The Question: If the Earth is rotating on its axis and also revolving around the Sun, why don’t we feel any motion?
When Do We Perceive Motion?
Don’t try this when you are the driver, but next time you are the passenger in a car, try a little experiment: on a straight stretch of road, ask the driver to drive at a constant speed. Roll up the windows to block any wind, and close your eyes to avoid seeing the scenery whiz by. As long as the driver is keeping the car at a constant velocity, you will not perceive any motion. If you throw a ball straight up it will fall back down into your hand. This effect is the same whether the car is traveling at 30 miles per hour, or 60 miles per hour – it even works in a jet traveling at ten times that speed.
Now, if the driver pushes down on the gas pedal so that the car accelerates, you will feel pushed back into the seat and perceive the effect of motion. The ball will not fall back into your hand, because the car is accelerating. If the driver either steps on the brake, or goes around a curve, you will also feel the same effects.
Acceleration, which includes increasing speed, decreasing speed, and changing direction, causes the perception of motion. We do not, however, perceive constant velocity motion unless we are in some way looking out at our surroundings to see the motion.
What are Velocity and Acceleration?
In everyday life the words velocity and speed are used interchangeably. In physics, however, they have distinct meanings. Velocity includes direction; speed does not. For example, a car might have a speed of 50 miles per hour and a velocity of 50 miles per hour towards the east.
In everyday life, acceleration often denotes an increase in speed. In physics, however, an acceleration is any change in velocity; an acceleration can be an increase in velocity, a decrease in velocity, or a change in direction.
In the example above, we perceive motion because the car has an acceleration – not because the car has a velocity. Even with the much faster speed of a jet airliner, we only perceive motion when the jet is accelerating. We do not even feel the much faster constant velocity motion of the jet.
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.