Perceiving Earth’s Motion
Now apply this example to Earth’s motion. Regardless of Earth’s speed, we would not feel Earth’s motion if it were at a constant velocity. People on the Earth, and Earth’s atmosphere, are part of the Earth, and move along with the Earth – hence, we feel no wind caused by Earth’s motion. We feel no constant velocity component motion of the Earth, because we are riding along with the Earth at the same velocity – just as we feel no motion while riding in a car at a constant velocity.
Earth, however, is not moving at a constant velocity. Earth’s rotation on its axis and revolution around the Sun are accelerated motions, because the direction is constantly changing. Just as we perceive motion from the acceleration of the circular motion on a merry-go-round, we should perceive motion from Earth’s approximately circular accelerated motions. So, the question remains: why don’t we feel the accelerations of Earth rotating on its axis or revolving around the Sun?
Earth’s Centripetal Acceleration
By way of comparison, if we drop an object near Earth’s surface, it will accelerate downwards at a rate of 9.8 meters per second squared. Physicists call that the acceleration due to gravity, and it is related to the gravitational force we feel from Earth, which is our weight.
A person standing on Earth’s equator will experience an additional acceleration of about 0.03 meters per second squared, caused by Earth rotating on its axis. The acceleration caused by Earth revolving around the Sun is considerably less; 0.006 meters per second squared. Both of these accelerations are so much smaller than 9.8 meters per second squared that we do not notice the additional effect. For a 150 pound person, these accelerations due to Earth’s motions have the combined effect of about 1/2 of a pound on the person’s apparent weight. Most people wear more than 1/2 pound of clothing, so the effect of getting dressed or undressed is greater than the effect on our apparent weight from Earth’s motion.
We don’t feel any motion from Earth’s speed as it rotates on its axis and revolves around the Sun because the acceleration is too small to feel.
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