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change in distance with respect to time. Speed is a scalar rather than a vectorvector,
quantity having both magnitude and direction; it may be represented by a directed line segment. Many physical quantities are vectors, e.g., force, velocity, and momentum.
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 quantity; i.e., the speed of a body tells one how fast the body is moving but not the direction of the motion. If during time t a body travels over a distance s, then the average speed of that body is equal to s/t. The speed and direction of a body's motion together determine the body's velocityvelocity,
change in displacement with respect to time. Displacement is the vector counterpart of distance, having both magnitude and direction. Velocity is therefore also a vector quantity. The magnitude of velocity is known as the speed of a body.
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The time rate of change of position of a body without regard to direction. It is the numerical magnitude only of a velocity and hence is a scalar quantity. Linear speed is commonly measured in such units as meters per second, miles per hour, or feet per second.

Average linear speed is the ratio of the length of the path traversed by a body to the elapsed time during which the body moved through that path. Instantaneous speed is the limiting value of the foregoing ratio as the elapsed time approaches zero. See Velocity

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(graphic arts)
The sensitivity of a photographic film, expressed according to one of several scales.
The time rate of change of position of a body without regard to direction; in other words, the magnitude of the velocity vector.
The light-gathering power of a lens, expressed as the reciprocal of the f number.
The time that a camera shutter is open.
In general, the rapidity with which a process takes place.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


an “illiterate loiterer”; slow-moving servant. [Br. Lit.: Two Gentlemen of Verona]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.


1. Physics
a. a scalar measure of the rate of movement of a body expressed either as the distance travelled divided by the time taken (average speed) or the rate of change of position with respect to time at a particular point (instantaneous speed). It is measured in metres per second, miles per hour, etc.
b. (not in technical usage) another word for velocity
2. a rate of rotation, usually expressed in revolutions per unit time
3. a gear ratio in a motor vehicle, bicycle, etc.
4. Photog a numerical expression of the sensitivity to light of a particular type of film, paper, or plate
5. Photog a measure of the ability of a lens to pass light from an object to the image position, determined by the aperture and also the transmitting power of the lens. It increases as the f-number is decreased and vice versa
6. a slang word for amphetamine
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


Early system on LGP-30. Listed in CACM 2(5):16 (May 1959).
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)


(1) Computer people may use the term for capability and time. For example, "not enough bandwidth to get the job done" means not enough staff or time to do it. Its true meaning follows.

(2) The transmission capacity of an electronic pathway such as a communications line, computer bus or computer channel. Digital bandwidth is the number of pulses per second measured in bits per second (bps). For example, Ethernet transmits at different speeds, including 10 Mbps, 100 Mbps and 1000 Mbps (see Mbps and baseband).

When transmitting alternating frequencies, as with all wired analog, many wired digital and most wireless communications, the bandwidth is the difference between the highest and lowest frequencies, measured in cycles per second, or Hertz (Hz). For example, 802.11n Wi-Fi transmits in 20 MHz and 40 MHz channels within the 2.4 GHz and 5 GHz bands. The 20 and 40 MHz channel frequencies are the bandwidths, and each channel is divided into subchannels.

From Hertz to Bits - A Complicated Process
When using alternating frequencies for digital transmission, the frequencies are modified (modulated) by the digital input. Using the 802.11n Wi-Fi example, the resulting bit rate can range from 6.5 Mbps to 600 Mbps. This extremely wide range is determined by the signal strength and interference in the environment at any given moment. Any one of more than 30 combinations of channel bandwidth (20, 40 MHz), modulation scheme, error correction rate, channel spacing and number of antennas may be selected, on a packet-by-packet basis. See modulation, video bandwidth, space/time and bandwidth junkie.

Bandwidth in Hertz to Bandwidth in Bits
This quadrature PSK (QPSK) example is one of the simplest modulation schemes. Each set of two input bits modifies the carrier into four phase angles. The amplitude remains constant, unlike QAM modulation, in which the amplitude is varied (see QAM).

data rate

(1) The speed at which data is transferred within the computer or between a peripheral device and the computer, measured in bytes per second. See transfer rate and space/time.

(2) The speed at which audio and video files are encoded (compressed), measured in bits per second (see bit rate).

(3) The transmission speed of a network. For example, 100Base-T Ethernet is rated at 100 Mbps (megabits per second). Also called "bit rate." See space/time.


The following units of measure are used to define digital capacities and speeds. For measurements of a meter, see metric system. See binary values, NIST binary and long scale.
************************************** S P A C E **************************************                      10 to theBits or Bytes         Power of:

 Kilo  (K)  Thousand       3
 Mega  (M)  Million        6
 Giga  (G)  Billion        9
 Tera  (T)  Trillion      12


 Peta  (P)  Quadrillion   15
 Exa   (E)  Quintillion   18
 Zetta (Z)  Sextillion    21
 Yotta (Y)  Septillion    24
 Bronto     Octillion     27
 Geop       Nonillion     30

 Storage Capacity Measured in:

 CPU word size        bits

 Disk/SSD/USB drive   bytes
 Memory (RAM)         bytes

 ****************************************  T I M E  ***************************************Fractions of a second    10 to theSecond                  Power of:

 Millisecond (ms) Thousandth     -3
 Microsecond (µs) Millionth      -6
 Nanosecond  (ns) Billionth      -9
 Picosecond  (ps) Trillionth    -12
 Femtosecond (fs) Quadrillionth -15
 Attosecond  (as) Quintillionth -18
 Zeptosecond (zs) Sextillionth  -21
 Yoctosecond (ys) Septillionth  -24

 Transmit/Transfer Measured in:

 Parallel bus/channel    bytes/sec
 Disk/solid state drive  bytes/sec
 Memory (RAM)            bytes/sec
 PCI Express bus         bytes/sec

 Cellular data plan      bytes

 USB                     bits/sec
 Network/communications  bits/sec

 Disk access time        ms
 SSD access time         µs
 RAM access time         ns
 Machine cycle           µs, ns
 Instruction execution   µs, ns
 Transistor switching    ns, ps, fs

 CPU Speed - Clock Cycles Per Second
 (1 cycle/sec = 1 Hertz)  See  Hertz.

 kilohertz (kHz) thousand
 megahertz (MHz) million
 gigahertz (GHz) billion

speed of light

All electromagnetic radiation, including light, radio transmission and electricity, travels at approximately 186,000 miles (300,000 kilometers) per second; more than seven times around the equator in one second. More precisely, the speed is 299,792,458 meters per second in a vacuum.

Never Fast Enough!
This inherent speed of Mother Nature is why computers work so fast. Within the tiny chip, electricity has to flow only a couple of millimeters, and, within an entire computer, only a few feet. Yet, as fast as that is, it is never fast enough. There is resistance in the lines, which slows down the current, and even though transistors switch in billionths of a second, scientific and multimedia applications are always exhausting the fastest computers. See software bloat.

speed of sound

Sound waves travel at approximately 750 mph at sea level, but slow down at higher altitudes. The speed of sound is called "Mach 1." Supersonic refers to speeds from Mach 1 to Mach 5 (1 to 5 times the speed of sound), and hypersonic ranges from Mach 5 to Mach 10 (5x to 10x).
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References in periodicals archive ?
But Reneman said she's "pretty sure the effects are due to methylphenidate and not something else," since prior research had produced similar results.
But in adults, amphetamines, bupropion, and methylphenidate all beat placebo.
Based upon this information, the researchers urge clinicians to be cautious in prescribing methylphenidate, and to weigh up the benefits and risks more carefully.
Recently, professional councils, the Ministry of Health and the National Health Council have started a movement that claims for efforts towards the construction of guidelines that support public policies for the rationalization of the use of the methylphenidate and the confrontation of situations of abuse (9).
Has amphetamine's neurotoxicity (relative to methylphenidate) been experimentally demonstrated?
Students using methylphenidate can experience numerous side-effects, such as hallucinations, anxiety, dry mouth and visual disturbances.
Elevated DaT levels result from up-regulation in the presence of chronic methylphenidate therapy, which accounts for early reports that demonstrated increased striatal DaT density.
Other than the concern for pregnancy loss, which could be explained in part by other characteristics of the mothers who were taking methylphenidate early in pregnancy, the limited data to date have been generally reassuring, with no suggestion of an overall increased risk for major congenital malformations or a specific pattern of defects.
"We are also aware of the possibility that methylphenidate could be diverted and abused, and for this reason we recommend that its use should be monitored carefully.
Each extended-release tablet contains 18 mg or 27 mg of methylphenidate hydrochloride as the active ingredient and is designed to be effective for 12 hours.
There have been reports over the past few decades about uses for stimulants, such as methylphenidate, for which they are not traditionally indicated and for which these drugs are not registered.
Methylphenidate is a stimulant drug that binds and inhibits the dopamine transporter (Gatley, Pan, Chen, Chaturvedi, & Ding, 1996; Schweri et al., 1985) and produces dopamine overflow in the striatum (Butcher, Liptrot, & Arbuthnott, 1991; Gerasimov et al., 2000; Hurd & Ungerstedt, 1989; Kuczenski & Segal, 1997; Volkow et al., 2001).