In the human body only? I searched on Google and all the hits were talking about some ant. Some candidates I can think of:
[ul]
[li]eyelids when you blink[/li][li]snapping fingers[/li][li]pitching a baseball[/li][li]whipping your hair back and forth?[/li][li]air in the lungs when you sneeze[/li][/ul]
I think nerve impulses are faster, but I’m only interested in movement of macroscopic members, thanks.
Pitching a baseball, the pitcher’s fingertips can move well over 100 MPH. But that’s the sum of much smaller relative velocities of body parts: each bone is moving relative to the next, from the most distal phalanx on down to the tibia/fibia, and those component velocities are much smaller.
Air in the lungs doesn’t move terribly fast when sneezing, but air in the narrow parts of the throat and mouth do. But that air isn’t really a body part.
I’d guess a fingersnap might get a fingertip moving relatively quickly: you hold it back with your thumb until your flexor muscles build up to full force, then you release the finger and it spends its entire length of travel accelerating until it slams into your palm. Estimate from this thread suggests the fingertip gets moving to about 19 MPH. But that still involves the three bones of the finger. So if we’re going to allow multiple links, then I’ll stick with an MLB player pitching a fastball.
Mrs. Jackmannii, when she sees a snake.
I think there are lots of movements that can add up to extremely fast motion. I don’t think a fast basebhall pitch has anything on a karate strike. This article on the Physics of Karate lists measured speeds over 14 meters per second, which is in excess of 32 miles per hour:
This more recent article gives body speeds of 19 m/sec, or more than 42 mph
Hmm if hair were long enough, could it break the sound barrier, like a whip?
Related: which body part experiences the highest acceleration, caused only by the body?
Ok blinks last at least 100 ms, so the speed isn’t more than 1 m/s. Not very fast.
Sneezes are about 40 mph.
Looks like the fastest might be a baseball pitch, at 100 mph. Unless someone can think of something else.
If I’m reading this correctly, you are asserting that a fastball pitch is slower than a karate strike.
Fastballs (and therefore the fingertips that launch them) are clocked at well over 100 MPH, which beats a 42-MPH karate strike by a wide margin.
No, I’m not saying the karate strike is faster, but that thye fastball pitch isn’t necessarily faster than the karate strike.
There’s quite a bit of variation in the speed of recorded karate strikes, which is one reason the Guinness book of records stopped listing them many years ago. There have been a lot more records of fast pitches than of people measuring karate strikes, and no one to my knowledge has really tried to measure the fastest karate strike. So I’ll concvede that you may have an indirect measure of the fastest body motion, but not that it exceeds the fastest possible body part motion due to a karate strike.
This was my first thought. Take one of those people who can grow hair to ridiculous lengths, braid a thin, properly formed whip, then actually see if it can be cracked while still attached to their head.
I looked for work on measuring pitching speed, and found this paper:
http://www.ewp.rpi.edu/hartford/~ernesto/F2010/EP1/Materials4Students/Cisneros/Theobalt2004.pdf
It gives bal speeds between 60 anf 70 mph, and talks of ball speeds of 8-0 mph and above. Curiously, although they talk in detail about how they measured body positions and speeds, they don’t actually tell you what any of them were, or give graphs.
I can’t find a reference so I’ll ask. How fast does the eardrum move back and forth when hearing sound? How fast do the vocal cords vibrate?
I can’t find the reference either, although I had been taught it was the bones of the middle ear.
I suppose it depends on the pitch. For A440, it would move back and forth 440 times per second, correct?
Velocity can’t really be equated to vibration frequency that way. Something small vibrating at 440 times per second may still only be moving a very tiny distance over that period.
That’ll depend on the frequency and volume level of the incident sound waves, as well as the compliance and inertia of the eardrum itself. Tough to say.
Saccadic eye movements reach rotational speeds (rotation of the eyeball) of about 500º per second. I am not sure how to translate that into mph, though.
According to the gradeschool joke, the fastest thing in the world is your butthole slamming shut after the last turd comes out.
I do not have a cite.
It certainly can. You multiply the frequency by the amplitude to get velocity*. I’ve done a quick search, and I find places that tell you how they measured the amplitude, but not what the values are.
Edgar Allemn Poe used this as an example of rapid movement by the body in his Thousand and first night of Scheherazade
*Multiplied by 2 times pi, to turn it into angular measure
Eyeball is about 1 inch in diameter, which makes it about 3.14 inches in circumference. So…
360 degree = 3.14 inches
therefore
500 degrees = 4.36 inches
500 degrees per second = 4.36 inches per second
4.36 in/s = 0.248 miles per hour. That’s…really not all that fast. 
If I shot somebody out of a cannon, all of his body parts would be travelling at SEVERAL hundred miles per hour.