Scalar involves only magnitude while vector involves magnitude in a certain direction. That's why there can be negative velocity, displacement and acceleration but no negative length. When it is negative it means the magnitude of the quantity is in the opposite direction. </span>
A scalar quantity is one which has only a magnitude - by contrast, a vector quantity has magnitude and a direction. The existence of a direction, on a simplistic level, can be ascertained by considering whether the quantity would make sense with a negative sign on the front.
A negative acceleration is simply a deceleration, so this must be a vector quantity. A negative displacement is a displacement backwards, so this is a vector too. A negative velocity is a speed backwards, so this is vector also.
The remaining option is length - length has to be positive because it refers merely to the distance between two points. The length of a iPhone is the same whether it is measured from top to bottom, or from bottom to top (i.e. no negative sign is used).
