#a = {dv}/dt#
#"So acceleration (a) is the derivative of the velocity v with respect"#
#"to the time t."#
#"v = {dx}/dt#
#"So velocity is the derivative of the position x and can be negative!"#
#"Velocity can also be a vector with a direction."#
#"Speed is like velocity but is always positive, so it can be seen"#
#"as the absolute value of velocity :"#
#"In the case of a velocity vector, speed is the norm of v."#
#s = | v |#