That low pressure pullsthe neck mass back in, and the result is an oscillation of the air in the neck.The rate of oscillation is determined by the mass of the air in the neck and thesize of the body of the bottle. (The air in the body of the bottle is thespring.) The flow of air across the top pulls the neck mass a little bit out ofthe bottle, which reduces the pressure within the body. When you blow across the bottle opening, the air in the neck behaves asa mass on the end of a spring. One harmonic motion machine that is particularly interesting is an ordinary popbottle. In the nextessay you will see that resonance is an important factor in the operation ofacoustic musical instruments. Sound waves can contribute to the simple harmonicmotion of an object if the period of the wave and period of the motion of theobject are about the same. In fact, any objectthat is even slightly movable, has mass, and has some tendency to return homewhen disturbed will do this. Now there are many objects that behave like swings in their motion (In classicalphysics this behavior is termed SIMPLE HARMONIC MOTION). Specificaly,the time between pushes must correspond with the period of the swing, and youmust push in the same direction the swing is already going. If you try this by giving the swing extra pushes, youvery quickly discover you must push at the right time and in the rightdirection, or you waste your push or perhaps slow the swing down. If you were to replace the energy lost to this friction, you could keepthe swing going forever. The only thing slowing the swing down is friction with the air and within therope. If you make a graph of the distance the swing travels crosswayswith time, the result is a sine wave. The amount of time ittakes the swing to complete its cycle depends on the length of the rope andnothing else. When the arc of swing is large, the swing movesquickly, when the arc is small, the swing moves slowly. This whole operationis periodic: each complete swing takes the same amount of time, regardless ofhow far the swing moves. When the swing has returned to the starting point, it can't go down anymore, but still has a lot of momentum to use up, so it moves beyond the centerand up on the other side until gravity slows it down and pulls it back to thecenter, but it still has too much momentum. If given one strong push, a swing will move back and forth because the earth'sgravity is pulling down while the rope is making the swing move in a partialcircle. Aplayground swing is a convienent example for a discussion of this kind ofmotion, because we have all ridden one, and more importantly, pushed one.
You have probably also noticedmost objects not only return to their original position but continue too far, sothat they may swing back and forth several times before they come to rest. You have noticed many objects have a tendency to return to their originallocation after they have been moved slightly. Simple Harmonic Motion SIMPLE HARMONIC MOTION The object oscillates around the equilibrium position, and the net force on the object is equal to the force provided by the spring. In simple harmonic motion, the acceleration of the system, and therefore the net force, is proportional to the displacement and acts in the opposite direction of the displacement.Ī good example of SHM is an object with mass m attached to a spring on a frictionless surface, as shown in (Figure).