So what exactly is a pendulum? A pendulum is a relatively massive object(usually round) that is hung by a string from a fixed support. Pendulums typically hang vertically in its equilibrium position. This massive object is referred to as a pendulum bob. When this bob is released, it begins to swing back and forth, or, to use more accurate terminology, it begins a back and forth vibration about its equilibrium position.
When the bob is about to be released, it is in a state of potential energy. Once it is released,while in a state of kinetic energy, the bob is dragged down by gravity, and pulled up again immediately by the tension in the string. These are the two dominant forces that are acting upon the bob. The gravity force is the predictable one- it always goes downwards, and it always has the same magnitude. However, the tension force is where it gets tricky. To quote from the article, "as the bob swings to the left of its equilibrium position, the tension force is at an angle - directed upwards and to the right. And as the bob swings to the right of its equilibrium position, the tension is directed upwards and to the left. "
The movement of a pendulum bob can be associated with three types of energy- kinetic, potential, and total mechanical energy. Kinetic energy is the energy an object possesses while in motion. Therefore, the faster an object is moving, the more kinetic energy it possesses. As we see in the diagram below, the kinetic energy of a pendulum bob is increased as it gets closer to its set equilibrium position, and it decreases as it gets farther away.
Potential energy is the stored energy of position. This simple pendulum uses gravitational potential energy, whose amount is based on the mass and the height of an object. To quote the article, "The height of an object is expressed relative to some arbitrarily assigned zero level. In other words, the height must be measured as a vertical distance above some reference position. For a pendulum bob, it is customary to call the lowest position the reference position or the zero level. So when the bob is at the equilibrium position (the lowest position), its height is zero and its potential energy is 0 J. As the pendulum bob does the back and forth, there are times during which the bob is moving away from the equilibrium position. As it does, its height is increasing as it moves further and further away. It reaches a maximum height as it reaches the position of maximum displacement from the equilibrium position. As the bob moves towards its equilibrium position, it decreases its height and decreases its potential energy."
The third type of energy, total mechanic energy, is a bit complicated, so I will not go into that. The article talks about it if you're interested in what it is and what effects it has on the pendulum.
Finally, the period of a pendulum. The period is the time it takes for a vibrating object to complete its cycle. In the case of the pendulum, it is the time it takes for the pendulum, looking at the above diagram, to get from G to A(to put it simply). The article has an interesting study in it on what factors affect the pendulum's period, and it seems that the only thing that can affect a pendulum's period is the length of the string. Nothing else works. Not the height that you drop it from, not the arc. Only the length of the string can change the period of the pendulum. Interesting, right?
So, the explanation as to why pendulums work wasn't as simple as you may have thought it to be, right? As you can see, physics played a huge part in why a pendulum works. Next time you look at a pendulum, look at it a little closer. Experiment a little. Maybe you'll find that no matter what angle you drop it at, or what height, the length won't change.
Sources: http://www.physicsclassroom.com/class/waves/Lesson-0/Pendulum-Motion
Its great to see how something so simple and so endearing can be so complicated. It goes to show how intriguing science can be. I thoroughly enjoyed this article and I learned something new about the transformation of energy through a pendulum!
ReplyDeleteWe can all look at something and just stare at it, a,azed, but how often do we ask why it this happens? Also, do we ever go and try to find out why? We are lucky for all the science in our world that explains all the simple thimgs we overlook amd take for granted, such as why a pendulum's bob swings the way it does. This article was very insightful and i learned a lot
ReplyDeleteI think you are right Andrea, a lot of times we don't think about why things happen. I do remember holding a pendulum when I was little and being mesmerized by the periodic movement. I never knew there was that much physics that was involved or the names of the energies. The article makes me want to start being more observant and curious as to why things happen around me. Good job ! :)
ReplyDeleteI was always amazed by pendulums and the way in which they work. I went to a science institute once and they had this enormous pendulum near the stairs. It was to cool to watch but it wasn't until later did I start to find out why pendulums work. There is a lot of physics that are involved and many experiments can be derived from the pendulum. I find that this article is very informative and causes us to notice the little details that complete the big picture.
ReplyDeleteThis article reviews the many types of energy and energy transformations that we learned in 7th grade. It's interesting to find out how complicated this seemingly simple contraption actually is. Is it true that pendulums are used in psychological therapies such as hypnosis?
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