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HELP with Oscillations PLEASE.

You are given a diagram which shows a toy consisting of a light plastic aeroplane suspended from a long spring.

First you're asked to draw a displacement against time graph which I did.

Its the next bit that confuses me;

"The aeroplane is replaced by a heavier model made of the same plastic having the same fuselage but larger wings. State and explain TWO changes which this substitution will make to the displacement against time graph that you have drawn for part i."

I know the the amplitude will decrease for a larger mass but Im not sure the reason WHY. And also, Im not too sure about the second reason - the markscheme says the frequency will decrease as acceleration decreases. They use a=(2pi*f)^2 * x but I know that a is proportional to x so as far as i can see x will also decrease - so why would the frequency decrease? Please could someone explain - much appreciated thank you :smile:
Reply 1
Avatar for phys981
phys981
10
Don't you have to think about how (ang) freq is related to mass? Like this http://ncalculators.com/images/formulas/simple-harmonic-motion-frequency.jpg

And the larger wings will produce a larger resistive force so cause damping?

That would be my two guesses - but I can't see the picture.
(edited 6 years ago)
Original post by MrToodles4
You are given a diagram which shows a toy consisting of a light plastic aeroplane suspended from a long spring.

First you're asked to draw a displacement against time graph which I did.

Its the next bit that confuses me;

"The aeroplane is replaced by a heavier model made of the same plastic having the same fuselage but larger wings. State and explain TWO changes which this substitution will make to the displacement against time graph that you have drawn for part i."

I know the the amplitude will decrease for a larger mass but Im not sure the reason WHY. And also, Im not too sure about the second reason - the markscheme says the frequency will decrease as acceleration decreases. They use a=(2pi*f)^2 * x but I know that a is proportional to x so as far as i can see x will also decrease - so why would the frequency decrease? Please could someone explain - much appreciated thank you :smile:

for the same spring...
frequency decreases (period increases) when mass increases
T=2π sqrt(m/k)

probably they're hoping you'll say something about damping for the larger wings.
Reply 3
Avatar for MrToodles4
MrToodles4
OP
10
Original post by phys981
Don't you have to think about how (ang) freq is related to mass? Like this http://ncalculators.com/images/formulas/simple-harmonic-motion-frequency.jpg

And the larger wings will produce a larger resistive force so cause damping?

That would be my two guesses - but I can't see the picture.


That makes sense thank you so much. And I do not have that equation for my OCR spec - so Im just wondering how is it exactly that frequency is related to that equation - so as mass increases frequency decreases? Is that always the case? thanks again :smile:
Reply 4
Avatar for phys981
phys981
10
Original post by MrToodles4
That makes sense thank you so much. And I do not have that equation for my OCR spec - so Im just wondering how is it exactly that frequency is related to that equation - so as mass increases frequency decreases? Is that always the case? thanks again :smile:


yes, that should always be the case.

The other response above mentions the equaiton in a different form, to find period T rather than frequency but they're different forms of the same equation. I'd be surprised if you didn't need this in some form for OCR (but I don't know much about OCR).

Edited to add - a quick look at OCR spec and you're right, I can't see this, which explains I suppose why the MS expresses it in the form it does. . It can be derived however if you have done shm and if you've done Force and spring constants (F = kx) elsewhere, so is worth knowing.
(edited 6 years ago)
Fwiw this is what you get on the AQA datasheet

oscillations aqa datasheet.JPG
Reply 6
Avatar for MrToodles4
MrToodles4
OP
10
Original post by phys981
yes, that should always be the case.

The other response above mentions the equaiton in a different form, to find period T rather than frequency but they're different forms of the same equation. I'd be surprised if you didn't need this in some form for OCR (but I don't know much about OCR).

Edited to add - a quick look at OCR spec and you're right, I can't see this, which explains I suppose why the MS expresses it in the form it does. . It can be derived however if you have done shm and if you've done Force and spring constants (F = kx) elsewhere, so is worth knowing.


alright ill just try and remember the form of equation you've given me. Thank you so much again :smile:
Reply 7
Avatar for MrToodles4
MrToodles4
OP
10
Original post by Joinedup
Fwiw this is what you get on the AQA datasheet

oscillations aqa datasheet.JPG


Thank youuuu
Can you please share the link to the markscheme for these questions. Thanks
Original post by 227334
Can you please share the link to the markscheme for these questions. Thanks

Why do you want the MS?

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