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Name: Michael
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
My student asked this question in class, and I couldn't answer it. We were talking about how the earth spins on its axis. We live in New York and a student asked, "If we're on an angle, why do we stand straight up and are not standing at an angle?"


Replies:
The earth's gravity pulls us directly toward the earth's center. Because the earth is round, the earth's center is straight down from wherever you are. Straight down for you in New York is at an angle to straight down for me in Chicago, but both point to the center of the earth.

Richard Barrans Jr., Ph.D.
Chemical Separations Group


--I went to HS in New York city. The reason people in New York do not fall over is that there is so much trash on the streets that you cannot help but stand up straight!! (ONLY KIDDING-but at least you guys have Mayor Giulani instead of Ed Koch and the garbage strikes to deal with).

Serious answer--as best I remember basic gravitational theory. The earth is roughly a sphere. Think of the mass being concentrated at a point in the center of the earth. Gravity acts outward perpendicular to the source. Gravity acts, therefore, radially out from the center of the earth (really simplistic model here) and is constant along those radial lines. A human is on average between 5 and 6 feet tall. The action distance out from the theoretical center of the earth is several thousand miles compared to the final 5-6 feet of the human's height at the surface. That is why you do not feel any perceptible difference on your body whether you are standing upright or reclining on your side (if you were in a centrifuge machine like they use to test astronauts and pilots, you would easily see the difference in body orientation) at the earth's surface. People are normally upright at the earth's surface due to gravity tending to keep them in a perpendicular orientation to the center of the earth along the radial gravity lines.

This is not an exact scientific answer, and I defer to experts on the subject. It should suffice for a grade school or HS class explanation. You can consult the physics dept. at Columbia or Princeton if you wish for a more detailed answer or you can try the Goddard Space Flight center of NASA in Greenbelt, MD, etc. Hope this helps a little.

John Suermann


The earth's axis is tipped relative to its orbit around the sun. The earth also rotates around its axis, and this tends to try and throw us off the earth. There is also the gravitational force of the moon which tries to pull us off the earth. In fact there are so many different forces acting on us I couldn't name them all.

However, the overwhelming force that acts on us is the gravitational force of the earth. This force is so strong compared to all the other forces that a lot of the time, we simply ignore the other forces completely.

This gravitational force acts on all objects and is fairly uniform around the whole earth. There are minor variations in the strength of the force of gravity at different locations around the earth, but humans are generally not sensitive enough to actually feel these variations.

Also, the force of gravity is always pulling us toward the center of the earth. That is why we don't fall off the bottom or anything like that--gravity always looks the same to us because we are on the top of a sphere being pulled to the center.

Anyway, I suppose the short answer to your question is that the earths axis may be on an angle, but gravity is not, and it is gravity that holds us to the earth.

Thanks,
--Eric Tolman


We are standing at an angle. A line starting at our head and running through our feet continues on through the center of mass of the earth. From space, people in New York appear to be leaning at something like a 45-degree angle with respect to an axis through the poles. That polar axis in turn makes an angle with the plane defined by the average orbit of the planets in the solar system because earth's spin axis is tilted.

We feel like we're standing straight up because our notion of up and down is defined by the direction of the gravitational force on us.

Tim Mooney


Michael,

In the big picture, we do stand at an angle. In the short run, though, we see ourselves as perpendicular to the horizon, so we see ourselves as standing straight up.

A good way to picture the situation on a larger scale would be the seeds of a dandelion. The seeds radiate around a seed head with each seed, per small area of the total seed head, appearing to be horizontal to that seed head surface. Those on the bottom appear to be perpendicular to the seed head itself. Those on each side are also perpendicular to the seed head, but in this case they point outward from the base of the seed head.

In pictorial form, we can depict a cross section:
             _\/_
               /\                 

Because gravity effectively pulls us toward the center of the earth, people on each side of the earth see what they call 'UP' as up or away from the ground, and "down" as down towards the ground. Simply put, we do seem to stand straight up, but if you see each human on the earth while in a spaceship distant from the earth, you would see humans radiating from the earth similar to the simple drawing on the left. With respect to another's 'horizon', others ARE standing at an angle; for them, however, they feel they are standing straight up, perpendicular to the ground, and everyone else is standing at some angle to their horizon.

Thanks for using NEWTON!

Dr. Rupnik



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