What's the difference between two identical objects traveling at different speeds? Nearly everyone knows that the one moving faster (the one with the greater speed) will go farther than the one moving slower in the same amount of time. Either that or they'll tell you that the one moving faster will get where it's going before the slower one. Whatever speed is, it involves both distance and time. "Faster" means either "farther" (greater distance) or "sooner" (less time). Doubling one's speed would mean doubling one's distance traveled in a given amount of time. Doubling one's speed would also mean halving the time required to travel a given distance. If you know a little about mathematics, these statements are meaningful and useful. (The symbol v is used for speed because of the association between speed and velocity, which will be discussed shortly.)
Combining these two rules together gives the definition of speed in symbolic form.
v =  s  (Note: this is not the final definition.)  
t 
Don't like symbols? Well then, here's another way to define speed. Speed is the rate of change of distance with time.
In order to calculate the speed of an object we must know how far it's gone and how long it took to get there. "Farther" and "sooner" correspond to "faster". Let's say you drove a car from New York to Boston. The distance by road is roughly 300 km (200 miles). If the trip takes four hours, what was your speed? Applying the formula above gives …
v =  s  ≈  300 km  = 75 km/h 
t  4 hour 
This is the answer the equation gives us, but how right is it? Was 75 kph the speed of the car? Yes, of course it was … Well, maybe, I guess … No, it couldn't have been the speed. Unless you live in a world where cars have some kind of exceptional cruise control and traffic flows in some ideal manner, your speed during this hypothetical journey must certainly have varied. Thus, the number calculated above is not the speed of the car, it's the average speed for the entire journey. In order to emphasize this point, the equation is sometimes modified as follows …
v =  Δs 
Δt 
The line over the v indicates an average or a mean and the Δ (delta) symbols indicate a change. This is the quantity we calculated for our hypothetical trip.
In contrast, a car's speedometer shows its instantaneous speed, that is, the speed determined over a very small interval of time — an instant. Ideally this interval should be as close to zero as possible, but in reality we are limited by the sensitivity of our measuring devices. Mentally, however, it is possible imagine calculating average speed over ever smaller time intervals until we have effectively calculated instantaneous speed. This idea is written symbolically as …
v =  lim  Δs  =  ds 
Δt → 0  Δt  dt 
or, in the language of calculus speed is the first derivative of distance with respect to time.
If you haven't dealt with calculus, don't sweat this definition too much. There are other, simpler ways to find the instantaneous speed of a moving object. On a distancetime graph, speed corresponds to slope and thus the instantaneous speed of an object with nonconstant speed can be found from the slope of a line tangent to its curve. We'll deal with this later in this book.
In order to calculate the speed of an object we need to know how far it's gone and how long it took to get there. A wise person would then ask …
"What do you mean by how far? Do you want the distance or the displacement?"
Your choice of answer to this question determines what you calculate — speed or velocity.
And for the calculus people out there …
Speed and velocity are related in much the same way that distance and displacement are related. Speed is a scalar and velocity is a vector. Speed gets the symbol v (italic) and velocity gets the symbol v (boldface).

average speed 

instantaneous speed 


average velocity 

instantaneous velocity 

Displacement is measured along the shortest path between two points and its magnitude is always less than or equal to the distance. The magnitude of displacement approaches distance as distance approaches zero. That is, distance and displacement are effectively the same (have the same magnitude) when the interval examined is "small". Since speed is based on distance and velocity is based on displacement, these two quantities are effectively the same (have the same magnitude) when the time interval is "small" or, in the language of calculus, the magnitude of an object's average velocity approaches its average speed as the time interval approaches zero.
Δt → 0  ⇒  v → v 
The instantaneous speed of an object is the magnitude of its instantaneous velocity.
v = v
Speed tells you how fast. Velocity tells you how fast and in what direction.
Speed and velocity are both measured using the same units. The SI unit of distance and displacement is the meter. The SI unit of time is the second. The SI unit of speed and velocity is the ratio of two — the meter per second.
⎡ ⎣ 
m  =  m  ⎤ ⎦ 
s  s 
This unit is only rarely used outside scientific and academic circles. Most people on this planet measure speeds in kilometer per hour (km/h or sometimes kph). The United States is an exception in that we use the older mile per hour (mi/h or mph). Let's determine the conversion factors so that we can relate speeds measured in m/s with the more familiar units.
1 kph =  1 km  1000 m  1 hour  = 0.2777 … m/s ≈ ¼ m/s  
1 hour  1 km  3600 s  
1 mph =  1 mile  1609 m  1 hour  = 0.4469 … m/s ≈ ½ m/s  
1 hour  1 mile  3600 s 
The decimal values are accurate to four significant digits, but the fractional values should only be considered rules of thumb (1 mph is really more like ^{4}/_{10} m/s than ½ m/s).
The ratio of any unit of distance to any unit of time is a unit of speed.
Sometimes, the speed of an object is described relative to the speed of something else; preferably some physical phenomena.
m/s  km/h  device, event, phenomena, process 

10^{−9} ~ 10^{−8}  continental plates, fingernail growth, hair growth  
10^{−4}  human sperm cells  
10^{−3}  snails  
0.013  0.045  ketchup pouring from a bottle 
10^{−1}  sloths, tortoises, turtles  
0.5–1.3  1.9–4.6  cockroaches 
1  3.6  nerve impulses, unmyelinated cells 
1  3.6  ocean currents 
1.14  4.10  manatees 
1.3  4.8  human, typical walking pace 
2.391  8.608  fastest human: swimming (César Cielo) 
8  30  maximum comfortable elevator speed 
10  40  dolphins, porpoises, whales 
10  40  falling raindrops 
10.438  37.578  fastest human: running (Usain Bolt) 
12  43  stadium wave 
16.447  59.211  fastest human: ice skating (Jeremy Wotherspoon) 
18  64  champagne cork 
20  70  rabbits, hares, horses, greyhounds, tuna, sharks 
30  100  typical freeway speed limit 
33  118  cheetahs 
37.022  133.28  fastest human: cycling (Sam Whittingham) 
40  140  falling hailstones 
33–83  120–300  hurricane, maximum sustained wind speed 
30–90  105–330  tornado, maximum sustained wind speed 
46.98  169.1  fastest human: baseball pitch (Aroldis Chapman) 
55  200  typical terminal velocity of a skydiver 
69.8325  251.397  fastest human: skiing (Simone Origone) 
73.06  263  fastest human: tennis serve (Sam Groth) 
80  290  peregrine falcon in a dive 
83  295  very fast golf ball 
100  360  nerve impulses, myelinated cells 
120  431  fastest streetlegal car (Bugatti Veyron Super Sport) 
142.89  511.11  fastest ship (Spirit of Australia) 
159.7  574.8  fastest train (Train à Grande Vitesse) 
168.249  605.697  fastest motorcycle (Top Oil/Ack Attack) 
180–1200  650–4,400  bullets 
200  700  tsunami 
250  900  commercial jet airplane 
331  1,190  speed of sound, STP 
340  1,225  speed of sound, sea level 
341.112  1,228.02  fastest experimental car (Thrust SSC) 
343  1,235  speed of sound at room temperature 
372.8  1,342  fastest human: sydiving (Felix Baumgartner) 
980.433  3,529.56  fastest airplane (SR71 Blackbird) 
1,500  5,400  speed of sound in water 
2,000  6,000  seismic waves 
6,900  25,000  detonation velocity of TNT 
8,000  29,000  space shuttle in orbit 
11,082  39,896  fastest humans (crew of Apollo 10) 
11,180  40,250  escape velocity 
15,090  54,320  New Horizons space probe 
15,440  55,584  Voyager 2 space probe 
17,045  61,362  Voyager 1 space probe 
29,790  107,200  earth in orbit 
220,000  790,000  sun moving through the milky way 
400,000  1,440,000  solar wind near earth 
600,000  2,200,000  milky way through the local super group 
124,000,000  446,000,000  speed of light in diamond 
299,792,369  1,079,252,530  protons and antiprotons in the Tevatron, Fermilab 
299,792,455  1,079,252,840  protons in the Large Hadron Collider 
299,792,458  1,079,252,850  speed of light in a vacuum 