Momentum of an object is the product of its mass and velocity (p=m x v). Since mass is a scalar quantity and velocity is a vector quantity, we can derive that momentum must therefore be a vector quantity as the product of a vector with a scalar is a vector. We can also see that momentum is a vector quantity as a vector quantity is one that has both magnitude and direction. If we picture a car driving down the road with momentum p: the momentum of the car has both magnitude and direction (the direction of this momentum is the direction in which the car is traveling).