When you are describing the rate that a race car goes around a track, should you use the term speed or velocity to describe the motion? Can the velocity of the car change without changing the average speed of the car?

2 answers

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Main Entry: ve·loc·i·ty
Pronunciation: v&-'lä-s&-tE, -'läs-tE
Function: noun
Inflected Form(s): plural -ties
Etymology: Middle French velocité, from Latin velocitat-, velocitas, from veloc-, velox quick; probably akin to Latin vegEre to enliven —more at WAKE
Date: circa 1550
1 a : quickness of motion : SPEED <the velocity of sound> b : rapidity of movement <[my horse's]g strong suit is grace & personal comeliness, rather than velocity —Mark Twain> c : speed imparted to something <the power pitcher relies on velocity —Tony Scherman>
2 : the rate of change of position along a straight line with respect to time : the derivative of position with respect to time
3 a : rate of occurrence or action : RAPIDITY <the velocity of historical change —R. J. Lifton> <the narrative leaps from one frantic episode to another with impressive velocity —James Atlas> b : rate of turnover <the velocity of money>

Main Entry: [1]speed
Pronunciation: 'spEd
Function: noun
Etymology: Middle English spede, from Old English spEd; akin to Old High German spuot prosperity, speed, Old English spOwan to succeed, Latin spes hope, Lithuanian speti to be in time
Date: before 12th century
1 : archaic : prosperity in an undertaking : SUCCESS
2 a : the act or state of moving swiftly : SWIFTNESS b : rate of motion: as (1) : VELOCITY 1, 3a (2) : the magnitude of a velocity irrespective of direction c : IMPETUS
3 : swiftness or rate of performance or action
4 a : the sensitivity of a photographic film, plate, or paper expressed numerically b : the light-gathering power of a lens or optical system c : the time during which a camera shutter is open
5 : a transmission gear in automotive vehicles or bicycles — usually used in combination <a ten-speed bicycle>
6 : someone or something that appeals to one's taste
7 : METHAMPHETAMINE; also : a related stimulant drug and especially an amphetamine
synonym see HASTE
- speed·ster /'spEd-st&r/ noun
- at speed chiefly British : FAST, RAPIDLY
- up to speed : operating at full effectiveness or potential

Now you can make your own choice. Most people use the word "speed."
P.S. I forgot to mention another word my brothers (who race) use:

Main Entry: [1]rev
Pronunciation: 'rev
Function: noun
Etymology: short for revolution
Date: circa 1890
: a revolution of a motor