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 bob , suspended from a rope that does not stretch. . The bob is so heavy compared to the weight of the rope that it can be neglected. When the string is attached to the retort holder, the bob will hang straight down. If the coil is pulled so that the string makes a small angle with the vertical, then released, the pendulum will swing back and forth, performing a simple harmonic motion. The bob will retrace its path at regular intervals and the time it takes for the pendulum to complete a complete cycle, or swing, is called a