# Terminal Velocity **URL:** \ **Category:** Factual Questions\ **Created:** [December 4, 2003, 9:56pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033 "2003-12-04T21:56:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Erik\_B.Johnson](https://avatars.discourse-cdn.com/v4/letter/e/f4b2a3/32.png) [@Erik\_B.Johnson](https://boards.straightdope.com/u/Erik_B.Johnson)\ **Post date:** [December 4, 2003, 9:56pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/1 "2003-12-04T21:56:07Z")
What is terminal velocity? If someone shot a bullet straight up in the air could it come back down with enough acceleration to go through a humans skull? ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [December 4, 2003, 10:05pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/2 "2003-12-04T22:05:36Z")
In a vacuum an object, under acceleration will continue to accelerate until it approaches c. With a drag inducing medium (like air) there is a resistive force proportional to the cross sectional area of the abject and the square(?) of it current velocity. Eventually the forces of acceleration and drag cancel each other out and the object stops speeding up. As for the bullet, I’m not sure. People have been killed by “bullets from the sky” though. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [December 4, 2003, 10:07pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/3 "2003-12-04T22:07:08Z")
I swear english is my first language. Sorry about the typos.:smack: ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 4, 2003, 10:07pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/4 "2003-12-04T22:07:23Z")
Terminal velocity depends on the shape and weight of whatever is object you’re talking about. For a bullet, **Cecil** answers your question, at least partially, in his clumn, [Can a bullet fired into the air kill someone when it comes down? (14-Apr-1995)](http://www.straightdope.com/classics/a950414b.html). There’s also been some discussion of this column in _Comments on Cecil’s Columns_ (with on thread being fairly recent) that might interest you. The search function’s pretty slow right now, otherwise I’d try to dig up the relevant links. ---
**Author:** ![BaldTaco](https://avatars.discourse-cdn.com/v4/letter/b/47e85d/32.png) [@BaldTaco](https://boards.straightdope.com/u/BaldTaco)\ **Post date:** [December 5, 2003, 2:08am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/5 "2003-12-05T02:08:00Z")
I saw a TV documentary once which claimed that bullets fired at an angle less than 45 degrees would be lethal, but those fired at a steeper angle would not be. Don’t know how variables such as muzzle velocity, wind, slope of ground, etc. affect that rule. Terminal velocity means that things fall faster until they reach a certain speed, and then stop accelerating. As Cecil said, the max. falling speed of a bullet is generally not enough to be lethal. A bullet fired straight up is decelerating until it reaches a speed of 0, then it starts speeding up again as it falls. All the energy imparted by the gun is lost. A bullet fired at an angle essentially has upward force, and sideways force. The upward force is cancelled out by gravity, leaving only the sideways force to kill. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 5, 2003, 2:29am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/6 "2003-12-05T02:29:54Z")
> [@](#): > > \*Originally posted by Grey \* > **In a vacuum an object, under acceleration will continue to accelerate until it approaches c.** No, it won’t. It will acelerate until it approaches the escape velocity of the body it is falling towards. That’s as fast as any object can fall towards a given body. Of course, if the body is a black hole, then you’re absolutely right. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [December 5, 2003, 3:09am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/7 "2003-12-05T03:09:52Z")
Fair enough, but I was trying to generalize the case of the acceleration. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [December 5, 2003, 3:36am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/8 "2003-12-05T03:36:12Z")
> [@](#): > > \*Originally posted by \*\*\*BaldTaco \*\* > > Terminal velocity means that things fall faster until they reach a certain speed, and then stop accelerating. Things don’t stop accelerating when terminal velocity is reached. The acceleration approaches zero as the velocity approaches terminal velocity. Terminal velocity is never actually reached. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 5, 2003, 4:00am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/9 "2003-12-05T04:00:54Z")
> [@](#): > > \*Originally posted by DrMatrix \* > \*\*Things don’t stop accelerating when terminal velocity is reached. The acceleration approaches zero as the velocity approaches terminal velocity. Terminal velocity is never actually reached. \*\* Are you sure about that? Once the force of wind resistance balances out the force of gravity, what’s left to cause acceleration? Of course the force of gravity changes with altitude, but so does the viscosity of the air. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [December 5, 2003, 5:37am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/10 "2003-12-05T05:37:49Z")
My point was that an object does not have a constant acceleration and then stop accelerating when terminal velocity is reached. The acceleration decreases; it does not stop. Terminal velocity is approached but never actually reached. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 5, 2003, 5:42am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/11 "2003-12-05T05:42:57Z")
Yes, I know what your point was. I’m saying I don’t think it’s entirely correct. Some math might be helpful at this point. ---
**Author:** ![Saranga](https://avatars.discourse-cdn.com/v4/letter/s/d6d6ee/32.png) [@Saranga](https://boards.straightdope.com/u/Saranga)\ **Post date:** [December 5, 2003, 6:43am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/12 "2003-12-05T06:43:55Z")
It’s mathematically true that terminal velocity is approached asymptotically, but here is a [site](http://www.vh.cc.va.us/ph1spring99/lectures/ph_981012_981030/ph1_1021/class_notes.htm) which says: > [@](#): > > If all the forces change very very smoothly this instant will, theoretically, never occur–we will approach terminal velocity but never reach it. However, after a time things like pressure variations and thermal fluctuations make the difference between actual and terminal velocity meaningless. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 12, 2013, 11:15pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/13 "2013-11-12T23:15:53Z")
A few months back I was replacing some wind damaged shingles on my roof. I found a bullet lodged point first in the roof. Looked to be a 9mm FMJ, as I could see the lead core. I suppose it could have been 40cal/ 10mm, but i have seen a fair few 9s… just not in my roof. Left it in place and tarred over it. Looked like it penetrated about two layers of fiberglass-asphalt shingles. Would sure smart if it hit you on the noggin, but it would need to hit just wrong to kill you to death. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [November 13, 2013, 12:49am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/14 "2013-11-13T00:49:14Z")
[QUOTE=Drmatrix] My point was that an object does not have a constant acceleration and then stop accelerating when terminal velocity is reached. The acceleration decreases; it does not stop. Terminal velocity is approached but never actually reached. [/QUOTE] This is kind of nitpicky, but terminal velocity will vary with altitude, due to the density of the atmosphere. So a bullet falling slower than terminal velocity gaining speed may cross terminal velocity as terminal velocity drops, briefly “reaching” it. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [November 13, 2013, 12:58am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/15 "2013-11-13T00:58:30Z")
> [@ZenBeam](#): > > This is kind of nitpicky, but terminal velocity will vary with altitude, due to the density of the atmosphere. So a bullet falling slower than terminal velocity gaining speed may cross terminal velocity as terminal velocity drops, briefly “reaching” it. Indeed, There is little point in claiming the falling object “never reaches” terminal velocity, of course there is wind and uplift and so on, and so there is a noise in the actual velocity, so its able to be said that its reached terminal velocity when it actually starts slowing down and speeding up again … when its in the noise zone. Also a bullet shot downward will de-accelerate and slow down to terminal velocity… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 13, 2013, 1:02am UTC](https://boards.straightdope.com/t/terminal-velocity/217033/16 "2013-11-13T01:02:44Z")
It’s misleading to say that an object never reaches terminal velocity, but it’s also tricky to answer the question “How long does it take to reach terminal velocity?”. If you use the simplest model, then the answer is “forever”, but if you’re using a more complicated model, then you have to specify how it’s more complicated. Much better to rephrase the question to something like “how long does it take to reach 90% of terminal velocity?”, or “99%”, or the like, but then you have to specify the percentage you want. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 13, 2013, 12:44pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/17 "2013-11-13T12:44:58Z")
> [@ZenBeam](#): > > This is kind of nitpicky, but terminal velocity will vary with altitude, due to the density of the atmosphere. So a bullet falling slower than terminal velocity gaining speed may cross terminal velocity as terminal velocity drops, briefly “reaching” it. Felix Baumgartner demonstrated this quite nicely with his high-altitude skydive earlier this year. Jumping from over 120,000 feet, he exceeded 800 MPH in freefall at an altitude of around 80,000 feet, at which point his velocity exactly matched the local terminal velocity; as he fell further down into more dense air, he was in excess of the local terminal velocity and actually began decelerating, reaching maybe ~140 MPH before finally deploying his parachute at a few thousand feet above sea level. For a bullet fired straight upward at sea level, I’m not sure how high it could travel before coming back down. A Glock with a muzzle velocity of 375 m/s could reach 16,000 feet if we neglect air drag - but we can’t neglect air drag. It won’t get nearly that high. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [November 13, 2013, 8:12pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/18 "2013-11-13T20:12:46Z")
> [@Q.E.D](#): > > No, it won’t. It will acelerate until it approaches the escape velocity of the body it is falling towards. That’s as fast as any object can fall towards a given body. Of course, if the body is a black hole, then you’re absolutely right. Well, summagun. Really? Hmmm. I believe you, but I’d appreciate an intuitive explanation. Not that orbital stuff is ever very intuitive, in general! > [@Grey](#): > > In a vacuum an object, under acceleration will continue to accelerate until it approaches c. With a drag inducing medium (like air) there is a resistive force proportional to the cross sectional area of the abject and the square(?) of it current velocity. Eventually the forces of acceleration and drag cancel each other out and the object stops speeding up. Cube, not square. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 13, 2013, 8:16pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/19 "2013-11-13T20:16:08Z")
> [@Learjeff](#): > > Well, summagun. Really? Hmmm. I believe you, but I’d appreciate an intuitive explanation. Might be hard to get. Q.E.D. is no longer with us. _Really_ no longer with us, RIP. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [November 13, 2013, 8:24pm UTC](https://boards.straightdope.com/t/terminal-velocity/217033/20 "2013-11-13T20:24:34Z")
Oh my. I didn’t spot the necropost. No pun intended. RIP, QED. [Next page](https://boards.straightdope.com/t/terminal-velocity/217033.md?page=2)