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Homework Help: Projectile Motion: Range proof

  1. Nov 1, 2005 #1
    Q. A projectile is fired with an initial speed [tex]V_0[/tex] at an angle [tex]\beta[/tex] to the horitontal. Show that it's range alnog a plane which it's self is inclined at an angle [tex]\alpha[/tex] to the horitontal [tex]( \beta > \alpha)[/tex] is given by:
    R = \frac{(2{V_0}^2 cos \beta sin(\beta - \alpha )}{g {cos}^2\alpha}

    A.So I`ve started off with
    [tex]\triangle x = (V_0 cos \beta) t[/tex]
    [tex]\triangle y = (V_0 sin \beta) t - frac{1}{2} g t^2[/tex]

    [tex]\triangle x = cos \alpha[/tex]and [tex]\triangle y = sin \alpha[/tex]
    so i rearranged [tex]\triangle x[/tex] to get [tex]t = \frac{cos \alpha}{V_0 cos \beta}[/tex] and sub it into [tex]\triangle y[/tex]
    now i have
    \triangle y = (V_0 sin \beta)\frac{cos \alpha}{V_0 cos \beta} - \frac{1}{2} g ( \frac{cos \alpha}{V_0 cos \beta} )[/tex]

    \triangle y= \frac {V_0 sin \beta cos \alpha}{V_0 cos \beta} - \frac{1}{2} g ( \frac{cos \alpha}{V_0 cos \beta})[/tex]
    sin \alpha = tan \beta cos \alpha - \frac {g {cos}^2 \alpha}{2 {V_0}^2 {cos}^2 \beta}
    now i`m stuck ... Any hint`s/tips would be great.
    Last edited: Nov 1, 2005
  2. jcsd
  3. Nov 1, 2005 #2


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    Staff Emeritus
    Science Advisor

    Somewhere there seems to be an 'R' missing.

    Also, one of the latex expression needs \ in front of frac.
  4. Nov 1, 2005 #3


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    Gold Member

    delta x = cos(alpha) ??
  5. Nov 1, 2005 #4
    alpha is the angle of the inclinded plane. it forms a right triangle with the horizontal and the point at which the projectile meets the inclinded plane. therefore delta x is equal to cos alpha.
  6. Nov 1, 2005 #5


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    Homework Helper

    This is where the R is missing. They should be,

    [tex]\triangle x = Rcos \alpha[/tex]and [tex]\triangle y = Rsin \alpha[/tex]
  7. Nov 1, 2005 #6
    ok so i`ve put the 'R' s in and get

    R Sin \alpha = R tan \beta cos \alpha - \frac {R g {cos}^2 \alpha}{2 {V_0}^2 {cos}^2 \beta}

    but i don`t see how this helps?
  8. Nov 2, 2005 #7


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    Homework Helper

    You're almost there. But the R in the last term should be R²
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