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Math Help - Moving Particle Equation

  1. #1
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    Moving Particle Equation

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

    The correct answer is 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 u:
    2ut = 735 \ \ \ \ \ \ \ \ \ \ \ \ \dots (1)
    0 = 3ut - 21 gt^2\ \ \ \ \dots (2). Mr F adds: => 0 = ut - 7gt^2.

    The correct answer is 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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  3. #3
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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 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 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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  5. #5
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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 P is projected with velocity (2u \b{i} + 3u \b{j}) \ \text{ms}^{-1} for a point O on a horizontal plane, where \b{i} and \b{j} are horizontal and vertical unit vectors respectively. The particle P strikes the plane at a point A which is 735m from O.

    Show that 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 u:
    2ut = 735 \ \ \ \ \ \ \ \ \ \ \ \ \dots (1)
    0 = 3ut - 21 gt^2\ \ \ \ \dots (2).

    The correct answer is 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

    0 = 3ut - \frac12 gt^2

    And I solved it to get u=24.49
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