Mechanics maths problem, please help me

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19 Replies - 2801 Views - Last Post: 08 December 2012 - 09:39 AM

#1 Coding in the name of  Icon User is offline

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Mechanics maths problem, please help me

Posted 06 December 2012 - 12:16 PM

Hi guys, I got a maths problem and I hope you'll be kind enough to help me. I didn't post it in the student lounge because it's not about programming.

In a relay race, a sprinter receives the baton 100m from the finish when his speed is 8.8m/s. He then accelerates uniformly for 2s until he reaches his top speed, which he maintains for the remainder of the race. His time for the 100m is recorded as 10.6s.

Calculate the sprinter's top speed.

I've set up a velocity/time graph like in the attached image.
I know the UVATS equations but I can only identify two terms with which to find v.

v = u + at
v^2 - u^2 = 2as
s = ut + (at^2)/2
s = t(u + v)/2

u = 8.8m/s
v = ?
a = ?
t = 2s
s = ?

I think I have to somehow determine the displacement in section B to find the displacement in section A. I can see that the final velocity must be determinable because the acceleration happens over 2 seconds and 8.6 seconds remain, and that if he accelerated to some high velocity he would have finished the 100m sooner, but I can't figure out the solution.

Thanks in advance.

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This post has been edited by Coding in the name of: 06 December 2012 - 12:16 PM


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Replies To: Mechanics maths problem, please help me

#2 lordofduct  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 12:27 PM

if you substitute in v = u + at into the v^2 - u^2 = 2as (for the v). And you substitute in the 's = ut + (at^2)/2' for the s in that as well. You have an equation of u, t, and a only. u and t is known, now you can get a.
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#3 Dormilich  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 12:29 PM

I think you’ll also need the 100 m and the 10.6 s overall distance and time. so-to-speak 100 m = A + B.
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#4 Coding in the name of  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 12:51 PM

I followed your idea lordofduct but I think I'm not seeing something.

(u + at)2 - u2 = 2a(ut + 1/2at2)
u2 + 2uat + a2t2 - u2 = 2aut + a2t2

Then everything cancels out to leave 0 = 0


Never mind I'm stupid I'ma try this.

EDIT

I thought if I substituted in the numbers I'd see it but
35.2a + 4a2 = 35.2a + 4a2

This post has been edited by Coding in the name of: 06 December 2012 - 12:55 PM

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#5 lordofduct  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 01:10 PM

I agree with Dormilich

I was just going off the equations you gave, but really even though it reduces it to 1 variable, we probably will get multiple results when reducing (I see some squares in their, so yeah, probably).

I don't really have the time to be looking into this right now though seeing as I'm juggling a bunch of stuff right now.
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#6 BenignDesign  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 01:37 PM

WTF is this in the Lounge? For all intents and purposes, it doesn't belong on these forums at all. We're here for programming help, not math homework. And, no, I'll not critique your creative writing project either.
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#7 jon.kiparsky  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 01:44 PM

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#8 Coding in the name of  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 01:54 PM

OK everyone post up what song they're currently listening to instead.
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#9 modi123_1  Icon User is online

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 02:02 PM

Quote

OK everyone post up what song they're currently listening to instead.

No.. we have a thread for that already.
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#10 jon.kiparsky  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 02:04 PM

View PostBenignDesign, on 06 December 2012 - 03:37 PM, said:

And, no, I'll not critique your creative writing project either.


I will: it sucks. Any other questions?
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#11 NeoTifa  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 02:57 PM

God damnit, I haven't even gotten accepted into college yet, and you're already making me hate the idea of going back. Thanks for deflating my hopes.
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#12 jon.kiparsky  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 03:00 PM

View PostNeoTifa, on 06 December 2012 - 04:57 PM, said:

God damnit, I haven't even gotten accepted into college yet, and you're already making me hate the idea of going back. Thanks for deflating my hopes.


Hope like a balloon. One prick and it's all over.
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#13 strawhat89  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 10:31 PM

*
POPULAR

And so it is with virginity.
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#14 raghav.naganathan  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 10:37 PM

View Postjon.kiparsky, on 07 December 2012 - 03:30 AM, said:

Hope like a balloon. One prick and it's all over.


View Poststrawhat89, on 07 December 2012 - 11:01 AM, said:

And so it is with virginity.


Haha...:D

Perfect example!!!

Couldn't be a more better example than that for 'One prick and it's all over' :^:

This post has been edited by raghav.naganathan: 06 December 2012 - 10:43 PM

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#15 jon.kiparsky  Icon User is offline

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Re: Mechanics maths problem, please help me

Posted 06 December 2012 - 10:43 PM

View Poststrawhat89, on 07 December 2012 - 12:31 AM, said:

And so it is with virginity.



Ah, you've heard that one before...
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