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fbmf
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### Physics Question

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Post by fbmf » Mon Jan 24, 2005 7:53 pm

Robin is running along a second story rooftop when he see a crime in progress in the alley below and elects to make his entrance in the style of his mentor. He leaps down, aiming both feet at one of the two perps.
Assuming Robin weighs 160 pounds, dropped 20-25 feet, and purposefully lunged down with the intent of taking someone down, at what approximate speed (MPH) did he impact said perp?
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Tae_Kwon_Dan
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### Re: Physics Question

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Post by Tae_Kwon_Dan » Mon Jan 24, 2005 8:23 pm

Do your own homework, .
Been a while since I've done this, but I get ~27 MPH.
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Tae_Kwon_Dan
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### Re: Physics Question

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Post by Tae_Kwon_Dan » Mon Jan 24, 2005 8:34 pm

I'll add that I assumed initial velocity was 0 and that wind resistance was neglible.
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Josh_Kablack
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### Re: Physics Question

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Post by Josh_Kablack » Mon Jan 24, 2005 10:39 pm

Robin never hits the perps.
Here's the relvent math:
s=1/2at^2
a (gravity) = 32.2 ft/s^s
s=20-25 ft (so -5 here)
-5 = 16.1 t^2
-5 / 16.1 = t^2
-0.31=t^2
Thus we can see that the time of impact is an imaginary number.
Seriously, going with a 25 feet drop, neglecting air resistance (which is likely insignificant) and the intial velocity of Robin's lunge (which is likely significant, but we don't have it to work with) I come up with 27.35 MPH
"But transportation issues are social-justice issues. The toll of bad transit policies and worse infrastructure—trains and buses that don’t run well and badly serve low-income neighborhoods, vehicular traffic that pollutes the environment and endangers the lives of cyclists and pedestrians—is borne disproportionately by black and brown communities."
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Username17
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### Re: Physics Question

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Post by Username17 » Mon Jan 24, 2005 10:40 pm

The weight of Robin is, in this case, completely meaningless, since you are asking for the speed at which Robin is effectively striking.
Assuming an initial velocity of zero, and assuming that Robin can leap 10 feet straight up if the mood strikes him, then we are simply adding the velocity of universal gravitation from a 20 foot drop and adding it to the universal gravitation of a 10 foot drop. Using those damned useless English measurements, that's 32 ft./s/s over 20 feet (35.8 ft./s) plus the same over 10 ft. (25.3 ft./s). So he's striking at 61.1 ft./sec. After converting to MPH (Each foot per second is .6818 MPH) - Robin is striking at 41.6423 MPH.
What the hell do you want to know that for? It's a pretty archaicly useless fact to discern, what with the fact that this in no way determines how much damage this will inflict...
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fbmf
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### Re: Physics Question

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Post by fbmf » Tue Jan 25, 2005 1:17 am

I'm writing a short story with a character about Robin's size executing this manuver and I wanted to be able to say something like "my feet connect with his collar bone at better than ??? MPH".
Thank for your help.
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rapanui
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### Re: Physics Question

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Post by rapanui » Fri Jan 28, 2005 9:00 pm

Wait until you get to rotational motion. Things start getting fun then. And by 'fun' I mean 'painful like a razorwire slide'.
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fbmf
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### Re: Physics Question

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Post by fbmf » Tue Feb 15, 2005 1:25 am

Another Physics Question:
A guy with a strength of ten (D&D rating) punches someone. How many pounds of pressure did the punch exert?
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Josh_Kablack
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### Re: Physics Question

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Post by Josh_Kablack » Tue Feb 15, 2005 1:52 am

Um, what?
Pressure is a measure of force (mass x acceleration) divided by area.
Common units of pressure include PSI (pounds per square inch), Atmospheres, MMHG and Millibars.
Now, it's possible for a str 10 character to lift up to 200 lbs, against the gravity of earth, so we know that under ideal conditions, such a character could extert 200 lbf (not a typo) due to muscle power alone. This is going to be a maximum value, not an actual value, since actual force delivered will depend on the the angle of the blow, the coefficient of friction between the attacker and the ground and a number of other factors which are not all determinable.
Divide this number of "something < 200" by the area in square inches of the knuckle striking the victim and you get the PSI of the blow - which lets you compare it directly to the inflation of your tires.
"But transportation issues are social-justice issues. The toll of bad transit policies and worse infrastructure—trains and buses that don’t run well and badly serve low-income neighborhoods, vehicular traffic that pollutes the environment and endangers the lives of cyclists and pedestrians—is borne disproportionately by black and brown communities."
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fbmf
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### Re: Physics Question

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Post by fbmf » Tue Feb 15, 2005 2:10 am

Okay, then I'd like to ask for advice. Here's the sentence in question:

> He almost has the sword two-thirds out of the scabbard before his hand hits the wall. I stiffen my left hand and send my fingertips into the soft ridges under his nose. They don’t have a name, but the depression between them does. It’s the philtrum, and although designed to protect a particularly sensitive area of the skull where three bones come together, it is not designed to receive XXX pounds of pressure all by its lonesome.

How can I reword that so that it it still gives awesome technichal pseudo-physiological fireworks like I intended?
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Username17
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### Re: Physics Question

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Post by Username17 » Tue Feb 15, 2005 6:10 am

> Josh wrote:Now, it's possible for a str 10 character to lift up to 200 lbs, against the gravity of earth, so we know that under ideal conditions, such a character could extert 200 lbf (not a typo) due to muscle power alone. This is going to be a maximum value, not an actual value,

Um... no. It's going to be a seed value which can be multiplied several times by speed. Consider the pounds you exert continuously standing on a scale. Now look at the pounds you exert while jumping up from a scale and landing again. No, the continuous lbf you can exert while holding an object is considerably less than the instantaneous force you can exert - even the force you obviously did exert while moving it to its present location above the ground!
When a 200 pound object is stationary it is exerting 200 pounds of force towards the center of the earth. And if it isn't moving, that means that something (usually the normal force from whatever it is resting on) is exerting exactly 200 pounds right back. If, however, it moves up, that means that more than 200 pounds are opposing its gravitational force. This causes it to have negative acceleration towards the earth and move up.
So if you can lift a 200 pound object, you can exert considerably more than 200 pounds of force. If you could only exert 200 pounds of force, you couldn't lift a 200 pound object.

> fbmf wrote:How can I reword that so that it it still gives awesome technichal pseudo-physiological fireworks like I intended?

While I'm sure you'd love to have an in-depth discussion of why a sudden acceleration from being hit by a large heavy object actually has a very low pressure and will kill you anyway, and why a virtually non-existant energy transfer with a high pressure from a bullet kills you just as dead in a completely different way... no wait, I'm pretty sure you don't care.
A jack-all novice who runs a boxing glove right into your face is giving you about 1600 Newtons.
Punches and their pressure component.
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fbmf
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### Re: Physics Question

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Post by fbmf » Tue Feb 15, 2005 6:05 pm

Doesn't 1600 Newtons roughly equal 360 pounds?
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Username17
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### Re: Physics Question

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Post by Username17 » Tue Feb 15, 2005 7:13 pm

Yeppers. A college student who is also an advanced boxer apparently throws punches of over 1000 pounds, but a normal person throwing a 360 pounder is incredibly reasonable.
On a more subtle note: present tense sucks my asshole. RPGs are usually played in present tense, but if somebody is reading a story, it generally wants to be in past tense. Present tense and future tense making for difficult reading. Even if stories take place in the far future, the action is usually stated as "he went down the corridor" and not "he is going down the corridor" nor as "he will go down the corridor".
It's not that you can't write, read, and comprehend a story in funky tenses, it's just that it's a major pain in my ass.
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Essence
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### Re: Physics Question

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Post by Essence » Sun Feb 20, 2005 8:47 pm

Frank, that link doesn't work for me.
fbmf:
One piece of martial-arts advice and one piece of literary advice:

> fbmf wrote:He almost has the sword two-thirds out of the scabbard before his hand hits the wall. I stiffen my left hand and send my fingertips into the soft ridges under his nose. They don’t have a name, but the depression between them does. It’s the philtrum, and although designed to protect a particularly sensitive area of the skull where three bones come together, it is not designed to receive XXX pounds of pressure all by its lonesome

First, the reason a strike to the philtrum is so devastating isn't the fixed joint it overlaps (a rigid-finger strike will break your fingers before it seperates that joint), but because it also protects the lower branches of the infra-orbital and external nasal nerve clusters. Just a severe poke upwards under the nose with your thumb will give you an idea of their sensivity. A hard blow there will numb your inner cheeks and nose for several seconds, and a severe blow will cause you to involuntarily close your eyes for a couple of seconds, not to mention causing enough pain to lead to muscle weakness and dizziness for a moment or two.
It's a great "combo starter", or a great thing to do to someone who you just need a couple of seconds to get away from.
As to the literary, first, Frank is mostly right. Secondly, compare your original quote to this:

> active is good! wrote:His hand hits the wall before he can finish drawing his sword. I jab my hand, fingers stiff, into the soft ridges under his nose. They don’t have a name, but the depression between them does. It’s the philtrum, and although it protects a place where two nerve clusters meet, it was never intended to protect from three hundred and eighty pounds per square inch.

By rewording the first two sentences into a more active voice, and removing some of the stumbling-block adjectives from the last sentence, you get an easier read without needless repetition (if you're jabbing under his nose, we already know it's the skull, and if you're talking about PSI, we already know it's pressure).
(Also, just as a side note, it's more dramatic to read a number written out than it is to see it in digits. Don't ask me why, just try it for yourself and see.)
I'm interested in reading more of this, if you're willing to share.
Can the US government be uncorrupted?
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fbmf
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### Re: Physics Question

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Post by fbmf » Mon Feb 21, 2005 2:29 am

Thanks for your input, Ess. The story is here.
Game On,
fbmf
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