A superheroic question...

[seawasp]

What effects WOULD you expect to see from a super-speedster at various speeds? I mean, we see -- for instance -- speedsters whipping past people and creating sonic booms that knock things over and shatter windows, or being able to run fast WITHOUT doing that, but I don't know if a human-sized object going at a given supersonic speed will create booms that will break anything or not.

Assume a speedster (A) isnt' subject to damaging effects of his or her speed (i.e., they can punch and hit things at their proportionate speed without hurting their hands more than they would hitting the same thing at normal speed, they don't burn up from friction, and they're not harmed by acceleration, etc. coming from their speed power), (B) can use their power to do things like run without (for instance) being limited by the ability of the surface they're running on to withstand the g-forces and friction, (C) can turn some effects on and off (i.e., run as silently as a normal person, OR interact with the air such that they produce sonic booms, throw up rooster-tails from water, etc) and (D) has all the appropriate reaction speed, etc., to be able to use it at proportionate speed (i.e., if a regular human could run through a given obstacle course, they could thread that exact same obstacle course at their running speed).

What would we see from said speedster running past, say, a double-line of people and vehicles on either side of a street at:

  100MPH (do we feel a breeze?)

  500 MPH

  Mach 1+ (say 780mph)

  Mach 5

  Mach 10

  Mach 100?


  Also, could they really pull off stunts such as making mini-tornadoes by running in circles?

Comments

[dsrtao]

Important additional assumptions that I will make for you:

Your superspeedster masses 100Kg and has a frontal surface area of 1 m^2.

[dsrtao]

100 Miles/hour = 161 Km/h = 45 meters/sec
500 => 223m/s
M1 => 340m/s
M5 => 1700m/s
M10 => 3400m/s
M100 = 34000m/s

[dsrtao]

Force is mass * acceleration.

A superspeedster who brakes themselves from full speed to zero in the course of half a second of contact with their target
dumps all their kinetic energy.

The most powerful commercial handgun load, .500 S&W Magnum, has a 30g bullet moving at 500m/s. Your superspeedster is 3300 times heavier.

A .50 BMG bullet from an antimateriel rifle is perhaps 50g at 900m/s. Your superspeedster is 2000 times heavier.

The Rheinmetall L55 120mm tank gun can fire a kinetic energy penetrator round, the M829A1, which has a penetrator weighing just under 5Kg and moving at 1600m/s, just about Mach 5, but massing 20 times less than your superspeedster.

So, there's a good comparison. At M5, your superspeedster kicks, punches or headbutts an opponent with an impact similar to 20 modern tank rounds.

[kjn]

Isn't force = mass x velocity?

Then you have the energy, which is mass x velocity x velocity. If your superspeedster goes from 340 m/s to zero, they will have to dump 100 kg x 340 m/s x 340 m/s, or 11.5 MJ.

[dsrtao]

F = MA.
MV = p, p being momentum.

Energy = work = F * distance, so energy is mass * acceleration * distance, or mass * distance * 1/time^2 * distance, or

mass * distance^2 / time^2, hence the megajoules.

[dsrtao]

That's if they dump it in one second. In half a second, it's 4 times more energy.

[bunny-m.livejournal.com]

'cause your everyman is 2m tall, weighs 100kg and can benchpress their own weight, naturally.

Ahh, the little absurdities of RPGs. =)

[dsrtao]

Realism levels are controlled by the GM (or author).

Suppose Usain Bolt (nominative determinism!) can coordinate himself at a record speed of 45Km/h, or 12.5 m/s. To coordinate at Mach 5, 3400m/s, implies reactions 272 times faster. What takes you a tenth of a second takes the speedster 3.5milliseconds.

At Mach 100, the speedster launches past orbit if they don't remember to follow the curvature of the Earth. Three seconds to space.

[seawasp]

Kengr's getting more at what I was most interested in, though; I could calculate impacts of fists and such at speed, but what about the wind of his passage, so to speak? At what speed do the people lining the two-lane street feel a wind as he goes by, feel a STRONG wind as he goes by, get blown off their feet as he goes by, and get hurled into buildings/blown apart as he goes by? And if he runs in a circle does he create a tornado?

[kengr]

Remember, "sonic booms" are what happens when the shock cone from a supersonic object passes over an observer.

Shock cones carry a lot of energy. Back when they were starting the USAF Academy in Colorado Springs, some hotshot made a low supersonic pass in a fighter and the overpressure was enough to damage the *framing* in a couple of buildings.

And that was likely at less than Mach 2.

Sure, the speedster is smaller. But he's also a lot *closer* (I think the fighter was a few hundred feet up) and going a lot *faster*.

at close range and high mach numbers his shock cone is liable to *homogenize* bystanders.

Somebody else will have to figure out the overpressure values for various speeds and distances.

[dsrtao]

"Sonic booms produced by aircraft flying supersonic at altitudes of less than 100 feet, creating between 20 and 144 pounds overpressure, have been experienced by humans without injury." -- http://www.nasa.gov/centers/dryden/news/FactSheets/FS-016-DFRC.html

I don't think she turns bystanders across the street into puree, but any contact is going to be fatal; windows will break, large flat surfaces will fall over. At Mach 5, that is.

Now assume that you can convert the full energy of a 100Kg person moving at Mach 100 into thrust; it's comparable to one of the Saturn V's F1 engines.

[muirecan.livejournal.com]

Yes exactly, I wonder if Ryk can find the mythbusters episode where they examined sonic booms and breaking glass. Which tuns out to be busted if I remember correctly. They had a great time with the Blue Angels flying about 200' above their test setup to create the sonic booms.

Myth: A supersonic jet can break any kind of glass by the sonic boom it generates.

Busted Jamie set up a test area filled with various glass items and products while Adam performed a flyby in an F/A-18 going supersonic. However, at flybys of 8,000, 2,000, and 500 feet (2,400, 610, and 150 m), the jet failed to break any of the glass. They then performed a series of low altitude flybys at 200 feet (61 m), but only managed to break a single window. Since the majority of the glass was still intact, the MythBusters declared the myth busted.

[kpreid.livejournal.com]

Not what you asked, but physics-brain has a complaint about the handwaving...

(B) can use their power to do things like run without (for instance) being limited by the ability of the surface they're running on to withstand the g-forces and friction,

Note that this is really a secondary effect, for flat surfaces. If they have arbitrary strength and control, then the conventional limit to their acceleration capability is how much they can push horizontally the ground — which depends on the force of gravity pushing them towards the ground. There's only so much force they can exert before they either slip (too much horizontal push) or leap into the air (too much vertical push); and either way, they stop accelerating horizontally.

Ways to avoid this problem without adding powers:

• Race cars can exceed the obvious limit by having spoilers and other aerodynamically significant elements which use the passing air to push the air against the ground. I don't think there's a reasonable body position that would help with that, but maybe a suitably designed costume could help.
  
• Skip the ground part and fly through the air like swimming through water (i.e. make up for lack of wings with brute force).
  
• Push horizontally off sufficiently strong structures (like in zero-G, because at the timescale of these movements gravity is small).
  

On the other hand, if the power set is inherently to Move Normally But Really Fast, they're going to have to a flight power (that is, force out of nowhere) which either pushes them towards the ground (can damage the surface unless reinforced) or moves them in the direction they want to go (contact with the ground is purely symbolic).

Perhaps anything they touch (including the ground) is automatically reinforced-to-scale in exactly the same way they are. That plus a downforce power allows normal locomotion. This would also mean their punches are effectively not superstrong (since what they hit is reinforced to match), just fast, which might be a useful limit on their power.

[seawasp]

Your puny physics is no match for the caveats.

Spoiler-style stuff only works when the force of your acceleration isn't sufficient to rip the surface you're pressed against to pieces.

Flight powers keep you flying. Superspeed keeps you running. I suspect you're a Champions kind of guy and would build this as "Flight, limited to contact with surface" which is the sort of thing that makes me detest Champions.

Selective reinforcement, not automatic, is my preference. If it's automatic and not controllable, then as you say his punches hit no harder than a regular human's, which makes him kinda useless against supers with any level of toughness/armor/invulnerability.

[kpreid.livejournal.com]

Spoiler-style stuff only works when the force of your acceleration isn't sufficient to rip the surface you're pressed against to pieces.

Oops, I forgot to connect that, yes. So, same caveats/fixes as in last two paragraphs.

Flight powers keep you flying. Superspeed keeps you running. I suspect you're a Champions kind of guy and would build this as "Flight, limited to contact with surface" which is the sort of thing that makes me detest Champions.

Can't say I've done any superhero RPG. What I'm going for here is a minimal, nailed-down set of differences from known physics. When I said “flight power” I was just explaining what a force out of nowhere looks like from a superhero powers perspective.

So, selective reinforcement, that works. (The bit about not being able to punch hard I saw as a convenient way to distinguish him from a brick-type, but evidently that's not what you want.) However, you still need some effect that provides downforce — or, it now occurs to me, sticking to the surface. Since we're doing magic at the interface with other materials anyway, how about we just say the traction is greatly increased? That is, they can produce more horizontal force per downforce before slipping. (They'd have to lean over really far into the wind, though.)

[bunny-m.livejournal.com]

I suspect you're a Champions kind of guy and would build this as "Flight, limited to contact with surface" which is the sort of thing that makes me detest Champions.

Why? The fact that you can build a character with "Flight, only when in contact with a surface" or for the more wushu/wirefu form "Flight, can only change direction when in contact with a surface" is part of why I've always loved Hero System/Champions.

Personally I'd just buy up my running speed* and spend some points in Clinging, unless there was a strong character/world reason for the special effects to work that way.


Skip the ground part and fly through the air like swimming through water (i.e. make up for lack of wings with brute force).
Push horizontally off sufficiently strong structures (like in zero-G, because at the timescale of these movements gravity is small).

*!!!*

Those little wrinkles never consciously occurred to me. Apparently my subconscious is smarter though.

(*) And a bunch of non-combat multipliers as well, 'cause a) it's really cheap, and b) sometimes straightline(ish) speed is much more important/useful/fun than having full combat values.

[ninjarat.livejournal.com]

I'm a Champions kind of guy and I probably wouldn't do that. I'd buy some Clinging so that the speedster will "stick" to non-horizontal solid surfaces, and I'd add two Limitations to that Clinging. One: must make a half-move each Phase. Two: cannot resist knockback. Because if the speedster stops moving then he stops "sticking" to the wall or whatever.

Because I did precisely that last week: I'm playing, coincidentally enough, a Juggernaut-inspired speedster in a Saint Seiya-inspired Champions game. Petra moves fast and hits like a ton of bricks. :)

[muirecan.livejournal.com]

Once they are generating a sonic boom it would be something you can feel. But generally it isn't going to knock you down. But a speedster at Mach 1+ isn't going to be stealthy.

[ninjarat.livejournal.com]

At 100mph: You'll feel a breeze at close range. If a cyclist at 35mph generates a noticeable draft then a runner running three times faster most certainly will.

At 500mph: The breeze becomes a significant wind. The Daisy Model 25 has a muzzle velocity of ~250mph. Detritus kicked up by the runner's wake will be moving around that fast so minor injuries could result. A speedster hitting a solid object at this speed will probably break all the bones in his hand and he might dislocate his shoulder. Note: human beings have survived 300mph car wrecks:
http://en.wikipedia.org/wiki/Richard_Hammond#Vampire_dragster_crash

At Mach 1: Detritus will be moving faster so injuries may be more serious. The pressure wave that generates the sonic boom will be noticeable and might knock some people down, might temporarily deafen some. The bones of the punching speedster's arm will shatter on impact. He might also suffer a broken shoulder and some broken ribs. At this point I'm ignoring damage to the actual surface that the speedster is running on: the impact energy of his feet, friction and so forth. I'm likewise ignoring the damage this causes to his legs (Newton's Third Law of Motion).

At Mach 5: Detritus causing serious or fatal injuries. Pressure wave knocks more people down, shoves lawn chairs, might break eardrums. The punching speedster is likely to have his arm ripped off or crushed into jelly on impact. Damage to the shoulder and torso would be more from tearing the arm off than from impact energy transfer.

At Mach 10: Detritus moving fast enough to cause lethal injuries and causing comparable damage to structures. I think that trumps anything that the wind or sonic boom might do directly. Punching impact can't destroy the arm any worse than a Mach 5 punch would. If his arm's off then his arm's off.

At Mach 100: Detritus is probably killing anyone in easy throwing distance. Escape velocity for Earth is Mach ~38 but escape velocity for the Sun near Earth is Mach ~142. He's fast enough to escape Earth's gravity but not the Sun's.

Rotating storms -- hurricanes, tornadoes -- have high/low pressure exchanges and temperature exchanges feeding energy into their systems which keeps them spinning. A speedster-created tornado will rotate as long as he keeps feeding energy into it. Once he stops, the mini-storm will dissipate.