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Homework Help: Projectile Motion gravity and acceleration

  1. Sep 29, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown horizontally from the top of a building 54.5 m high. The ball strikes the ground at a point 61 m from the base of the building.

    The acceleration of gravity is 9.8 m/s^2 (9.8 meters per second squared)

    1. Find the time the ball is in motion. (in units of s)
    2. Find the initial velocity of the ball (in units of m/s)
    3. Find the x component of its velocity just before it strikes the ground. (in units of m/s)
    4. Find the y component of its velocity just before it strikes the ground.

    HELP! please!!!

    2. Relevant equations

    Delta X = VX T

    R = (Vsquared sin (2 x angle)) / g

    Delta Y = (1/2) (Vyi + Vyf) (t)

    Vyf = Vyi - (g)(t)

    Delta Y = (Vyi)(t) - (1/2)(g)(Tsquared)

    Vyfsquared = Vyisquared - (2)(g)(Delta Y)

    3. The attempt at a solution

    If someone could just tell me which formulas to use, that would be great. I know I need to solve for t for #1, and need to solve for Vyi for #2.
  2. jcsd
  3. Sep 29, 2010 #2
    First if you were to draw a visual of this problem you would get a triangle that would be easier for you to se ethat you need to find the value of the hypotenuse which is the velocity of the time in teh air before it gets 61m from the building. Then you can use that to sole for t in the equation.
  4. Sep 29, 2010 #3
    I did draw a diagram.

    So my hypotenuse is V? And then what formula should I use to solve for t?
  5. Sep 29, 2010 #4


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    Giving formulas is not very educational, and is basically doing the work for you. Here are some thinking points.

    #1. The ball is thrown horizontally, that is in the +x direction. Since it is thrown horizontally, what is the initial velocity in the vertical direction? What does that tell you about the time it takes the ball to reach the ground, given that the x and y velocities are independent of each other?

    #2. Since you know the time from #1, this is a very simple formula. You are given a distance (61 m from the base of the building) and you have calculated the time it takes to hit the ground. You should be able to calculate a velocity from that.

    That should get you started, at least.
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