Newton's Third Law of Motion: Newton's third law of motion states that action and reaction are equal and opposite.PrinciplesA simple pendulum experiment consists of a weight suspended from a string which does not stretch. The weight is so heavy in comparison that the weight of the rope can be ignored. When the string is tied to the retort holder, the weight will hang straight down. If the pendulum is pulled out so that the string makes a small angle with the vertical and then released, the pendulum will swing back and forth, performing a simple harmonic motion. The weight will retrace its path at regular intervals and the time it takes for the pendulum to complete one complete cycle, or swing, is called