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โš™๏ธย ap physics c: mech

ย ย >ย ย 

๐ŸŒŠย Unit 6

ย ย โ€ขย ย โฑ๏ธ3 min read

6.1 Simple Harmonic Motion, Springs, and Pendulums ๐Ÿ•ฐ๏ธ

Daniella Garcia-Loos

daniella garcia-loos

โฑ๏ธ November 5, 2020

๐Ÿ“…

6.1: Simple Harmonic Motion, Springs, and Pendulums ๐Ÿ•ฐ๏ธ

Let's begin by defining what simple harmonic motion is!

Firstly, periodic motion is the type of motion that repeats itself over and over.

Simple Harmonic Motion is periodic motion that follows this very general equation:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EOkWKWFcIGXh.png?alt=media&token=b777281c-e5e5-44a3-a628-35396f830a12

Where x is position as a function of time, A is the amplitude of the wave function, omega is angular frequency, and phi is the phase shift/angle.

Additionally, we can find equations to describe velocity and acceleration in simple harmonic motion by taking derivatives! Which gives us:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-3sDNmyU4tKHH.png?alt=media&token=9ad4ad6d-3a6b-4208-bbb0-f243ac840fcb

Let's take a look at some graphs depicting these relationships!

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-NOJSvEr73vWw.png?alt=media&token=b8c2a52b-2039-4890-b813-103d28815611

Image taken from Physics Stack Exchange

You should be able to notice the calculus-based relationships between the slopes, where they should be zero, where directions are changing, and where the equations are reaching local maxima/minima.

Now, let's discuss a common part of a wave function. Since we have angular frequency, we can also describe simple harmonic motion with a period.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-5bBQ1JoR3Kw1.png?alt=media&token=51b41fcb-d287-4713-9674-6a14db4fa162

I bet you thought the calculus was over, get ready for the generic differential equation version of simple harmonic motion!

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Y5qKowDj7dLu.png?alt=media&token=3ba32822-7aa4-44ae-8f11-d11395146dd2

This equation stems from Newton's Second Law (which I'll show later) and can be used to describe any simple harmonic motion (SHM) scenario. If something is in SHM, it should be able to be described in a manner similar to this. X does not necessarily need to be displacement or distance for it to be applicable, it could be theta or arc length, or any other crazy unit of measurement!

Let's try to find period using the relationship above for two common SHM scenarios: springs and pendulums.

Springs:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EFk6j9GzRYMi.png?alt=media&token=3144dea5-ac65-41ed-a44c-c7522f7eb0cc

There we have it! This should look similar to something in your formula chart.

โš ๏ธNote: Any system that creates a linear restoring force (F=-kx) will display the characteristics of SHM!

Pendulums:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-klWiocKNeQPR.png?alt=media&token=05cb6581-35cd-410b-b079-c62e0380fa9f

Which is another formula you should be familiar with!

You didn't think we forgot about energy did you? Let's analyze some relationships in SHM with energy!

Total mechanical energy in SHM is always conserved, and it is the sum of the kinetic energy and potential energy(which comes from the restoring force, like gravity or spring).

ME = K + U

in which kinetic energy is:

K = .5mv^2

and potential energy is

Us = .5kx^2 or Ug = mgh

Let's take a look at a graph representing these relationships:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-N79fGjuHEzVO.png?alt=media&token=eb3d1880-87d9-457a-bb4c-13b372c885d6

Image taken from LibreTexts

As you can see, the maximum potential energy occurs at maximum displacement, in which velocity is zero and kinetic energy is zero because the object is changing directions. Kinetic energy is at its maximum at the equilibrium position where velocity is at its max and displacement is zero.

Practice Questions

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-kBtptVMLXNLJ.png?alt=media&token=384d5a2b-c48a-4f8f-ae16-47fd40fe6c5e

Taken from College Board

Answers

The beginning of this FRQ involves the application of previous units, like momentum and energy into an SHM scenario. Then it involves correctly identifying the proper SHM formula from the formula chart.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-CX11q2r7N2PR.png?alt=media&token=69031c28-7130-4859-8b87-c9ba36ed5639

The next two parts involve recognizing relationships in equations! From a simple increase or decrease in accordance to another variable, to finding local maxima and minima, you should be prepared to analyze SHM situations in the context of calculus.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-32JLCc2feRTl.png?alt=media&token=69fbd827-146e-4cc6-95f7-159a6c60f1d0

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