Author Topic: Physics Question about a Simple Harmonic Motion Question  (Read 449 times)

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Offline Skiffy Wolf

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Physics Question about a Simple Harmonic Motion Question
« on: March 30, 2015, 03:16:15 am »
So we had this worksheet to do in Physics today. Which read:
With this table at the beginning.


"Example 2.
A pendulum has a bob of mass .15kg. To get it moving with SHM, the bob is pulled aside until it has gaines a height of 0.902m.
1. Calculate the total energy.
2. Calculate the maximum speed."

So first of all I understand the first example. But this second one I'm not entirely sure about. I was going to use 1/2KA^2 for the first question until I realised that didn't work. Then I went ahead and decided that I would use 1/2mv^2 which I realised also wouldent work. My teacher said that for that question we need to use Ep = mgh.
Well how was I supposed to know that when the question says calculate total energy and I have a table of equations with a column of total enery equatins right above which dosen't include that one? And for the second question I'm apparently supposed to use v = √2EK/m
or 1/2mv^2 (which is the one I'm going to use) but I don't understand where v = √2EK/m came from. I swear I have never seen that equtaion before.
« Last Edit: March 30, 2015, 03:24:10 am by Skiffy Wolf Skiffington »

Offline ArtificialSweetener

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Re: Physics Question about a Simple Harmonic Motion Question
« Reply #1 on: April 05, 2015, 01:18:28 am »
For your the first part you find the potential energy because you have just pulled it to the top and have not yet released it. It is not in motion so the energy is only the gravitational potential energy their is no kinetic energy.

The second equation to calculate the velocity is derived from 1/2mv^2 that is just to find the kinetic energy so the equation would actually be Ek=1/2mv^2. You have the mass and you know the total energy of the system (from the previous part). Velocity would be at the maximum when kinetic energy is max so that is just what you found previously but now it is the kinetic energy. You know this because the law of conservation of energy. The Kinetic and Potential always equal the total E. So if there is more potential there is less kinetic (sorry I might be explaining badly ask if you're confused). You get the equation v = √2Ek/m from Ek=1/2mv^2. you just simplify here are the steps.
Ek=1/2mv^2 (multiply be 2 to cancel out the half)
2Ek = mv^2 (divide by m to cancel that out)
2Ek/m = v^2 (Now find the square root since v is squared)
v = √2Ek/m (the final equation)

You input your values and solve for v and your done! I would do the math but I don't have a calculator now.

I hope this helps sorry for the late response. If you need me to elaborate or explain something else just ask!
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Offline Skiffy Wolf

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Re: Physics Question about a Simple Harmonic Motion Question
« Reply #2 on: April 07, 2015, 04:44:22 am »
Oooooh. Right. Having the table with a column of 'Total Energy' made me super confused. But that does make sense with the potential energy. And with rearanging the equation to v = √2Ek/m, it makes more sense to me to just leave it at Ek=1/2mv^2 and subsitute in numbers and solve from there.

And I really don't mind the late reply. I'm just glad someone actually replied.

Offline Eagle God-Heart

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Re: Physics Question about a Simple Harmonic Motion Question
« Reply #3 on: April 09, 2015, 11:51:14 am »
TACO! TACO! TACO!
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