# Wha is speed?

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Definition from Columbia University Press:

Speed, change in distance with respect to time. Speed is a scalar rather than a vector 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 d, then the average speed of that body is equal to d/t. The speed and direction of a body's motion together determine the body's velocity v.

*原文中的distance notation是s不是d﹐但係in mathematical notation, distance係用d代表﹐所以velocity formula係v = d/t。

s是second的notation。為免誤會﹐我將原文中的s改為d。

Definition from Sci-Tech Encyclopedia:

Speed

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.