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Math Help - 1st & 2nd Derivative Physics word problem

  1. #1
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    1st & 2nd Derivative Physics word problem

    I'm stuck on this problem. Can someone help me with where to start?

    A body moves in such a way that the space described in the time t is given by s=t^n, where n is a constant. Find the value of n when the velocity is doubled from the 5th to the 10th second; find it also when the velocity is numerically equal to the acceleration at the end of the 10th second.
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  2. #2
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    Hello, dbakeg00!

    You're expected to know that: . \begin{array}{ccc}v &=& \dfrac{ds}{dt} \\ \\[-3mm] a &=& \dfrac{dv}{dt} \end{array}


    A body moves in such a way that its position at time t is given by: . s\:=\:t^n, where n is a constant.

    (a) Find n when the velocity is doubled from the 5th to the 10th second.
    We have: . v(t) \:=\:n\,t^{n-1}

    Then: . \begin{Bmatrix}v(5) &=& n\,5^{n-1} \\ v(10) &=& n\,10^{n-1} \end{Bmatrix}


    v(10) is twice v(5)\!:\;\;n\,10^{n-1} \:=\:2\cdot n\,5^{n-1} \quad\Rightarrow\quad \frac{10^{n-1}}{5^{n-1}} \:=\:2 \quad\Rightarrow\quad 2^{n-1} \:=\:2^1

    Therefore: . n-1 \:=\:1 \quad\Rightarrow\quad n \:=\:2




    (b) Find n when the velocity is numerically equal to the acceleration at the end of the 10th second.
    We have: . \begin{Bmatrix}v(t) &=& n\,t^{n-1} \\ a(t) &=& n(n-1)t^{n-2} \end{Bmatrix}

    Then: . \begin{Bmatrix} v(10 &=& n\,10^{n-1} \\ a(10) &=& n(n-1)\,10^{n-2} \end{Bmatrix}


    These two quantities are equal: . n(n-1)\,10^{n-2}\;=\;n\,10^{n-1}


    Divide by n\,10^{n-2}\!:\;\;n-1 \:=\:10


    . . . . . Therefore: . n \:=\: 11

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  3. #3
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    Thanks for the quick reply and for your answer, it helped a lot. One more thing...


    These two quantities are equal: . n(n-1)\,10^{n-2}\;=\;n\,10^{n-1}


    Divide by n\,10^{n-2}\!:\;\;n-1 \:=\:10


    . . . . . Therefore: . n \:=\: 11
    So when you divide n\,10^{n-1} by n\,10^{n-2}, how do you come up with 10 ?(sorry, I'm a little rusty on division by exponents..I know you should subtract them, but I am having a difficult time seeing this one)
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  4. #4
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    Nevermind...I'm an idiot! I see it now. Thanks again for the help.
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