According to newton's second law: F=MA
Or, force equals mass times acceleration.
Scalar quantities are measurements WITHOUT a direction. Example, speed. Speed measures how fast you are going, but not what direction you are going. Therefore mass is considered a scalar since you can measure how much mass an object has, but since the mass doesn't have to be moving it does not have an associated direction.
Conversely, velocity is a vector. Vectors are measurements WITH a direction. Velocity by definition is speed in a direction therefore by definition it is a vector. Acceleration is defined as change in velocity over a change in time and therefore is also a vector.
