Is momentum a vector quantity? Why?

Yes - Momentum is a vector quantity. A vector quantity possesses both a magnitude and direction. A scalar quantity possesses only a magnitude and no direction. Mass is an example of a scalar quantity (mass doesn't point in any direction!) whereas velocity is a vector quantity. p = m x v Momentum is the product of mass and velocity, and the product of scalar and vector quantities produces a vector quantity (i.e. you can't lose direction!). SCALAR x VECTOR = VECTOR This means that momentum must be a vector quantity, with its direction pointing in the same direction as the velocity (i.e. if a ball is travelling eastwards, its momentum is also pointing eastwards). (N.B. Speed is a scalar quantity and Velocity is a vector quantity. These always have the same magnitude but speed has no direction. SPEED IS VELOCITY WITH NO DIRECTION ASSOCIATED WITH IT. This means that you can't have a negative speed value. If the velocity of an object is -5m/s, then its speed is 5m/s. A good test to see if a quantity is vector or scalar is to check if you can have negative values of such quantity; if you can't then it's a scalar!)