Re: Why do I spin along with the earth?

Hello Nathan,

Objects (and people) in motion keep moving in the same direction at the same speed, unless a force acts on them to speed them up or slow them down. Standing on the surface of the Earth, you're moving around in a circle with radius of about 6000 km at a speed of about 1500 km/hr (depending on latitude). If there were no forces acting on you, you'd keep going in a straight line, which means you need a force to keep you going in a circle. Gravity does this nicely, pulling you straight toward the center of the Earth, which is just what it takes to keep your motion circular. This is similar to what happens when you tie an object to a string and swing it around over your head. The string keeps it going around in a circle, and if you let go of the string it flies off in a straight line. There's a nice description of this at http://explorezone.com/101/centrifugal.htm Since the whole Earth is rotating with you, there isn't any frictional force trying to slow you down, so you keep moving at the same speed. At the surface of the Earth, gravity is much stronger than the force needed to keep you moving in a circle, so you feel the extra force pulling you straight down (towards the center of the Earth).

When you're in space, gravity doesn't suddenly stop, and your motions don't suddenly go away, so you don't "see the Earth spin below you" unless you've done something to change your own velocity. Of course, in order to get into space in the first place you generally need to use a rocket to change your velocity by quite a lot. When the space shuttle is in orbit, for example, it is usually at a height of about 300 kilometers, circling the earth in about 90 minutes at about 27000 kilometers per hour. At 300 kilometers altitude, the pull of gravity is just enough to keep it moving in a circle at 27000 kph. Thus, once the shuttle is up to speed it can turn off the engines and coast. Since the force of gravity is just enough to keep the orbit circular, the shuttle is actually "falling" in a circle. The astronauts feel weightless because they're falling. It's the same effect that makes your stomach lurch on a roller coaster or a falling elevator. There's a good explanation of this at http://www.si.edu/harcourt/h_si/GAL109/NEWHTF/HTF611A.HTM Since the shuttle normally takes off towards the East (to take advantage of the 1500 kph it starts with from the Earth's rotation), astronauts in the shuttle usually see the Earth turning backwards, about one turn every 90 minutes as they orbit around it. When the shuttle lands, it fires rockets to slow down its speed, and uses friction with the atmosphere to slow down some more, until it eventually matches speeds with the rotating Earth and lands on the (moving) ground. It's not much different from the way an airplane lands. When you fly East in a commercial airplane at 1000 kph over the rotating Earth, your speed (as seen by a distant observer who isn't rotating with the Earth) is about 2500 kph, and when you fly West, it's only 500 kph. Either way, the airplane pilot doesn't have to worry about the Earth's rotation. All that matters is the 1000 kph speed of the airplane over the ground.

By the way, there's a neat trick used for some satellites, called "geosynchronous satellites". If you put a satellite in orbit at just the right distance (about 35000 km), gravity can keep it in a circular orbit that takes exactly 24 hours to go around the Earth. This means that if it's orbiting over the equator, it appears stationary as seen from the moving Earth. (So you can point your microwave receiving dish at it and receive television signals without having to move the dish around.) There's an explanation of this and other satellite orbit stuff at http://www.hughespace.com/sat101.html

I hope this answers your question.