Re: How fast is the earth spinning, and how come we can't feel it?

Your question is "How fast is the earth spinning, and how come we can't feel it?"

If you think about how long a day is, I think you will figure out that the earth spins one revolution in about 24 hours, so if we put a rotational rate on the earth it would be 360 degrees (one revolution) per day = 360 degrees per 24 hours = 15 degrees per hour = 15 degrees per 60 minutes = 0.25 degrees per minute.

Now, the actual speed that you have depends where on the earth you are, and the reason for that is that your speed depends on how far from the rotational axis you are. The two extremes are at the equator and at the poles, because at the equator you would be as far from the axis as you can be, and at the poles you would be right on the axis and therefore zero distance from it. For a spinning body the (linear) speed is equal to the rotational rate (which we calculated in the previous paragraph) times the distance from the axis, but the rotation has to measured in radians. There are 2 Pi radians in one revolution, so there are Pi / 180 radians in one degree. So, 0.25 degrees per minute is equal to about 0.004363 radians per minute, or about 0.0000727 radians per second.

So at the equator you would be going about 6380 km (the radius of the earth) times 0.0000727 radians per second, which is equal to about 0.464 km per second, or 464 meters per second! Considering that the speed limit on Interstate highways is only about 29.1 meters per second, you are going fast at the equator by just standing still! At the poles you have zero linear speed, but you are spinning like a top.

There are other motions that we haven't talked about yet, such as the (almost) circular path we take around the sun every year.

Okay, so how come we don't feel all this motion? What our bodies actually sense are accelerations, which are *CHANGES* in velocity. As long as we have a nearly constant velocity we don't feel any motion. Think about what it feels like in a car on a very smooth road if the car is neither speeding up nor slowing down. If you close your eyes can you tell that you are moving? You probably can because of other clues such as sounds, but you might not *FEEL* the motion. That's the way it is with our motion on the earth; it's constant (more or less) and it's smooth!

Note added by Marc Herant, astro moderator:

In addition, the centrifugal force which tends to pull us away from earth when standing at the equator (same as the force that pulls you to the side of your car in a turn) is small compared to the force of gravity that keeps you down.