Direction

　　The property of a vector that distinguishes it from a scalar: while scalars have only a magnitude, vectors have both a magnitude and a direction. When graphing vectors in the xy-coordinate space, direction is usually given by the angle measured counterclockwise from the x-axis to the vector.

　　Directly proportional

　　Two quantities are directly proportional if an increase in one results in a proportional increase in the other, and a decrease in one results in a proportional decrease in the other. In a formula defining a certain quantity, those quantities to which it's directly proportional will appear in the numerator.

　　Dispersion

　　The separation of different color light via refraction.

　　Displacement

　　A vector quantity, commonly denoted by the vector s, which reflects an object’s change in spatial position. The displacement vector points from the object’s starting position to the object’s current position in space. If an object is moved from point A to point B in space along path AB, the magnitude of the object’s displacement is the separation of points A and B. Note that the path an object takes to get from point A to point B does not figure when deining displacement.

　　Distance

　　A scalar quantity. If an object is moved from point A to point B in space along path AB, the distance that the object has traveled is the length of the path AB. Distance is to be contrasted with displacement, which is simply a measure of the distance between points A and B, and doesn’t take into account the path followed between A and B.

　　Doppler shift

　　Waves produced by a source that is moving with respect to the observer will seem to have a higher frequency and smaller wavelength if the motion is towards the observer, and a lower frequency and longer wavelength if the motion is away from the observer. The speed of the waves is independent of the motion of the source.

　　Dot product

　　A form of vector multiplication, where two vectors are multiplied to produce a scalar. The dot product of two vectors, A and B, is expressed by the equation A · B = AB cos .

　　Dynamics

　　The application of kinematics to understand why objects move the way they do. More precisely, dynamics is the study of how forces cause motio