# Free Fall

As Clark notes, here is a very nicely written piece on parabolic flight and weightlessness. Rare is the reporter (even science reporters) who get the physics right on this, because (as he points out) they get confused by the phrase “zero gravity,” which doesn’t really exist anywhere in the universe. Only one quibble:

Each period of ‘weightlessness’ is limited to half a minute or so; otherwise we ‘zeronauts’ would continue freefalling right into the Nevada desert at 600mph. As it is, during half-a-minute’s power- dive we drop nearly 20,000ft – although inside the plane we are completely unaware of this.

This gives the impression that weightlessness only occurs when you “drop” (i.e., descend in altitude). But it actually happens on the way up as well. In both cases, you are “falling” (in the sense that there is no force acting on you other than gravity). First you fall up, then hit the top of the trajectory, then fall down, weightless all the while, and unable to discern your direction of motion. If this seems counterintuitive, it is. But consider an elliptical orbit. As you approach perigee you’re heading down (toward the earth), and once you reach it, you start heading back up (away from the earth) to apogee, but you’re in orbit, and free fall the entire orbit. A parabola in an aircraft is an orbit that, if continued, would intersect the earth’s surface (which is why it is wise to not continue it). And of course, to be more technical yet, it is only parabolic in an approximate sense (assuming flat earth). In reality, it is a tiny section of an ellipse, because the contents of the aircraft are (briefly) in orbit, within the atmosphere.

I should also note that the phrase “power dive” is also misleading. “Power dive” implies that you are diving with engines at full thrust to get down as fast as possible, but in fact, the engines are barely running above idle throughout (until the pullout). Their only function is to overcome wind resistance so that the aircraft can approximate a cannon ball falling in vacuum.

## 9 thoughts on “Free Fall”

1. Ear Wax says:

But why shouldn’t this be intuitive to roller-coaster riders everywhere? You start feeling the butterflies on the way to the top of the hump, as the track curves more tightly than the parabola of your unimpeded path.

2. Mike Puckett says:

I think I understand this. You are on a ballistic trajectory flying in an arc. The plane is simply flying ‘around’ you for a half minute or so before pulling out of the dive.

3. Daniel L says:

The analogy to the apogee of an elliptical orbit is very helpful to understanding this. So, at all times the plane’s path can be regarded as a partial or sectional orbit – and thus the passenger is always weightless, even when going “up” to “apogee”. – However, I understand this only if the plane isn’t actually accelerating upwards to “apogee” – otherwise you’d be pressed back in your seat. I assume Rand you mean a kind of free-fall “upwards”, where the energy for this must have come from some previous initial acceleration. Anyway thanks for this.

4. However, I understand this only if the plane isn’t actually accelerating upwards to “apogee” – otherwise you’d be pressed back in your seat.

Again, you have to understand the proper meaning of “acceleration.” You are accelerating during the entire time — the acceleration is that of gravity (essentially a constant for this case), and it’s always toward the center of the earth. On the way up, this acceleration results in a reduction of velocity, and on the way down, it results in an increase. But the sensation is the same both ways, because we have no way of sensing velocity, absent wind in our face or visual cues.

5. Daniel L says:

Thanks, I do get your point about constant acceleration of gravity towards the center of the earth. (It’s like in the simple vector diagram of projectile motion, where the vertical vector arrows get progressively smaller towards apogee, then progressively bigger again towards ground – despite their changing size, the arrows are always there, and the particle is always falling.) – I simply meant though, don’t you need extra thrust from the plane’s engines to get you up to apogee in the first place – that’s the acceleration I referred to. I guess you are referring to the picture after this intial thrust – just as the elliptical orbit analogy factors out the original blast-off to orbit. In both examples, after the initial thrust, total energy PE + KE remains constant (allowing for energy loss due to air drag). The short version of this long-winded reply is that I probably misunderstood you on context. Regards .. Daniel

6. I think I understand this. You are on a ballistic trajectory flying in an arc. The plane is simply flying ‘around’ you for a half minute or so before pulling out of the dive.

Exactly. The same forces are acting on both you and the plane, while neither you nor the plane are interacting directionally with one another.

Manipulating gravity, if it ever were possible, would be about the only way a spaceship could accelerate from a standing start to lightspeed and beyond in an instant like we see in movies and on TV — somehow the ship creates a closed gravitational environment around itself that propels it and everything in it at exactly the same acceleration, so that to the passengers it seems they’re standing still and the universe is moving past.

And I say “standing” rather than “floating” because of course a technology that can manipulate gravity to that extent could create an internal gravity slope to approximate earth-normal.

7. don’t you need extra thrust from the plane’s engines to get you up to apogee in the first place – that’s the acceleration I referred to.

Yes, you pull a couple of gees downward going into the parabola, as the plane does a dive and then pull up under full power to get up to its mach limit at an upward angle, just before throttling back to enter the free-fall maneuver.

8. Karl Hallowell says:

Manipulating gravity, if it ever were possible, would be about the only way a spaceship could accelerate from a standing start to lightspeed and beyond in an instant like we see in movies and on TV — somehow the ship creates a closed gravitational environment around itself that propels it and everything in it at exactly the same acceleration, so that to the passengers it seems they’re standing still and the universe is moving past.

There are ways. Suppose we took the Earth, plated it in unobtainium (so it won’t slosh around when we move the entire Earth) and strapped on a mobile home upside down. A person inside would appear to be standing on the ceiling. Now accelerate the entire Earth at 2 G opposide the direction of that mobile home. Now the person feels two forces, the 1 G field of gravity pulling towards the center of the Earth and the 2 G push from the acceleration of the mobile home (which is strapped to the Earth). The net force on the person is only 1 G, but he is accelerating at 2 G. The point here is that gravity is not being manipulated, though other problems make it similarly intractable.

This can be taken to extremes. For example, there’s a collection of science fiction short stories by Charles Sheffield titled “The McAndrew Chronicles”. In it is described a vehicle that consists first of an enormous disk of condensed matter and a habitat which can be moved along the axis of the disk to experience a gravity field between 1 G and 49 G. The whole system can be accelerated at a continuous rate of 50 G (it takes matter from the disk and converts it magically to high speed reaction mass). So the crew accelerates along at 50 G, but the disk leads the vehicle and pulls them along at 49 G, so they only experience a pull of 1 G.

9. The same forces are acting on both you and the plane, while neither you nor the plane are interacting directionally with one another.

Actually, not quite right. The forces acting on the plane are aerodynamics, thrust and gravity. The plane “flies around” its contents to ensure that the only force acting on them is gravity.