How can we measure the position of an object? We can say where an object is, relative to some reference point. We need to give the distance from the reference point, and the direction-position is a vector quantity. How can we tell if the position is changing? Measure the position x0 at time t0. Then measure the position x at time t. If the two position vectors (the original position x0 and the new position x) are the same, then the object is not moving. If they are different, the object is moving. The amount that the position changes by is called the displacement. The displacement is given by: Note that the displacement is a vector quantity. If we only interested in the distance moved, and not the direction, the distance is the magnitude of the displacement. The SI unit of position, displacement, and distance is the metre (m).