If you reduce the time interval between measurements of position both the distance moved and the time required are reduced. If the speed is constant, then the ratio of the two does not change. Instantaneous speed is defined as the limit of distance divided by time interval when the time interval is reduced to zero. In practice you canâ€™t reach the limit, but it is possible to measure positions every thousandths of a second. There are indirect methods of measuring instantaneous speed. For example, police use the Doppler shift (that will be discussed later in this book). That is, the change in frequency that occurs when the radio or light wave is reflected from a moving object. Automobile speedometers often use the turning force (torque) on an aluminum disk produced by a magnet that is rotated by the turning car axle. This force will also be discussed later in the book.