It appears from the above that forces do not obey the rules of addition defined in ordinary arithmetic or algebra. For example, two forces acting at a right angle to each other, one of 4 lb and the other of 3 lb add up to a force of 5 lb, not to a force of 7 lb. Forces are not the only quantities which follow the parallelogram law of addition. As you will see later, displacements, velocities, accelerations, and momenta are other examples of physical quantities possessing magnitude and direction that are added according to the parallelogram law. All these quantities can be represented mathematically by vectors, while those physical quantities which have magnitude but not direction, such as volume, mass, or energy, are represented by plain numbers or scalars.