IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: hesho21 on May 20, 2010, 12:43:33 pm

Title: Last question in the mechanics paper!
Post by: hesho21 on May 20, 2010, 12:43:33 pm

Alright the paper was easy , but I have a doubt on the last question
So A was 4 and B was 3375
so the question said something had two motions, one for 15s and one greater than that , and they satisfy these equations:

V=4t-0.05t²   0<=t<=15

v= 3375/t² t>15

Distance travelled during the first 15s (first motion is 225m)

now if u integrate the second motion how do u find C?

Title: Re: Last question in the mechanics paper!
Post by: [Spy] on May 20, 2010, 12:49:34 pm

the integrated expression is -3375/t + c=225
you know t is 15

so its -225+c=225
c=450

so it asked what is the speed where distance is 315
you do -3375/t+455=315
t=25

then 3375/25^2 which is equal to 5.4

:)

Title: Re: Last question in the mechanics paper!
Post by: vserian on May 20, 2010, 01:11:43 pm

Quote from: Spy on May 20, 2010, 12:49:34 pm

the integrated expression is -3375/t + c=225
you know t is 15

so its -225+c=225
c=450

so it asked what is the speed where distance is 315
you do -3375/t+455=315
t=25

then 3375/25^2 which is equal to 5.4

:)

But it already travelled 225 m with the first expression for the last forumla to be vails

it shuls be equal to 315-225 = 90

Title: Re: Last question in the mechanics paper!
Post by: hesho21 on May 20, 2010, 01:30:26 pm

Quote from: Spy on May 20, 2010, 12:49:34 pm

the integrated expression is -3375/t + c=225
you know t is 15

so its -225+c=225
c=450

so it asked what is the speed where distance is 315
you do -3375/t+455=315
t=25

then 3375/25^2 which is equal to 5.4

:)

yeh thanx for that clarification , i did it wrong i just put c as 225 , the distance it travelled in motion A :S anyways thnk u :D

Title: Re: Last question in the mechanics paper!
Post by: neno on May 20, 2010, 01:39:12 pm

yup i did the exact same mistake as u spy...my bad anyways sooo i need to ask u guys sumthing is it true tht this year thrs not gna b a curve its gna b graded on the bases of percentages??

Title: Re: Last question in the mechanics paper!
Post by: hesho21 on May 20, 2010, 01:52:52 pm

Dude u should check this out..

Title: Re: Last question in the mechanics paper!
Post by: MAF on May 20, 2010, 03:05:43 pm

Quote from: Spy on May 20, 2010, 12:49:34 pm

the integrated expression is -3375/t + c=225
you know t is 15

so its -225+c=225
c=450

so it asked what is the speed where distance is 315
you do -3375/t+455=315
t=25

then 3375/25^2 which is equal to 5.4

:)

erm....mate, this is the formula for the distance it covers after t=15 sec
so when t=15 s=0....and not s=225  :)

Title: Re: Last question in the mechanics paper!
Post by: [Spy] on May 20, 2010, 03:08:11 pm

hahahah so whats C?

and anyways i didnt even put C cause i always do this stupid mistake when i integrate! godddddd!!! :@

i hope they only take away part two's marks and not the third one too

Title: Re: Last question in the mechanics paper!
Post by: MAF on May 20, 2010, 03:15:00 pm

But the answer is the 5.4 anyway....cos if you put in 0 as s the the constant 'k' comes out as 225....so the same answer....
man, i dont yth i did this but i messed up question 4..:S
i squared the horizontal and vertical components of the vectors given before finding the resultants horizontal and vertical components....:(
how many marks are they gonna cut for this.....it was a mistake in one step i guess... ??? :-[