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DOJO
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#1
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A particle is moving with uniform acceleration. If it starts from rest and 5 seconds later has a speed of 18 ms-1. Find the distance it has travelled.

My method:

u = 0 [initial velocity]
t = 5s [time]
a= 18ms-1 [acceleration]

so I used the following formulae to find the distance it had travelled:

s=vt+1/2at^2

as we do not know the initial velocity, I marked that as "0" and just equating the coefficent aboves into their letters for that formulae.

s = 0 + 1/2(18)(5)^2 = 187.5 m

which is wrong it should be 45m

Any idea, where I am going wrong...I have a feeling I used the speed variable incorrectly.
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Gaz031
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(Original post by DOJO)
A particle is moving with uniform acceleration. If it starts from rest and 5 seconds later has a speed of 18 ms-1. Find the distance it has travelled.
u=0, v=18, t=5, s=?
s=(u+v)/2 . t
s = 9x5
s = 45m.

You do know the initial speed (rest) and don't know the acceleration.
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DOJO
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(Original post by Gaz031)
u=0, v=18, t=5, s=?
s=(u+v)/2 . t
s = 9x5
s = 45m.

You do know the initial speed (rest) and don't know the acceleration.
Thanks for you answer, I can see how you have got that answer, but what is confusing me is that the formulae for s = ut + 1/2at^2 .

so I am slightly unsure on how you came to that conclusion because if you equate the co-efficents U,V, and T to find S, everything cancels out because the initial velocity is 0 and the acceleration is 0.

Thanks.
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Gaz031
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(Original post by DOJO)
Thanks for you answer, I can see how you have got that answer, but what is confusing me is that the formulae for s = ut + 1/2at^2 .

so I am slightly unsure on how you came to that conclusion because if you equate the co-efficents U,V, and T to find S, everything cancels out because the initial velocity is 0 and the acceleration is 0.

Thanks.
The acceleration isn't 0.
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GoPat
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#5
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who said accel. is 0
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DOJO
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(Original post by Gaz031)
The acceleration isn't 0.
Yeah true, acceleration is 9 right? a=v-u/t which is a = 18/2 = 9

therefore s= ut + 1/2at^2

which is s=0(5) + 1/2 9(25) = 225/2 = 112.5 m , which is incorrect
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GoPat
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a=v-u/t

= (18-0)/5

=3.6 ?
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Gaz031
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(Original post by DOJO)
Yeah true, acceleration is 9 right? a=v-u/t which is a = 18/2 = 9

therefore s= ut + 1/2at^2

which is s=0(5) + 1/2 9(25) = 225/2 = 112.5 m , which is incorrect
The acceleration isn't 9 either..
Just use the other appropriate formula.
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DOJO
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#9
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(Original post by GoPat)
a=v-u/t

= (18-0)/5

=3.6 ?
oops! :dito: got the right answer now :tsr:
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Juno
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You don't knoiw the acceleration from the question. You know that u=0, t=5 and v=18. You want to find s. So you should really use a formula with those 4 in, which is s= ((u+v)t)/2

If you want to find the acceleration, then use v=u+at, which means it comes out at 3.6

I think you need to look at the formulae again, as that's where you seem to be going wrong
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magiccarpet
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#11
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(Original post by DOJO)
A particle is moving with uniform acceleration. If it starts from rest and 5 seconds later has a speed of 18 ms-1. Find the distance it has travelled.

My method:

u = 0 [initial velocity]
t = 5s [time]
a= 18ms-1 [acceleration]

so I used the following formulae to find the distance it had travelled:

s=vt+1/2at^2

as we do not know the initial velocity, I marked that as "0" and just equating the coefficent aboves into their letters for that formulae.

s = 0 + 1/2(18)(5)^2 = 187.5 m

which is wrong it should be 45m

Any idea, where I am going wrong...I have a feeling I used the speed variable incorrectly.
im a bit rusty but i'll try..
v=18 m/s
u=0 (at rest)
x=?
t=5 seconds


x= 1/2(u+v)t
= 1/2 * (18+0) * 5
=45
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DOJO
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#12
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(Original post by juno_the)
You don't knoiw the acceleration from the question. You know that u=0, t=5 and v=18. You want to find s. So you should really use a formula with those 4 in, which is s= ((u+v)t)/2

If you want to find the acceleration, then use v=u+at, which means it comes out at 3.6

I think you need to look at the formulae again, as that's where you seem to be going wrong
The speed bit in the question confused me at first, which gaz thankfully corrected.

I have got the right answer so i think:

s=(0)(5)+1/2 3.6(25)= 45m
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Juno
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(Original post by DOJO)
The speed bit in the question confused me at first, which gaz thankfully corrected.

I have got the right answer so i think:

s=(0)(5)+1/2 3.6(25)= 45m
Yeah, that's right. But as the question didn't ask you to find the acceleration, you could've used the other equation I said. It's just a bit quicker for if you're short of time in exams, but if you've got enough time to do it that way then that's fine
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DOJO
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(Original post by juno_the)
Yeah, that's right. But as the question didn't ask you to find the acceleration, you could've used the other equation I said. It's just a bit quicker for if you're short of time in exams, but if you've got enough time to do it that way then that's fine
The thing is that the M1 book only teaches you that equation in the form I used...So i was not aware of that other formulae until now. Maybe that other formulae was in the book , but i probably skimmed over it. Thanks all :tsr:
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magiccarpet
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#15
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can i say, im fairly certain you'll get given the equations in the formula book especially if OCR or Edexcel Math. you can also dervive all the equations using one of two ways. if u know one u can gett the rest mathematically as here:

http://www.plus2physics.com/linear_m...apter=1&page=3

but i dont understand it, or you can do the area under a graph using speed/time, distance time, findint the tangents etc i think thats here too or else pm me for mor hepl
good liuck!
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magiccarpet
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v=u+at also known as a=(v-u)/t

s=0.5 (u+v)t


v^2= u^2 +2as
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Juno
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#17
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(Original post by DOJO)
The thing is that the M1 book only teaches you that equation in the form I used...So i was not aware of that other formulae until now. Maybe that other formulae was in the book , but i probably skimmed over it. Thanks all :tsr:
You should have 5 equations, so that there's one for whichever bit of information you're missing.

v=u+at (no s)
v^2=u^2+2as (no t)
s=ut+1/2at^2 (no v)
s= ((u+v)t)/2 (no a)
s=vt-1/2at^2 (no u)

So you just pick the formula that contains the 3 things you know, and the 1 you want to find out.
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DOJO
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#18
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(Original post by magiccarpet)
can i say, im fairly certain you'll get given the equations in the formula book especially if OCR or Edexcel Math. you can also dervive all the equations using one of two ways. if u know one u can gett the rest mathematically as here:
I have looked in the formulae booklet for AL EDEXCEL Maths and does not give you any of the formulaes in the applied unis. Is there a seperate formulae booklet for the applied units? The formula booklet I have is c1-fm6.
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