What is Motion?
Dynamics, the study of motion, is today assisted in many ways by modern electronic instruments and high-speed photography. One of these instruments, called a stroboscope, produces very sharp flashes of light at a steady rate so that a moving object is illuminated many times in a second. If a photograph of the object is taken with the camera shutter open for as long as a second, a large number of positions of the object are recorded one after another on the film.
For example, these photographs reveal many details of the motion of the human body which cannot be seen under normal viewing conditions. The movements of a dancer photographed under stroboscopic light flashing at twenty times per second produce a flowing pattern of arms, legs and body positions. When photographing things that happen very quickly, such as a golfer swinging his club, the number of flashes per second, or the 'frequency' has to be increased.
The term frequency is used in many branches of science: the frequency of rotation of a motor car wheel is the number of turns it makes a second: the frequency of an alternating electric current is the number of cycles or changes which occur per second.
It was later decided to measure all frequencies in a new unit called a hertz (hz for short) after the German physicist Heinrich Hertz (1857-1894).
Stroboscopic photographs of moving objects, or bodies as the scientist calls them, show that there are many different kinds of motion. The simplest, but not the easiest to produce, is a constant speed. The body moves an equal distance in every interval between the flashes of light. Quite a different motion occurs when a body falls freely through the air. The increasing distances traveled between flashes indicates that the body is accelerating, that is, increasing its speed in each interval of time. If the body is thrown upwards, the distances between flashes decrease, indicating de-acceleration, or slowing down. Photographs of a ball thrown forward at an angle show that projectile motions of this kind combine acceleration, de-acceleration and constant speed. To understand how all these motions can occur at the same time requires some study of the types of motion involved.