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VERY HARD SUVAT QUESTION - need urgent help please! Watch

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    A motorist travelling at x m/s joins a motorway. On the motorway she travels at a constant acceleration of 0.07 m/s-2, until her speed has increased by 2.8m/s. Calculate the time for the increase in speed.
    (NOTE : IT DOES MENTION LATER IN THE QUESTION THAT THE DISTANCE TAKEN FOR THE CHANGE IN SPEED IS 1050M).
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    S = ?
    U = x m/s
    V = x + 2.8 m/s
    A = 0.07 m/s^2
    T = ?

    Now use one of the equations and rearrange to find the time.

    Good luck
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    Pretty simple.

    u=x, v=x+2.8, a=0.07

    You know s=\frac{1}{2}(u+v)t

    So use v^2=u^2+2as=u^2+a(u+v)t and solve for t
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    (Original post by cazual)
    S = ?
    U = x m/s
    V = x + 2.8 m/s
    A = 0.07 m/s^2
    T = ?

    Now use one of the equations and rearrange to find the time.

    Good luck
    I laid it out exactly like you, yet i achieved an incorrect answer. Could you do it and see what answer you would get. I used the s=ut + 0.5at2, but surely you cant use U as you would then have two unknowns?
    Thanks
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    (Original post by RDKGames)
    Pretty simple.

    u=x, v=x+2.8, a=0.07

    You know s=\frac{1}{2}(u+v)t

    So use v^2=u^2+2as=u^2+a(u+v)t and solve for t
    Im pretty sure thats wrong as how would you be able to solve for T if U is unknown?
    Thanks
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    (Original post by IGCSETANK)
    Im pretty sure thats wrong as how would you be able to solve for T if U is unknown?
    Thanks
    The x goes away with basic manipulation if you go through it. I got a nice answer.
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    S = ?
    U = x m/s
    V = x + 2.8 m/s
    A = 0.07 m/s^2
    T = ?

    V = U + AT

    x + 2.8 = x + 0.07t
    2.8 = 0.07t
    t = 40 s
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    (Original post by RDKGames)
    The x goes away with basic manipulation if you go through it. I got a nice answer.
    Can you display what youve said in different steps and order of logical working, with an answer at the end, because i'm confused as to what your getting at here.
    Thank you
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    40 seconds
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    (Original post by IGCSETANK)
    Can you display what youve said in different steps and order of logical working, with an answer at the end, because i'm confused as to what your getting at here.
    Thank you
    I'm sure you understood the post above, it's simple substitution.

    So it becomes:

    (x+2.8)^2=x^2+0.07t(2x+2.8)

    \Rightarrow x^2+5.6x+7.84=x^2+t(0.14x+0.196)

    \Rightarrow 5.6x+7.84=(0.14x+0.196)t

    \displaystyle \frac{5.6x+7.84}{0.14x+0.196}=t

    \displaystyle \frac{5.6x+7.84}{\frac{1}{40}(5.  6x+7.84)}=t

    and finishing it off leaves you with t=40

    It is easier to notice that the fraction cancels if you turn decimals into fractions, then put everything under a single fraction. Otherwise, use your calculator to investigate what multiple you need.

    There is a solution above with v=u+at which is really simple, I just over-complicated it
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    may be a quadratic
    (Original post by IGCSETANK)
    Im pretty sure thats wrong as how would you be able to solve for T if U is unknown?
    Thanks
 
 
 
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