Factors that Affect Pendulums? By Jun. 1

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wuggymarshall
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Factors that Affect Pendulums? By Jun. 1

Post by wuggymarshall »

Hi,
I am doing a project on pendulums and other simple harmonic oscillators. Right now, I'm working on a section where I'm discussing the factors that affect a pendulum's swing (speed, etc.). So far, I have length, starting angle, force of gravity, friction, air resistance, and sympathetic vibration. Have I missed any? Also, I'm curious about any factors that DON"T have an effect (other than the weight of the bob). Thank you so much! I really appreciate your help.
wuggymarshall
wuggymarshall
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Another Pendulum Question

Post by wuggymarshall »

Hi again,
I have been looking for information about what forces are at work when a pendulum swings. All of them have been in language that is very advanced, and since I'm a beginner to this stuff, didn't make much sense. I would very much appreciate if someone could explain a pendulum's forces in language that a complete beginner could understand. Thank you!
wuggymarshall
stephaniebie
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Re: Another Pendulum Question

Post by stephaniebie »

Hi wuggymarshall,

That's a great question! For this discussion, I will use the example of a ball attached to a string to represent a pendulum.

Before we dive into the answer to your question, it is important to understand this concept about motion: according to Newton's First Law of Motion (more on it here: http://www.physics4kids.com/files/motion_laws.html), an object set in motion will move in a straight line in the direction of that motion until something else, like a second force, stops it. For example, when you drop a ball straight down from a building, it will go straight down because the force due to gravity causes it to fall down. But let's say there's some wind pushing it to the left, the ball would not fall down at a completely vertical path now but rather a curved path because of a second force acting on the ball in a different direction.

Now back to the pendulum: If we only drop the ball to allow the pendulum to swing by itself, there are only ever two forces acting on it - the force due to gravity (AKA the weight) and the centripetal* force (in this case, tension). Why? The easy way to look at it is to look at each little part of the pendulum (refer to the diagram below). When we first drop the pendulum, the ball will naturally want to fall down due to gravity (like the example with the building). That is why there is a downwards arrow representing the weight of the ball (in the picture, the weight is mg, meaning mass times the acceleration of gravity).

Image

However, in a pendulum, we see the ball follow a curved path - it doesn't just fall straight down. That means there must be another force acting on it. Since nothing is pushing it to the right or left, the only other force acting on it must be from the string. This force, T (meaning tension), since it is not parallel to the force due to gravity at every point can move the ball either towards the left or right because there is a component, or part, of that non-vertical force that is directed towards the left or right. See in the diagram below how the tension forms a triangle with the dotted line. It is that red, horizontal component of tension which causes the ball to move to the left. (Think of that horizontal component like the wind in our example with the ball falling down from a building.)

Image

So to recap, a simple pendulum only experiences two forces: the gravitational force (due to the object's weight) and tension (an exception would be if you push the ball; then that extra force needed to push the ball would count as a third force).

*more information on centripetal force here: https://www.khanacademy.org/science/phy ... -intuition

Hope this helps. Good luck and feel free to write back with any more questions! :D
Stephanie Bie
dcnick96
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Re: Factors that Affect Pendulums? By Jun. 1

Post by dcnick96 »

Hi, there. Please keep all questions in one topic thread so the experts can best help you by seeing what has already been discussed.

Thanks for using Science Buddies, and good luck!
Deana
wuggymarshall
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Occupation: Student

Thank You!

Post by wuggymarshall »

Thank you so much Stephanie, I can understand it perfectly now!
stephaniebie
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Re: Factors that Affect Pendulums? By Jun. 1

Post by stephaniebie »

No problem, wuggymarshall! Glad I could help :D
Stephanie Bie
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