Physics Simple harmonic oscillations Watch

fiftythree
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Im stuck on 4b (iii)
https://www.ocr.org.uk/Images/61757-...nian-world.pdf

firstly, im confused on how we know to use the x=Acoswt graph and not x=Asinwt

secondly, where did the 15.5 m come from.

Thanks
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Shafi_haque
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You use x = Acoswt when the object begins to oscillate from its max positive amplitude, and the sine version when it begins oscillating from equilibrium.
So when t=0, use the cosine version if it begins from amplitude, and sine for if it begins from equilibrium (think about your sine and cosine graphs).

Im guessing the 15.5m is the average height of the surface of the water. (18+13/ 2) = 15.5m
Hope that helps!
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fiftythree
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(Original post by Shafi_haque)
You use x = Acoswt when the object begins to oscillate from its max positive amplitude, and the sine version when it begins oscillating from equilibrium.
So when t=0, use the cosine version if it begins from amplitude, and sine for if it begins from equilibrium (think about your sine and cosine graphs).

Im guessing the 15.5m is the average height of the surface of the water. (18+13/ 2) = 15.5m
Hope that helps!
Ty for the reply.

What information in the question suggests that the water starts from maximum amplitude
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fiftythree
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Can someone help me visualise this
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Physics Enemy
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(Original post by fiftythree)
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You're right they don't give start position of the tide, cos or sin is fine. Or they assume high tide at t = 0 without stating.

d = 15.5 corresponds to x = 0, the equilibrium of oscillation; d = 18 to x = A, d = 13 to x = -A.

t = 0, high tide: d = 15.5 + x = 15.5 + 2.5cos(wt)
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Eimmanuel
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(Original post by fiftythree)
Im stuck on 4b (iii)
https://www.ocr.org.uk/Images/61757-...nian-world.pdf

firstly, im confused on how we know to use the x=Acoswt graph and not x=Asinwt

secondly, where did the 15.5 m come from.

Thanks
For such a question, you can use either Acos(2πft) or Asin(2πft). There is no info within the question that says you must to use Acos(2πft).
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fiftythree
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(Original post by Physics Enemy)
You're right they don't give start position of the tide, cos or sin is fine. Or they assume high tide at t = 0 without stating.

d = 15.5 corresponds to x = 0, the equilibrium of oscillation; d = 18 to x = A, d = 13 to x = -A.

t = 0, high tide: d = 15.5 + x = 15.5 + 2.5cos(wt)
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