Speed, Velocity and Acceleration

When a car accelerates at a constant rate (a),
its velocity (v) increases steadily,
while the total distance travelled (x) increases exponentially.

The concepts of speed, velocity and acceleration are vital to describing the motion of all bodies in mechanics. Speed and velocity are commonly treated as if they are interchangeable, but strictly speaking, speed is a measure of a body's rate of motion in any direction, while velocity is a measure of a body's rate of motion in any direction, while velocity is a measure of motion in a specific direction. Both can be measured in the same units (such as metres per second), but speed is a directionless or 'scalar' quantity, while velocity is a directed or 'vector' quintity. In most situations, it's far more useful to know an object's velocity than its speed.



Acceleration is another vector quintity - it measures the rate of change in an object's velocity (in units such as metres per second per second, often written m/s2). Acceleration occurs only when an external force is applied to the object, and depending on the direction of that force, can result in a reduction in its velocity (sometimes termed deceleration) or a change in direction as well as magnitude of its velocity.

Source: Physics in Minutes