What does this mean? Pendulum physics

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Freddy12345
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When you have measured the oscillations you should use your data to find a value for k. Use your value for k to find the number of oscillations it takes for the aamplitude to halve, that is the number of oscillations for 75% of the initial energy to dissipate?

What is that question saying? can someone explain please for me step by step.

Then it says assuming the pendulum will be wound up when it loses 75% of it's enrgy you can find the time elapsed between windings?



HOW?
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Ari Ben Canaan
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(Original post by Freddy12345)
When you have measured the oscillations you should use your data to find a value for k. Use your value for k to find the number of oscillations it takes for the aamplitude to halve, that is the number of oscillations for 75% of the initial energy to dissipate?

What is that question saying? can someone explain please for me step by step.

Then it says assuming the pendulum will be wound up when it loses 75% of it's enrgy you can find the time elapsed between windings?



HOW?
You haven't supplied the full question hence nobody will be of help to you. I suggest you upload the question before your thread is relegated to the 'land of no return'.
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Freddy12345
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Can you check your in box please?
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Freddy12345
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Have you found it?
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Freddy12345
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Type in google: Breifing damped pendulum, then click on quick view link of edexcel gce physics. and once loaded scroll down to the damped pendulum question.

this is to get the original quesition
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Ari Ben Canaan
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The experiment is rather simple.

You displace your pendulum a certain value and this becomes your A0

You then let go of the pendulum and measure the maximum displacement after each successive oscillation. This will obviously decrease with time due to air resistance.

Each successive measured is your A value.

Now you plot a graph of A on the y axis and 'no. of oscillations on the x axis'.

The reason for this is because :

A= A0e-kn

When you take natural logs of each side :

lnA = ln(Ao) - kn

So the gradient of your graph will be equal to -k.
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Ari Ben Canaan
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Use your value for k to find the number of oscillations it takes for the amplitude to halve, that is the number of oscillations for 75% of the initial energy to dissipate.
Read the part in bold above. You can use the formula provided and your value for k to determine the number of oscillations after which the amplitude halves.
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Ari Ben Canaan
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The pendulum is re wound after the energy of its oscillations falls to 75% i.e. its amplitude halves.

Now this means, the entire system is reset such that the pendulum is set back to its initial maximum displacement A0.

If this was not done the pendulum would eventually stop and your clock would not read time at all.

The question is asking you to now determine the time elapsed between setting the pendulum to oscillate and the point at which it is rewound.

To put it in other words.... Find the time between the start of oscillations and the point where the amplitude of the oscillations is halved.

You know the time period (2 seconds as dictated in the question) and you can use the formula and k to determine the number of oscillations.

Should be easy from here on...
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brijmohan3
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(Original post by Ari Ben Canaan)
To put it in other words.... Find the time between the start of oscillations and the point where the amplitude of the oscillations is halved.

You know the time period (2 seconds as dictated in the question) and you can use the formula and k to determine the number of oscillations.

Should be easy from here on...
I see what you're saying and had that thought myself. I had one question though. If by theory (the equation) the number of oscillations comes out to be say 15.3 (to 3sf), would you take it as it was on the 15th oscillation that the half initial amplitude was reached, or the 16th? Mathematically I'd choose 15 and therefore do 15 x time period for one oscillation. But I don't know why my teacher keeps banging on to think about it... and so is making me think it should be 16... although I don't see why. Any help?

Edit: Or would you just do 15.3 x time period of one oscillation?
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Freddy12345
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16, because we can't see the 15.3 swings. meaning we will only be able to view that the amplitude has halfed at the 16th swing.
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brijmohan3
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Ooooooooooo. Thanks!
So in my conclusion, I'd also be writing that it's the 16th swing for the initial amplitude to halve? If it is 15.3.
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Freddy12345
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yes
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VeeOMGlob
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this is why the studentroom is brill!
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