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Why can't we feel the acceleration of the earth?

  1. Jul 25, 2010 #1
    I used my intuition and decided that acceleration is only felt on certain levels. Say if you were to super-size a car to the size of a big city. If you were to drive in that car as a normal sized human you would "feel" less acceleration than you would in a regular sized car with the same actual acceleration.

    Could I be right, or is there another reason why we cannot feel earth's acceleration?
     
  2. jcsd
  3. Jul 25, 2010 #2
    You do feel it...it's that pressure (force) against the bottom of your feet when you are standing. It's analagous to the force you feel against your back when you accelerate in an automobile or an airplane.
     
  4. Jul 25, 2010 #3

    mgb_phys

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    The earth isn't accelerating - it's going at the same speed.
    There is a force you feel from the earth spinning which pushes you outward (like on a roundabout) if you are on the equator this means the spin is lifting you up against gravity.
    At the equator you weigh a 1/4 % less than at the poles.
     
  5. Jul 25, 2010 #4

    DrGreg

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    Sorry to nitpick. This would be true if the Earth were spherical, but in fact the ellipsoidal shape means that gravity is slightly stronger at the poles which exactly cancels out the effect mentioned.

    Nevertheless this doesn't negate the thrust of the reply: the effects of acceleration are quite tiny compared with gravity. Also the effects don't vary over time so your body has nothing to compare your apparent weight against.
     
  6. Jul 25, 2010 #5
    The earth IS constantly accelerating towards the sun. The reason you don't feel it is because the acceleration is due to gravity, which you don't feel. It's the same reason you're weightless when in orbit. The earth is also in orbit, and hence as far as it's concerned, it's also weightless.

    When you're accelerated by a car, it pushes against your back, which in turn compresses the rest of your body, which is what you feel. However, gravity pulls on each and every atom in your body with practically the same force, so even though you might be accelerating quite rapidly, your body doesn't feel it. There's nothing getting compressed or pulled apart.

    I thought that the effect of gravity is weaker at the equator. Isn't that why they launch rockets as close from it as possible?
     
  7. Jul 25, 2010 #6
    My answer to the OP: the acceleration due to the Earth's rotation does exist, but is in effect bundled together with the acceleration due to gravity. We cannot distinguish these accelerations, but we know of them by theoretical reasoning.

    Are you sure they exactly cancel out?
     
  8. Jul 25, 2010 #7

    HallsofIvy

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    No, they lauch rockets as close to the equator as possible- and launch to the east. in order to add speed of the earth's rotation to the launch speed of the rocket.
     
  9. Jul 25, 2010 #8
    I think it is because of our everyday life experience that we cannot feel anything about the earth's acceleration in particular and the earth's motion in general. From the day we're born to the day we comment on this thread, we have been living on the earth. If we were born in the space, brought up in the space, went to school in the space where gravity doesn't exist, our experience would be different. In that situation, our feeling when we went on a trip to earth would be somehow "the same" as our actual feeling when we go from the earth into the space (by "the same", I mean it's a weird feeling, out of our everyday life experience). The physical effect does exist, there is gravity, there is spin, and we undergo gravity every moment on earth. But IMHO, it cannot account for our feeling which is so biased and subjective.
     
  10. Jul 25, 2010 #9
    Last time I checked, acceleration is defined as change in "velocity" over change in time. Velocity is a vector which contains speed and direction. The earth is always changing its direction, not speed because of centripetal acceleration. So the earth is always accelerating.
     
  11. Jul 25, 2010 #10
    I cannot see how you came to this conclusion. It has nothing to do with the size, the earth's acceleration is very small. One revolution around the sun every year is slow, if the sun is an inertial frame.
     
  12. Jul 25, 2010 #11

    D H

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    Very small indeed: 0.59 cm/s2, or 605 micro-g. However, you can't feel that because you cannot feel gravitation. What you can feel is that the difference between the gravitational acceleration of the Earth as a whole toward the Sun and your gravitational acceleration toward the Sun. At high noon you are closer to the Sun than the center of the Earth, about 6378 km closer if by chance your location is exactly along the line connecting the centers of mass of the Earth and the Sun. Your gravitational acceleration toward the Sun is a bit larger than that of the Earth as a whole. The difference between these accelerations is very, very small; it is about 0.05 micro-g.
     
  13. Jul 26, 2010 #12

    mgb_phys

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    No the effects add, at the equator you are about 20km further from the center of the Earth - which reduces your weight slightly, but the centripedal acceleration also reduces the downward force - further reducing your effective weight.

    You can work it out but IRC the effect of the spin s about twice the effect of the larger radius
     
    Last edited: Jul 26, 2010
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