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    I cannot find out to find out what C is when you intergrate.
    In this following question during the intergration part of getting displacement how do i find out what c equals?
    here the question:
    A particle moves in a straight line. Its velocity at time t s is v ms-1, where v= 0.3t^2 + 4t
    Find:
    (i) find an expression for the acceleration of the particle in terms of t.
    The particle passes through the point A with velocity 20.8ms-1
    (ii) Find the displacement of the particle from A when its acceleration is 7ms-2
    I can nearly do all of this question its just the bit in the middle where I dont understand how to find what C equals.
    thanks
    btw final answer to this question is 24.1metres

    sorry i copied the question out wrong, just updated it
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    You need to know some conditions in order to find c. Did the particle start from rest, if so then v=0 at t=0.
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    sorry i copied the question out wrong
    ive updated it now
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    You just need to call the point where it starts x=0. Since x=0 at t=0, then c=0
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    (Original post by Bezza)
    You just need to call the point where it starts x=0. Since x=0 at t=0, then c=0
    hmm ok...when it the point where X=O, at point A it has already been moving for awhile hasnt it?
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    (Original post by hero)
    hmm ok...when it the point where X=O, at point A it has already been moving for awhile hasnt it?
    Yeah, but because the question asks you about the displacement from the point A it's easiest just to call A the origin so at A, x=0, then x will be the displacement from A.
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    Calculate the value of t at the point A where the velocity is 20.8. you can't just say that it started at v=0 cos you dont know that.
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    (Original post by queen-of-pain)
    Calculate the value of t at the point A where the velocity is 20.8. you can't just say that it started at v=0 cos you dont know that.
    I said let x=0 initially, and having read the question I've realised it doesn't start at A :rolleyes: crap. sorry! You do know that v=0 initially cos it gives you the eqn for v as a function of t.

    You just find how far A is from the start point, and how far it is from the start point when a = 7 m/s^2, then subtract one from the other to calculate displacement from A.
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    s = 0.1t³ + 2t² + c
    v= 0.3t² + 4t
    a = dv/dt = 0.6t + 4

    at A, v=20.8 m/s
    => t = 4 secs
    =========
    (using v= 0.3t² + 4t)

    when a = 7m/s²
    t = 5 secs
    ======
    (using a = 0.6t + 4)

    dist travelled is,

    s(t=5) minus s(t=4)
    = 0.1(5)³ + 2(5)² + c - 0.1(4)³ - 2(4)² - c
    = 12.5 + 50 - 6.4 - 32
    = 24.1
    ====
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    Ic thank you
 
 
 
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