Talk:Free fall

Errors in section “Inverse-square law gravitational field”

Both formulas for y(t) are suspect. The first does not lead to the second (I checked with Mathematica), and neither can be found in the mentioned references [7] and [8]. Reference [8] has a different series. The first y(t) is also missing a factor of y_0.

Both formulas should be removed.

A contradiction in the article!

The first line says: "Free fall is motion with no acceleration other than that provided by gravity, and no deceleration other than that caused by the aerodynamic drag of the object". Then a few lines down it says: "Examples of objects not in free fall: ... Jumping from an airplane" What!? ExitLeft (talk) 00:46, 7 December 2007 (UTC)

Excellent point. I've tried to clean up the intro by distinguising between the technical, physics definition (falling under the influence of gravity only), and the nontechnical, skydiving usage in which drag from the air is also acting. What do you think? Physicsman1965 (talk) 04:55, 30 January 2008 (UTC)

The contradiction is still there, but I tried to resolve it by changing the intro to unify the "physics" and "skydiving" definitions of free fall as motions that are both "initiated" by the force of gravity alone. While a skydiver is not "free falling" under the physics definition, I think the article should acknowledge the common usage of the term. Tweesdad

Michael Holmes

I think its wrong to have his story here. His parachute was open, but entangeled. He had spinn on the parachute. Yes, he had fast vertical speed but far from free-fall. you can see the video here: http://www.dagbladet.no/nyheter/2007/02/13/491864.html (norwegian)

Czechoslovakia

An anon just ammended the reference to Vesna Vulovic, in what was probably a well meaning attempt to avoid nationallistic bias [1]. However, I suspect the plane didn't blow up over both the Czech Republic and Slovak Republic. Does anyone know the more precise location based on today's boundaries? -- Solipsist 21:51, 11 Jun 2005 (UTC)

Here: Srbska Kamenice. Article updated. Pavel Vozenilek 22:02, 11 Jun 2005 (UTC)

Someone should comform the information provided in this section with the information provided in the Vesna article. They aren't consistent. Rklawton 02:45, 5 February 2006 (UTC)

Unfortunately another nationalist insists on Croatian terrorists blowing up the plane despite lack of proof. Can we watch out for this?Interista (talk) 17:20, 14 October 2008 (UTC)(interista)

Spelling

I've just gone through this article and changed most of the spellings to 'free fall', but I may not have done this right. I did a bit of checking with online dictionaries that suggested the version as two separate words was most common, see for example [2]. More acurately it seems that

• 'Free fall' - is the noun, the fall of a body within the atmosphere etc.
• 'Free-fall' - is the intransitive verb, or the act of falling
• 'Freefall' - not used at all

In general, we should use a consistent spelling within the article. However, this suggests that we should use both 'Free-fall' and 'Free fall' depending on whether it is used as a verb or a noun. We should also consider moving the article to 'Free fall' (I think we are principally discussing the noun). -- Solipsist 19:43, 24 November 2005 (UTC)

I checked with the United States Parachute Association's website. They use "freefall" approximately 98% of time time (out of nearly 300 references). The USPA does not use the hyphenated version at all. Rklawton 02:39, 5 February 2006 (UTC)

Should we create a separate article for "freefall" that refers only to a human falling with nothing more than the wind to offer resistance? Parachutists use the word "freefall" to describe the period between the jump and the open canopy. (see: USPA) Rklawton 20:58, 11 February 2006 (UTC)

I'd say the two are similar enough that it is best to discuss and handle the difference on the same page. If you like, both are in freefall for most of the flight but the later uses a parachute to come out of freefall, whilst the former uses the surface of the Earth :)

Slightly more problematic is the use of Wingsuit flying to control and direct the drag. There is at least one guy who has a suit with webbed flaps between the arms and legs so that he can glide horizontally a little - at which point the parachutists are arguably not in freefall. -- Solipsist 08:46, 26 May 2006 (UTC)

Lots of people use wingsuits. Even skydivers aren't sure what to call skydiving with a wingsuit. To wit, world records. It doesn't seem very sporting to count wingsuit records against non-wingsuit records. However, both wingsuit fliers and freefallers need their parachutes to survive in the end. Rklawton 13:38, 26 May 2006 (UTC)

Shayna Richardson Accident

The mentioned accident was not a good example in this case. I read on http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=1973796 that she opened her reserve parachute, however it failed to inflate properly. Such accidents (hard landings because of parachute malfunction) happen from time to time in skydiving and this particular one was not something very unique or at least not as unique as the Yugoslavian girl that survived plane crash. Shayna had partially inflated parachute what reduced her terminal velocity and thus it was not a pure free fall. I suggest removing this part from this article.

The video makes it clear she wasn't in freefall, so I removed the reference. Rklawton 20:51, 11 February 2006 (UTC)

Tiny Broadwick

I was going to add a See also to Tiny Broadwick, an article I just started, based on Googling up some research... Actually, that article could use some expansion, so if anyone is interested, please make some changes. -HiFiGuy 19:39, 1 March 2006 (UTC)

Problem with the drag equation

The equation is derived from the assumption that the acceleration is dependent on mass and gravity (m * g) whereas in reality in only depends on gravity (g). Secondly, there is a problem with the latex implementation. I'd rather someone with a better math/physics background fix the equation before I attempt to myself. --216.170.141.157 22:51, 25 May 2006 (UTC)

I've moved this comment to the bottom, because Wikipedia convention is to add new comments to the bottom of the page. Now, I was under the impression that free-fall under constant gravity without drag is only dependent on gravity, but free-fall with drag is dependent on the shape, size, and mass of the object as well, which is why a lead ball falls faster than a feather. I can't confirm the correctness of the equation, but I can say that acceleration with drag is NOT dependent only on gravity. TomTheHand 23:31, 25 May 2006 (UTC)

Righto, but remember, we're mixing apples and oranges. That is, there are two different definitions of freefall in play here. One relates to skydiving and the other relates to physics. Let's just make sure the article is clear on the difference - and keeps them separate. Rklawton 06:24, 26 May 2006 (UTC)

There also appears to be an inconsistency with the "real" drag equation, where the drag is proportional to the square of the velocity, not as given here where it is proportional to the velocity.

The first equation of the "With air drag" section is wrong. The first member should be a force but is instead an acceleration (the derivative of the velocity). It should be m * ay = -(k * v) + (m * g). Sorry for my bad English, but I'm not a native speaker.

What is this constant number 5000 of Vy=...? How did you get it? --Shoons 15:12, 15 May 2007 (UTC)

The drag equation is:

F_drag= 1/2 ρv²C_dA

The drag coefficient is dimensionless. Consider that the object moving through the fluid is displacing the mass of fluid at a rate of ρ*A=m/d. The right hand side of the eq. has the dimensions of a force:

energy/distance = F*d/d = m*a = dp/dt

Note that dp/dt reflects that the force is proportional to the velocity and the mass of the fluid accelerated out of the way. The mass accelerated is contained in the swept volume/t:

(V/t)(1/V)m = (A*d/t)(1/V)m = m/t

So the drag force can be seen as a momentum transfer to the fluid per time, and is proportional to v: F=v*m/t The proportionality to v² appears in the drag eq., because of the proportionality of the force to m/t. Spunkets1 (talk) 21:35, 19 February 2008 (UTC).

I changed the statement that with turbulent friction, the drag equation "has no analytical solution". It certainly does: v(t) = v_T tanh(gt/v_T) where v_T is the terminal speed. And you can integrate this to get y(t). My equation editor skills aren't up to making this look pretty but I'll leave it up for others. Tweesdad 00:04, 28 September 2008 (UTC)

The solutions to the drag equations are all wrong. Try the symbolic integrator at integrals.wolfram.com .

v(t) = v_T arctanh(gt/v_T)

v_infinity is missing the A area term, follow the link to terminal velocity immediately above.

y = something else as the solution here is the integral of the wrong v(t) function above.

I'll leave it to someone else who knows how to edit equations to fix the page, please. 80.177.4.46 (talk) 18:16, 19 August 2009 (UTC)

OK, sorry, I was wrong about v(t) and y = something above. I got my integrals the wrong way round. v_infinity is still wrong though so I'll try to fix that. 80.177.4.46 (talk) 00:24, 28 August 2009 (UTC)

Something else: the meaning of the letters in the equation is not explained! It says "where:" and then nothing. Can someone who understand Latex please fix that? Thank you! (Also: willpower?!?) — Preceding unsigned comment added by Octonion (talk • contribs) 20:48, 9 August 2011 (UTC)

Freefalling and parachutists

I would say 'freefalling' in the parachutist sense is not freefall. It is 'dropping' or 'falling'. It doesn't hurt until you land :). Plenty of air-resistance acting on you. Whereas freefall would when there is no net force acting on you. ie: when forces are balanced (ie: zero-g, gravity is countered by your orbital speed). If you were outside the Sun's gravity well, you might be effectively in free-fall... I guess nowhere are you outside of the reach of gravity. But the lack of sensation of forces pulling on your mass == freefall.
-- ~ender 2005-03-13 12:00:MST

1. Note that zero-g is independent of gravity. A mass is in zero-g when no force other than gravity is acting on that mass. You, the reader are presently prevented from experiencing zero-g for significant periods because your chair is firmly pushing on your butt. Jumping up will temporarily place you in zero-g – until you hit something. Rklawton 22:52, 3 June 2006 (UTC)
2. That's one way to use the word freefall. It might even be a good definition for physicists. However, here in the skydiving world, freefall means just you and the wind. Rklawton
3. The science community might consider skydivers not as freefalling but in a maneouver called "aero braking." Because they can change roll, pitch, and yaw as well as horizontal and vertical velocity (within limits), skydivers often think of themselves as "body pilots" or "in flight." Rklawton 22:52, 3 June 2006 (UTC)
4. To prevent confusion, and since skydivers aren't actually in freefall as defined in science, and since skydivers (see the USPA website) spell freefall differently, I think we should create a separate freefall article for skydiving. The two can point to each other to aid disambiguation. Any takers? Rklawton 22:52, 3 June 2006 (UTC)

Splitting the article might be an alternative, but it might be better handled by having separate sections on this page and discussing the destinctions in the different use of the word. -- Solipsist 08:47, 4 June 2006 (UTC)

The article should be split so the definitive definition of free-fall (physics) is a body experiencing solely the force of gravity (with other relevant sections) and a separate article for sky diving freefall and its relevant sections

I think we're really talking about the difference between a technical physics definition and a nontechnical usage among skydivers. I've tried to make that distinction clearer in the introduction, and it might be desirable to make that distinction clearer throughout the existing article, rather than splitting the article into two. On another technical note, 'zero-g' refers to a situation in which the force of gravity is vanishingly small (far away from any massive body such as a planet, star, or asteroid). What one experiences in free fall (technical sense) is apparent zero-g (or apparent weightlessness). One experiences all of the indications of zero-g, even though (ironically) one is falling precisely because gravity is acting. One does not experience apparent weightlessness in free fall (nontechnical, skydiving sense) after the first few moments of falling, by the way. Physicsman1965 (talk) 04:45, 30 January 2008 (UTC)

Record free fall speed

I have found multiple sources saying different speeds at which Kittinger fell. This USAF site says it was 714 mph and this Time magazine article says it was 614 mph. I assume one is a typo since they're only off by one digit. Which would people suggest we use? This obviously has implications in many other articles too.

--W00tfest99 19:02, 19 February 2007 (UTC)

==It's 614... close to the speed of sound at that altitude and enough for Kittinger to claim he broke the speed of sound. At 714, he would have clearly done so. Rklawton 20:59, 19 February 2007 (UTC)

I'm confused (a familar sensation for me). If the terminal velocity for a human body is approximately 124mph, how did Kittinger reach 614mph? Was he using some form of external propulsion (which wouldn't be a 'free fall' in my mind)? Or is it because he was falling in a rocking-chair position, which offers less drag than an arms-and-legs-outstreached, stomach-down position?Tesseract501 (talk) 19:06, 9 February 2013 (UTC)

Photos don't match article

Quick question ... there are two photos on the page. The first is a photo of skydivers, but the article states: Jumping from a plane, as there is a resistance force provided by the atmosphere. Why then is there a photo of parachutists saying they are in free fall? Second, there is a photo of Joseph Kittinger saying it's a record free fall, yet the article says Eugene Andreev holds the record. Should these photos be removed? Pgrote 14:40, 21 September 2007 (UTC)

First, I believed clarified.

Second-- Keep. Kittinger made the higher jump, but the drogue chute lowered his terminal velocity and provided stabilization. Andreev had no such chute, so he fits the term free fall as used by skydivers. MMetro 18:57, 28 October 2007 (UTC)

Acceleration Due to Gravity Versus Acceleration Due to Other Forces

I'm probably just thick, but when you are in free fall, accelerating under only the influence of gravity, you feel weightless (zero G).

On the other hand, when your acceleration is caused by any force other than gravity, you feel a sense of weight (G forces).

This keeps me awake at night.

What gives?

125.161.130.92 (talk) 21:51, 17 November 2007 (UTC)H&T

No, you're not thick. It's complicated but not complex!

a) Acceleration is a change in velocity. Velocity is speed with direction. If you are travelling at a velocity and you change speed or you change direction then you change velocity. A change in velocity is acceleration.

b) Free-fall, zero-g, weightlessness etc is simply when you and your observable surroundings are either accelerating at the same rate, or not accelerating. JustJimWillDo (talk) 13:47, 31 October 2008 (UTC)

LOL didn't get it!

The article says that jumping from an airplane is not a free fall. Several paragraphs below the cases of people jumping from the planes are given as examples of free falls. Can you make it somehow... more clear... —Preceding unsigned comment added by 95.58.136.113 (talk) 14:39, 7 April 2009 (UTC)

Free-fall in scholastic mechanics error

The section 'Free-fall in Newtonian mechanics' should be 'Free fall in scholastic mechanics', according to which gravitational fall was a uniformly accelerated motion because they thought gravity was a constant, as did Galileo in his scholastic mechanics and radically mistaken law of free-fall, whereas in Newtonian mechanics gravitational fall is increasingly exponentially accelerated. See Proposition 32 of Book 1 of Newton’s Principia for his distance-times law for gravitational free-fall.

I flag the title for clarification.

--Logicus (talk) 18:18, 23 July 2009 (UTC)

"Surviving falls" anecdotes each need to specifically say free fall

The "Surviving falls" section anecdotes all need to specifically say whether the person actually free-fell to the ground. Currently they are all maddeningly vague about this point: Was each of them strapped into an aircraft seat as he/she free-fell? Or was it more of a skydiving free-fall? Tempshill (talk) 07:42, 27 November 2009 (UTC)

James Boole flew a wingsuit when he hit the snow and, as a skydiver, I would not call that a free fall. 62.95.76.2 (talk) 14:05, 13 April 2010 (UTC)

Physics definition

Where does this Physics definition come from: "Free fall describes any motion of a body where gravity is the only or dominant force acting upon it, at least initially. Since this definition does not specify velocity..."?

Halliday & Resnick; Fundamentals of Physics, 3rd Ed states "Free fall means falling in a vacuum, so that the frictional resistance and buoyant effect of the air do not affect the motion." p21

The significant difference here is that the definition Halliday & Resnick define "free fall" as actually involving falling. Who defines it in such a way as to not involve falling?

In common sense terms, the name "free fall" would suggest that it is a subset of falling. —Preceding unsigned comment added by 96.52.231.179 (talk) 01:56, 28 February 2010 (UTC)

Free fall time

Norbeck who has contributed to section Inverse square law gravitational field has answered my question[3] on free fall time.The question was What is the free fall time from Earth orbital radius (~150 million km.) to Sun. The answer can readily be found by numerical methods. But using Norbeck's equation it is much simpler. It is 64.56 days and it is related to the period of revolution around the Sun. I found $\mathbf {t} ={\sqrt {\frac {1}{32}}}\cdot T$

where t is the free fall time and T is the period of revolution.

(Sbharris also contributed to discussion by pointing out that the same relation can be found by using semi major axis of infinite eccentricty ellipse instead of radius in Kepler's third law. ) Nedim Ardoğa (talk) 11:24, 10 March 2010 (UTC)

Merger proposal

Equations for a falling body and this article (Free fall) are basically the same thing.sanpaz (talk) 21:57, 9 April 2010 (UTC)

I just found another article about the same thing...Falling (physics).sanpaz (talk) 22:08, 9 April 2010 (UTC)

I'm fine with the merge... I'd leave the main page at Free fall and redirect the rest. Qfl247 (talk) 22:43, 24 May 2010 (UTC)

Agreed. I don't see much unique to Falling (physics). I'm just going to nuke it now. If you liked anything there, feel free to recover it to this page. —Ben FrantzDale (talk) 12:49, 9 June 2010 (UTC)

paragraph 2.3 needs a modification ?

--Guerinsylvie (talk) 14:34, 18 August 2010 (UTC) : good morning ; i am french locuter , so i apologize for my bad english . it is the first time I read this article. I would point this : the huge development is exact but is not pertinent because ... it is not calculated at the good point : If you want a series at x=0 , you don't take the series at z= 1, and put x = -1+z ! it seems easyer to take the series at x= 0.

• So, In the present case, I suggest this sort of redaction :

Let be the time of free fall t , such as h = 1/2 gt^2 ; if you measure t and h, you have g : this is an operation of gravimetry. The gravity of earth is ~ 981 gals, an old unit nammed in honor of Galilée.

• But, in fact, the gravity is depending of altitude-de-chute z = - Z : g(z) = g . R^2/(R-z)^2 : when the body is falling from z= 0 , g(z) is growing up, and you plan that the time t is less than t = sqrt(2h/g). And if you need precision, you have to solve the equation $^{{\ddot {z}}\,=g(z)=g+g.2z/R+...}$ (be careful with sign ; we choose axe downward ) : this equation has a rigourous solution (see note-1):

$t=f(h)={\sqrt {2h/g}}[1-h/6R-...]$

, less than sqrt(2h/g) as expected,

• by inverse-series, you obtain :

$h=1/2gt^{2}+1/3R.(1/2gt^{2}/R)^{2}+11/45R.(1/2gt^{2}/R)^{3}+...$

exactly as expected : the effect is tiny at the beginning !! (see note-2 for an elementary derivation : you need not sophisticated math ) ,

""in my opinion"", 

the huge development showed in the article miss its target ; the task is hard but it don't present real evaluation, because it presents the problem seen by the bad side ! that is to say, at the final term of collision instead of the beginning : you must have at the beginning : z = 1/2 gt^2 !!

Is it useful to take this huge development in this article ? it is not to me to decide. 

In any case, I suggest we change.You never need seven terms ( and as I have shown, these are not the good ones ! ) even in the most precise technology. You need the correction I have given, only if precision is asked : for instance, if you need 1/100000 précision and a fall of 50 m . But for main uses, it is useless : the correction is too very-little ( and it is not indicated by this huge and frightened presentation ).

Of course, for the all fall, the time would be Kepler-demi-period : Pi.sqrt(R/2g) and you have to use the formula I have given , and after the collision the formula given in the article .

• [ note-1 : the rigourous solution is given by Kepler's formula :

$\omega t=u+\sin u\quad {\text{and}}\quad z=R\,\sin ^{2}(u/2)$, with $\omega ^{2}R^{3}=8g.R^{2}$ , that is to say : $\omega t=2\arcsin({\sqrt {z/R}})+2{\sqrt {z/R-z^{2}/R^{2}}}$ ]

• [note-2 : it is perhaps more simple if you want just the first terms to plan a simple perturbation method :

${\ddot {z}}=g+2{\frac {g}{R}}\cdot z+...=>z={\frac {1}{2}}gt^{2}+2{\frac {g}{R}}\cdot \int _{0}^{t}\int _{0}^{\tau }z(t)dtd\tau +...={\frac {1}{2}}gt^{2}+{\frac {1}{12}}g^{2}t^{4}/R+...]$

• ¤¤¤ : please, correct english mistakes ; best respects.

What is fatal height for adults?

Could someone with access to the article Buckman 1991 (PMID 2003254) in the journal The Surgical clinics of North America - or will to search for any other potential source - check if there is any specific height given that causes mortality in 50% in cases for adults - or any other equivalent measure of fatal height?

The thing is, according to Mosenthal 1995 (PMID 7760404) "The 50% mortality for free falls in children occurs at five to six stories (60 to 72 feet), while for adults it occurs at four stories (48 feet)". However, it cites, in turn, Buckman 1991 as well as Barlow 1983 (PMID 6620098). However, when looking into Barlow 1983, it actually states "between 5th and 6th floors", which, assuming US convention, corresponds to between 4 and 5 storeys above the ground, which would rather correspond to something like 40 to 50 feet. It doesn't give what is the case for adults, so I assume Mosenthal got it from Buckman. However, I don't have access to that original article by Buckman myself, but it should still be checked to see what Mosenthal really meant. Mikael Häggström (talk) 14:20, 2 January 2011 (UTC)

Record Free fall section has an error

In the Record Free Fall section, it says Eugene Andreev jumped from 83,523 in 1962. Though "later" jumpers would ascend higher.

Then talking about Captain Kittinger, it says his record jump at 102,800 feet occured in 1960.

Either the word later should be "other" or the dates are wrong. You can't jump later and have it be in the past :-) — Preceding unsigned comment added by 207.171.191.60 (talk) 01:49, 10 July 2012 (UTC)

Contradiction (or simply unclear) about terminal velocity in "Surviving Falls" section

I don't see an explanation anywhere of why extremely-high-altitude jumpers reach a higher velocity than regular jumpers (at least in the non-mathematical sections). (Would they slow down to the regular terminal velocity after hitting regular-jump height?) This makes the following paragraph confusing:

"Another factor to mention is that a falling person will reach terminal velocity after some 12 seconds or so, falling some 450 m (about 1,500 ft) in that time. That person will not then fall any faster, so it makes no difference what distance they fall if it is more than 1,500 ft - they will still reach the ground at the same speed."

Genepoz (talk) 23:19, 8 October 2012 (UTC)

Yes, high altitude jumpers actually slow down before they hit, due to being slowed by thicker atmosphere. At 18,000 ft your air density is half as much as sea level, which means you fall about square-root 2 = 41% faster than terminal velocity at sea level. But if you jump at 20,000 ft, by the time you get to sea level altitude, you go to terminal velocity for high altitude and (as you fall) slow down 40% to terminal velocity at sea level, before you impact the ground. SBHarris 23:35, 8 October 2012 (UTC)

History

I think this section needs to be rewritten. It says that no one tested Aristotle's ideas but on the page for John Philoponus, who lived a millenium earlier, it quotes him as having carried out the test. I'd prefer if someone who's more familiar with editing Wikipedia asnd who knows how to create a link would correct this rather than attempting it myself.  — Preceding unsigned comment added by Jjc2002 (talk • contribs) 14:07, 5 February 2014 (UTC) 

"Fall"

The usage and primary topic of Fall is under discussion, see talk:Fall (disambiguation) -- 70.51.202.113 (talk) 04:04, 2 September 2015 (UTC)

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Removing off-topic sections

IMO, the concept of free fall in physics should not be mixed up with arbitrary material on skydiving and plane crash survivors. I've moved the skydiving section to Parachuting, copied the survivor section into Falling (accident) and will remove the latter here once it's no longer linked on the main page. Any objections? OnceAndFutureFlopsy (talk) 21:45, 28 August 2019 (UTC)

Original?

At the moment I come across a warning This article possibly contains original research.' I find it very hard to find any research in the affected paragraph - let alone original research. Most of the stuff is as old as - ok not Metusalah - but Galileo ? So ... may we laugh ? Sjoerd22 (talk) 15:49, 2 July 2022 (UTC)