One of the first things we learn is that the Earth rotates very quickly (about once a day). We, as very imaginative 4/5/6 year old children, quickly realize the dangerous implications of this. However, as quickly as the fear beset us, we are calmed by our teacher/parent/television. "Gravity holds us to the Earth so we don't fly off." Whew! Crisis averted. What we feared as children was that our bodies would resist the angular acceleration demanded of us by the rotation of the Earth and that we would fly off into space. We then learn that the gravitational pull on our bodies easily counters the "centrifugal force". But I've been thinking lately... The angular acceleration we feel and resist isn't always the same. Relative to the Earth's axis, someone close to the north pole wouldn't feel nearly as much centrifugal force as someone at the equator. And it's not only the Earth! The Earth revolves around the Sun, the Sun around the center of the galaxy, and the galaxy around god-knows-what. Surely, our bodies resist all of these accelerations. If you think about it even more, these orbits could compound or diminish the centrifugal force we feel if they are synchronized in a particular way. Like constructive/destructive interference in waves. For example, the Earth rotates counter-clockwise and orbits the sun counter-clockwise (as viewed from the North Star). Therefore, person A standing on Earth and facing the sun will feel the centrifugal force from being hurled around the Sun, but the centrifugal force from the Earth rotating will be in felt the opposite direction, so the net centrifugal force is diminished. However, person B standing on the opposite side of the Earth, away from the Sun, will resist the rotation of the Earth and the Earth's orbit around the Sun in the same direction (out away from the sun), so the net centrifugal force is compounded. The angular acceleration we undergo, and therefore resist, is always changing, depending on how all the Earth's/Sun's/Galaxy's orbits are interacting, and different points on the Earth's surface (Relative to the Earth's axis, someone standing on the equator should feel twice the centrifugal force as someone at +/- 60 degrees latitude). If you factor into a person's free body diagram the resistance to all of these celestial spins, the normal force acted on us by the Earth isn't always the same. Factor in the gravitational pull by these masses on a person and the change is even more dramatic. Our weight isn't always the same! It changes depending on the time of day, the day of the year, even how far you live from the equator! Out of curiosity, I did some numbers. I live in Ohio, which is at 40 degrees latitude, so I revolve about the Earth's axis at about .766 times the radius of the equator and therefore at .766 times the speed. I found that I feel a centrifugal force of .026 N due to Earth's rotation. Keep in mind that I feel that force pushing me normal to the Earth's axis, not away from Earth's center. Gravity, however, does pull me to the Earth's center with a force of 1067 N. So... not really comparable... but still interesting! After adding the vectors, I find that The normal force of the earth on me is actually 1066.98 N. What's more interesting is that, because the centrifugal force isn't normal to Earth's surface, I also constantly feel a force of .017 N to the South! Can't really say I've ever noticed it, but cool! What about forces from the Sun? I found that the Sun and I pull on each other with a force of .6 N and I feel a force of .005 N directly away from the sun because of the revolution of Earth around it. Let's say that it's 12:00 noon in the middle of summer in Ohio (wishful thinking). This means that all 23.4 degrees of tilt of the Earth's axis are toward the Sun and that Ohio is facing the Sun more directly than at any other point in the 24 hour day. At this moment, the orbital plane of the Earth is offset from Ohio's latitude by 16.6 degrees. I'm going to ignore the angle between the orbital plane and the Ohio-Sun line, because it makes things a lot more complicated and it's negligible (.0006 degrees). Earth is pulling me toward it's center with 1067 N. I feel a force of .02 N away from Earth because of it's spin. The Sun is pulling me away from the Earth with .6 N and I feel a force of .005 N into the Earth because of Earth's orbit. After adding all the vectors, I find that the normal force of the Earth on me is 1066.385 N. Now, let's say the opposite. It's the 12:00 midnight in the middle of winter in Ohio (that's more like it). In this situation, the normal force of the Earth on me is 1067.035 N. That's a .65 N difference! That's .146 pounds! Okay, so it's not that much, but I think it's cool. That's just from gravity from the Sun and Earth and our motion through space, relative to the Sun! I haven't even begun to incorporate all of the bodies we revolve around and all the masses that pull on us! I don't claim to be an astronomer, and I know Newtonian physics gets a little funky on certain scales, so I don't doubt that I could be wrong. What do you think?