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Homework Help: Denote the initial speed of a cannon ball fired from a battleship

  1. Sep 17, 2005 #1
    denote the initial speed of a cannon ball fired from a battleship as Vo. when the initial projectile angle is 45 degrees with respect to the horizontal, it gives a maximum range of R.
    the time of flight of the cannonball for this maximum range R is given by
    1.t=(1/3^1/2)vo/g
    2.t=3^1/2(vo/g)
    3.t=2^1/2(vo/g)
    4.t=2(vo/g)
    5.t=1/2(vo/g)
    6.1/(2^1/2)(vo/g)
    7.t=4(vo/g)
    8.t=(1/4)(vo/g)
    9.t=(2/3)(vo/g)
     
  2. jcsd
  3. Sep 18, 2005 #2

    VietDao29

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    So, what have you done? Which option to you is correct?
    Because you just need to find the time of flight, you don't need the x-component of your initial velocity, you just need the y-component of the initial velocity. So first, you can try to find out the y-component of the velocity.
    Then, note that the object has the acceleration of g (downard). Can you find out the time in flight of the object?
    Viet Dao,
     
  4. Sep 18, 2005 #3
    thanks

    thank you viet dao, i figured it out.
     
  5. Sep 18, 2005 #4
    ?

    but now how do you find max height?
    do u use y=voyt + 1/2gt^2?
     
  6. Sep 19, 2005 #5

    VietDao29

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    The y-component of the velocity makes the object go up or down.
    Because the object has the acceleration of g (downward), so the y-component of the initial velocity decreases, and the object moves upwards slowlier and slowlier, finally when the y-component of the velocity is 0 (m / s). The object's at its max height, because, right after that, it will start accelerate downwards (ie, it no longer moves upwards).
    You can use:
    vf2 = vi2 + 2ad
    Here a = -g (if you choose the positive direction upward).
    vi is the object's initial velocity.
    vf is the object's final velocity.
    Here, you just need the y-component, so the vi is the y-component of the initial velocity.
    vf = 0 m / s. It's when the object's velocity has no more y-component.
    You can use that and solve for d, which's the object's max height.
    Viet Dao,
     
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