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Homework Help: Calculus involving accln and vel

  1. Feb 20, 2007 #1
    1. The problem statement, all variables and given/known data
    A point moves rectilinearly with a deceleration whose modulus depends on the velocity v of the particle as

    a=(alpha) X (square root of velocity v), ​

    where alpha is a positive constant. At time t=0, the particle has initial velocity v1.

    Then, what are the following:
    i. Time taken by the particle to come to rest
    ii. The distance travelled before it stops
    iii. The time at which the instantaneous velocity is (v1)/4
    iv. Distance travelled at the time the velocity becomes (v1)/4.

    3. The attempt at a solution
    I tried to integrate the equation but ended up no where because of the v term in the given equation, which kept getting in my way...PLEASE HELP!!
    I want to learn how to solve such problems which involve such variables in their equations.....
  2. jcsd
  3. Feb 20, 2007 #2


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    Writing x' and x" for dx/dt and d^2/dt^2 (Latex takes too long!)

    Your equation is x" = ax'^2

    The "trick" for solving this is to substitute y = (1/2)x'^2

    y' = x'x"
    x" = y'/x' = (dy/dt) / (dx/dt) = dy/dx

    So the equation becomes dy/dx = 2ay.
  4. Feb 20, 2007 #3


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    No, the equation is NOT x"= ax'^2: it is x"= -a sqrt(x').
    (negative a (or alpha) because you are told that the deceleration is given by alpha sqrt(v) where alpha is a positive number.)

    I would have done it just slightly differently. Your base equation is
    [tex]\frac{dv}{dt}= -\alpha \sqrt{v}= \alpha v^{\frac{1}{2}}[/tex]

    That's a "separable" equation- you can write it as
    [tex]v^{-\frac{1}{2}}dv= -\alpha dt[/itex]

    That should be easy to integrate. Once you have solved for v(t), of course
    [tex]v= \frac{dx}{dt}[/tex]
    so you have a second integration for x(t).
  5. Feb 20, 2007 #4
    Hmmm....i got the idea....
    but my problem now is how to apply the limits...on integrating, what i got was:

    [tex] 2 \sqrt{v} = \alpha t[/tex]

    <i hope the latex is right, i am using it for the first time!!

    now for the above equation, what will my limits be and where do i bring in the v1 term?????
  6. Feb 20, 2007 #5

    [tex] - \alpha \sqrt{v(t)}= \frac {dv}{dt} [/tex]

    [tex] - \alpha t = 2 \sqrt{v(t)} + 2\sqrt{v(0)} [/tex]

    [tex] - \alpha t = \{2 \sqrt{v(0)} [/tex]

    The above is because the velocity is 0 (in the first sub division of the question)

    and thus,
    [tex] t = \frac {2 \sqrt{v(0)}}{\alpha} [/tex]

    Now how do i proceed???
  7. Feb 21, 2007 #6


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    Yes, [itex] t = \frac {2 \sqrt{v(0)}}{\alpha} [/itex] is the time until the particle comes to rest (v= 0).
    Now, since you know that [itex] - \alpha t = 2 \sqrt{v(t)} + 2\sqrt{v(0)} [/itex] you can solve that equation for v(t): [itex]2\sqrt{v(t)}= -2\sqrt{v(0)}- \alpha t[/itex] so [itex]\frac{dx}{dt}= v= (-\sqrt{v(0)}-\frac{\alpha}{2}t)^2[/itex].
  8. Feb 21, 2007 #7
    is this true??

    [tex]\frac{d}{dt} \-v(0) = 0 [/tex]????

    is that right???
  9. Feb 21, 2007 #8
    okay...here is my working:

    [tex] 2 \sqrt{v} = - 2\sqrt{v(0)} - \frac {\alpha t}{2} [/tex]

    On squaring,

    [tex] v = v(0) + \frac {\alpha^2 t^2}{4} + \sqrt{v(0)}\alpha t [/tex]

    On writing v as [tex] \frac {dx}{dt} [/tex], and then integrating:

    [tex]x = 0 + \frac {\alpha^2 t^3}{12} + \frac { \sqrt{v(0)} \alpha t^2}{2} [/tex]

    and i am going to continue in a separate post becos i am new to laTex and i hope all that i have typed isnt a mess.....:rolleyes:
  10. Feb 21, 2007 #9
    Now the body comes to rest at

    [tex] t = \frac {2 \sqrt{v(0)}}{\alpha} [/tex]

    Thus, i substituted the above expression for 't' in the expression for 'x'.

    and i have gotten the following value for x, but it is not one of the options.

    i got:

    [tex] x = \frac {8 \sqrt{v(0)}^3}{3 \alpha} [/tex]

    Where am I going wrong?????????
  11. Feb 25, 2007 #10


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    You were NOT told that the initial velocity was 0! You were told
    You have determined that
    [tex]2\sqrt{v(t)}= -\alpha t+ C[/tex]
    Taking t= 0 [itex]2\sqrt{v1}= C[/itex]. That gives
    [tex]2\sqrt{v(t)}= 2\sqrt{v1}- \alpha t[/tex]
    squaring that equation,
    [tex]4 v(t)= 4 v1- 4\sqrt{v1}\alpha t+ \alpha^2 t^2[/itex]

    The particle will "come to rest" when that is equal to 0.

    Now you have
    [tex]v(t)= \frac{dx}{dt}= v1- \sqrt{v1}\alpha t+ \frac{\alpha^2}{4} t^2[/itex]

    That should be easy to integrate for x(t).
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