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Thread: Moving Particle Equation

  1. #1
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    Moving Particle Equation

    Q: With the following two equations, find the value of $\displaystyle u$:
    $\displaystyle 2ut = 735 \ \ \ \ \ \ \ \ \ \ \ \ \dots (1)$
    $\displaystyle 0 = 3ut - 21 gt^2\ \ \ \ \dots (2)$.

    The correct answer is $\displaystyle u=24.5$. I'm getting an incorrect answer. Can someone do this question and see if they obtain the correct answer so I can compare with mine. Thanks in advance.
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  2. #2
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    Quote Originally Posted by Air View Post
    Q: With the following two equations, find the value of $\displaystyle u$:
    $\displaystyle 2ut = 735 \ \ \ \ \ \ \ \ \ \ \ \ \dots (1)$
    $\displaystyle 0 = 3ut - 21 gt^2\ \ \ \ \dots (2)$. Mr F adds: => 0 = ut - 7gt^2.

    The correct answer is $\displaystyle u=24.5$. I'm getting an incorrect answer. Can someone do this question and see if they obtain the correct answer so I can compare with mine. Thanks in advance.
    Substitute ut from (1) into (2), take g = 9.8 and solve for t (keeping only the positive value): t = 2.31 seconds.

    Substitute t into (1) and solve for u: u = 159.1 metres.

    Then again, I'm terrible with arithmetic ....
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    Quote Originally Posted by mr fantastic View Post
    Substitute ut from (1) into (2), take g = 9.8 and solve for t (keeping only the positive value): t = 2.31 seconds.

    Substitute t into (1) and solve for u: u = 159.1 metres.

    Then again, I'm terrible with arithmetic ....
    But that contradicts the marks scheme answer as $\displaystyle u=24.5$.
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  4. #4
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    Quote Originally Posted by Air View Post
    But that contradicts the marks scheme answer as $\displaystyle u=24.5$.
    Could you completely specify the problem (as in originally how it was stated)?
    I think that will help
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    Quote Originally Posted by Isomorphism View Post
    Could you completely specify the problem (as in originally how it was stated)?
    I think that will help
    Q:
    A particle $\displaystyle P$ is projected with velocity $\displaystyle (2u \b{i} + 3u \b{j}) \ \text{ms}^{-1}$ for a point $\displaystyle O$ on a horizontal plane, where $\displaystyle \b{i}$ and $\displaystyle \b{j}$ are horizontal and vertical unit vectors respectively. The particle $\displaystyle P$ strikes the plane at a point $\displaystyle A$ which is $\displaystyle 735m$ from $\displaystyle O$.

    Show that $\displaystyle u=24.5$.
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  6. #6
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    Quote Originally Posted by Air View Post
    Q: With the following two equations, find the value of $\displaystyle u$:
    $\displaystyle 2ut = 735 \ \ \ \ \ \ \ \ \ \ \ \ \dots (1)$
    $\displaystyle 0 = 3ut - 21 gt^2\ \ \ \ \dots (2)$.

    The correct answer is $\displaystyle u=24.5$. I'm getting an incorrect answer. Can someone do this question and see if they obtain the correct answer so I can compare with mine. Thanks in advance.
    Well why did you write the second equation like that?

    The correct equation is

    $\displaystyle 0 = 3ut - \frac12 gt^2$

    And I solved it to get $\displaystyle u=24.49$
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