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Homework Help: Projectile Motion

  1. Jul 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A canon ball is fired from a canon upwards at an angle above the horizontal.

    a. Sketch a graph showing the height of this canon ball over time if it was fired at 30°, 45° and 60° above the horizontal.
    b. Sketch showing the acceleration of the canon ball over time if it was fired at 30°, 45° and 60° above the horizontal.
    c. At what point in the flight of the canon ball is the velocity at a minimum?

    2. Relevant equations
    h*sin(angle) = hy
    h*cos(angle) = hx

    3. The attempt at a solution

    I have sketched 3 triangles but how do I work out the components with just an angle? Or is there another way of doing this?
  2. jcsd
  3. Jul 16, 2010 #2


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    - projectile motion isn't triangles
    - once launched, whats the only force on the projectile? therefore the acceleration is?
    - at what point in time will the vertical velocity be zero?
  4. Jul 16, 2010 #3
    - I'm under the impression that a canon ball being fired is projectile

    - the acceleration in the y direction is -9.8, but what use is that when I don't know the initial velocity or how high it goes?

    - I don't know when it will be zero because I don't have any other information.

    If I'm doing it wrong please just say so.
  5. Jul 16, 2010 #4


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    -Therefore your acc'n -time graph will be a straight line.
    -A greater angle (from 0 - 90) represents a greater time in the air. (as well as greater height achieved) (why?)
    -therefore, you can use relative times.

    -you can assume each projectile has the same initial velocity, Vo because it was fired from the same cannon
    -vertical velocity is zero at 1/2t
  6. Jul 16, 2010 #5
    thank you so much :)
  7. Jul 16, 2010 #6


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    another hint:

    If you fire a cannon ball with the following velocities for ex:
    Vy = 10, Vx = 10
    Vy = 10, Vx = 100
    Vy = 10, Vx = 10000
    ALL will fall to the ground at the SAME time.

    You're Vy, however, is changing because the cannon fires at the same initial velocity. You have to break that initial velocity into its components. You can safely ignore the x-component. A greater angle = A greater Vy component = A greater time in the air.

    hope that helps
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