Physics is the fundamental natural science concerned with matter, its motion and behavior through space and time, and the related concepts of energy and force. To describe these phenomena quantitatively, physicists rely on a shared system of measurement: the Système International d'Unités, or SI. The seven base SI units cover the fundamental dimensions needed in physics. Length is measured in metres, mass in kilograms, and time in seconds. Electric current is measured in amperes, thermodynamic temperature in kelvins, amount of substance in moles, and luminous intensity in candelas. All other physical quantities can be expressed as combinations of these base units.
A central distinction in physics is between scalar and vector quantities. Scalars have only magnitude, such as mass or temperature, while vectors have both magnitude and direction, such as velocity or force. Vectors are often represented by arrows whose length indicates magnitude and whose orientation indicates direction. Closely related to this is the difference between distance and displacement. Distance is a scalar representing the total length of the path traveled, while displacement is a vector pointing straight from the initial position to the final position. Because displacement depends only on the endpoints, it can be zero even when the distance traveled is significant, as in any journey that ends where it began.
The description of motion, or kinematics, is built on the concepts of speed, velocity, and acceleration. Speed is the scalar rate at which distance is covered, while velocity is the vector rate of change of displacement. Average speed equals total distance divided by elapsed time, whereas average velocity equals displacement divided by time. Acceleration is the rate of change of velocity with time, and because velocity is a vector, acceleration can arise from changes in magnitude, in direction, or in both. When acceleration is constant in magnitude and direction, three equations of motion apply. In these equations, \(u\) is the initial velocity, \(v\) is the final velocity, \(a\) is the acceleration, \(s\) is the displacement, and \(t\) is the time, so that \[v = u + at,\] \[s = ut + \tfrac{1}{2}at^2,\] and \[v^2 = u^2 + 2as.\]