Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!qt.cs.utexas.edu!cs.utexas.edu!uunet!lhdsy1!yzarn From: yzarn@lhdsy1.chevron.com (Philip Yzarn de Louraille) Newsgroups: rec.skydiving Subject: Re: Graph of osc'n due to high windforce Message-ID: <1023@lhdsy1.chevron.com> Date: 30 Jun 91 02:44:06 GMT References: <1991Jun24.153922.12763@rodan.acs.syr.edu> <1000@lhdsy1.chevron.com> <1991Jun26.162046.8601@athena.mit.edu> Organization: Chevron Oil Field Research, La Habra, CA. Lines: 88 In article <1991Jun26.162046.8601@athena.mit.edu> jnrees@athena.mit.edu (Jim Rees) writes: >No laws of physics are being broken here. Without atmosphere, there is >only one force on the skydiver, so he accelerates without bound until >he hits the ground. With an atmosphere, the skydiver's acceleration >starts at 9.8 m/s^2 and gradually reduces to zero when he hits >terminal velocity. At this point, the skydiver's kinetic energy is no >longer increasing, but his potential energy is decreasing as he falls. >Where is this energy going? Since the skydiver isn't getting any of >it, it must be going into the air in the form of heat, sound, >turbulence, etc. Now in between exiting the aircraft and hitting >terminal velocity, the skydiver's kinetic energy is increasing, but >not as fast as his potential energy is decreasing. The difference is >transferred to the air. > We both agree that the difference is transfered to the atmosphere, we disagree on how. My point is that the potential energy is first transfered into kinetic then lost to the atmosphere. Yours is that the potential energy is transfered to the atmosphere directly. (In both cases, the objects/skydivers have reached terminal velocity, this is written in case new people are reading this and wondering what we are talking about.) I guess we won't be able to convince each others. A final point from me, *potential* energy is just that, a "reserve" of energy belonging to the skydiver (in this case), not the atmosphere. In order to be transfered to the atmosphere, it *has* first to become "real" (kinetic) to the skydiver who can then loose it (or transfer it.) >>>assumption. Maybe I'm too used to feeling that 1 g upward acceleration under >>>my feet every day. :) >> >>Really? Are you feeling 1 g all the times? (or are you just kidding?) ;-) >>-- > >Yes, one *feels* the push of the ground. Have you ever made a balloon >jump? You know that weird feeling when you first step off when there >is negligible air friction. That is the *feel* of no forces, even >though there is a net force of 1g. The reason why gravity cannot be >felt is because it acts uniformly on all the particles in our body. >Our entire body accellerates uniformly in a gravitational field. On >the ground, however, the normal force the ground provides to prevent >us from accellerating towards the center of the earth does not act >uniformly on our bodies. There is much more pressure in your feet, >for example, than your head. > A "g" is an acceleration, not a force. It seems to me that you are using both intermittently. When you are on the ground, standing still (or moving at a constant velocity), you are *not* feeling (or pulling) a g. No way. I have made ballon jumps, I have made helicopter jumps and I have been on roller-coasters and for the first two cases, when I jumped, I could feel the acceleration of *one* g at the very beginning of the jump. In the case of the roller-coaster (or in a swing), when the machine is reaching the top of an arch and starts to go down, people feel less than one g (but more than zero!) because the machine does not go straight down but at a steep (but not 90 degrees) angle. >When you see pictures of astronauts in the space shuttle, they float >around, apparently experiencing a zero-g environment. But in fact, >there is still a force of 1g on them (ok, since they're further from >the center of the earth by a couple percent, the force is a little >less than 1g if 1g is defined as the gravitational force at ground >level). They are, at all times, accelerating uniformly towards the >center of the earth, but never get any closer since they're in orbit. >Other than the force of gravity, there are no other significant forces >on the astronauts, and they feel like they're in a zero-g environment >(which, by the way, would feel the same way). > Allow me to disagree! astronauts are not "apparently experiencing a zero-g environment"! They are experiencing zero-g! The reason for that is that they are experiencing a centrifical force which exactly compensate for the gravitational attraction of Mother Earth. The source of this centrifical force is the revolution of the spaceship about the Earth. Now even though (in the case of a circular orbit) the ship would have a constant speed, it would not have a constant velocity. It is this change of velocity which creates the acceleration which compensates for the gravitational acceleration. Actually, it is a little bit more complicated than that, because it is not the accelerations which cancel each others but the forces! "They are, at all times, accelerating uniformly towards the center of the earth..." No they are not. The concept is wrong. You cannot accelerate and not move. What is going on is that they are subjected to a gravitational force which is cancelled by the centrifical force. The resulting force (henceforth, the acceleration) on them is zero! > -- Philip Yzarn de Louraille Internet: yzarn@chevron.com Research Support Division Unix & Open Systems Chevron Information & Technology Co. Tel: (213) 694-9232 P.O. Box 446, La Habra, CA 90633-0446 Fax: (213) 694-7709