velocity
or dx/dt, the rate of change of x with respect to t. The velocity is positive when the particle is moving forwards (in a positive direction) and negative when moving backwards.
If we assign the unit vector i in the line's positive direction then r = xi and the velocity is seen to be a vector equal to 
.
When the motion is in two or three dimensions, vectors are used explicitly. The velocity v of a particle P with position vector r is given by 
. When Cartesian coordinates are used, r = xi + yj + zk, and then 
.
Velocity has the dimensions LT−1, and the SI unit of measurement is the metre per second, abbreviated to ‘ms−1'.