The Student Room Group

A level differential equations help

Scroll to see replies

Original post by e123c
Is that just H = e^(0.1sin(0.25t) + c) ?


Yes, but you can split into a simpler form (which is something you should get used to doing).

e0.1sin(0.25t)+c=e0.1sin(0.25t)ece^{0.1\sin(0.25t) + c} = e^{0.1\sin(0.25t)}\cdot e^c

Now since cc is an arbitrary constant of integration, then so is ece^c. We can denote it as A=ecA = e^c.

Thus the equation reduces to H=Ae0.1sin(0.25t)H = Ae^{0.1\sin(0.25t)}.

Now use the initial condition to determine AA.
Reply 21
Original post by Kvothe the Arcane
Initial height is 5. I.e. H is 5, when t=0 but you need to have a workable equation to figure out what the constant of integration is first.


Original post by RDKGames
Yes, but you can split into a simpler form (which is something you should get used to doing).

e0.1sin(0.25t)+c=e0.1sin(0.25t)ece^{0.1\sin(0.25t) + c} = e^{0.1\sin(0.25t)}\cdot e^c

Now since cc is an arbitrary constant of integration, then so is ece^c. We can denote it as A=ecA = e^c.

Thus the equation reduces to H=Ae0.1sin(0.25t)H = Ae^{0.1\sin(0.25t)}.

Now use the initial condition to determine AA.

I see, so if H = 5 when t = 0, 5 = A x e^0 = A x 1 so A = 5 and that makes the proof
Thanks for your help :biggrin:
Reply 22
Something I find simpler is to substitute H=5 and t=0 (which is given) into the equation
lnH= 0.1sin(0.25t)+C

This would give you ln5= C since sin(0.25*0)=0

So you get lnH= 0.1sin(0.25t)+ln5

Rearranging this for H gives you H= e^(0.1sin(0.25t)+ln5)
= e^(0.1sin(0.25t)) x e^(ln5)
because e^(ln5)=5

you get H=5 x e^(0.1sin(0.25t))
Original post by e123c
Oh no, it's okay I was trying to differentiate it :colondollar:
Now I get lnH = 0.1sin(0.25t) which makes a lot more sense, do I then just show H = e^(0.1sin(0.25t)) ? And then just put the 5 in as that's the initial value or what should I do about that bit?

Have you been taught this topic? I'd go back to easier examples if you are struggling this much - RDKGames is doing far too much of this question for you.
Original post by RDKGames
Yes, but you can split into a simpler form (which is something you should get used to doing).

e0.1sin(0.25t)+c=e0.1sin(0.25t)ece^{0.1\sin(0.25t) + c} = e^{0.1\sin(0.25t)}\cdot e^c

Now since cc is an arbitrary constant of integration, then so is ece^c. We can denote it as A=ecA = e^c.

Thus the equation reduces to H=Ae0.1sin(0.25t)H = Ae^{0.1\sin(0.25t)}.

Now use the initial condition to determine AA.

You are giving FAR too much help here - this student needs to go back to the beginning of this topic.
Original post by Muttley79
You are giving FAR too much help here - this student needs to go back to the beginning of this topic.


Ok? Just let OP know that, no need to quote me lol...
Original post by RDKGames
Ok? Just let OP know that, no need to quote me lol...

I did but you didn't seem to notice that your help was way over the top.

Quick Reply

Latest