pretty much. With respect to the sun then the plane is pretty much motionless but what about with respect to the earth. In order to stay in daylight all the time the plane (let's put it over the equator just to simplify) has to fly over the earth at the rate of one full circumference of the earth in 24 hours. If a person on the earth looked up at the plane it would look like it's flying really really fast and would think they are not moving but the person in the plane would see the earth flying below them really really fast as they would think they were not moving.

    This is why people speed in well made cars. Once you reach a constant speed and you don't have a lot of road noise and engine noise you don't feel any different than you would if you weren't moving at all - unless you ran into something fixed on the earth like a wall.

    So neither the plane, nor the car, nor the earth nor the person are moving each other. They are just moving with respect to each other quite frequently.

    If you carry a ball and run down the street and just drop the ball at your side as you run, you will think that ball is dropping in a straight line. But have a friend who is standing still with respect to the earth, watch, and the friend will say the ball is moving forward with you at the same speed you are moving (assuming it's a heavy ball and not influenced by the wind). When the ball hits the ground and bounces it will bounce forward, not in one place.

    A friend was hit by an egg that was thrown by someone in a moving car. It almost broke her collar bone and cause a bruise that took a year to heal. Why? Because the egg was moving with the speed of the car, plus the speed from the throw plus the speed it had from gravity pulling on it. It was a missile.

    so nothing is the absolute stationary point. Not the sun, not the earth, nothing. YOU can PICK a stationary point and solve your problems with your chosen stationary point. That is called your "frame of reference" or the thing you will compare all other speeds to. it's totally arbitrary and you pick the thing to be stationary that makes the problem the easiest. And it's usually the earth we choose as stationary - but not always.
  5. Jan 23, 2012 #4
    From the point of view of the observer in the car, the Earth is indeed moving. The car is not making the Earth move: indeed, nothing is needed to make the Earth move, the simple fact is that in the frame of reference of this observer, the Earth is already moving.

    From the point of view of the observer in the car, the energy of the car engine is preventing the car from moving with the Earth.
  6. Jan 24, 2012 #5
    It might be inutitive to assume that the earth is fixed and everything is moving with respect to it, but that's just because the earth is so big and heavy. Every motion you, a car, or a plane makes it ultimately pushes itself off of the earth, which seems to sit still.

    When you consider other things, though, it becomes less clear which is "moving" and which is not. You see a plane flying fast overhead....but if you're on the plane, you seem to be sitting still. Indeed if you were knocked out and woke up on a plane, you wouldn't even know you're moving 600 mph unless someone told you. If you're driving a car, it doesn't really matter whether you get hit by an 18-wheeler or the 18 wheeler hits you. The effect is physically and mathematically identical.

    Likewise with the "moving pavement". You are moving relative to it as much as it is moving relative to you. This might be hard to grasp because the earth has appeared to be "fixed" all your life. Perhaps then you have to leave the earth (at least in your mind) to overcome this. Imagine being out in space flying around. You might be moving 20,000 mph relative to earth, but relative to the sun you might be moving ~80,000 mph. Likewise the earth is moving relative to the sun at 60,000 mph. Relative to the galaxy ~200,000 mph? What if you're an alien that came from another planet moving at yet another speed? A So which one is right? The short answer is that the're all right...it just depends on which frame of reference you look at. Usually we pick the one we're about to crash into.
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