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Homework Help: Projectile motion: find distance a ball would land

  1. May 2, 2016 #1
    1. The problem statement, all variables and given/known data
    I'm trying to come up with an equation to determine where a ball would land (basically the distance it moves) from a spring loaded projectile launcher set up on a table. I'm looking for "d", and I know the spring constant, compression, mass of the ball, height the ball starts at, angle the launcher is set at, and whatever else I can measure using a meter stick, balance, and protractor. There aren't any numbers just known variables

    2. Relevant equations
    Conservation of Energy eqn (at least the version I learned in class): Efinal-Eintial=Einput-Eoutput
    KE: 1/2mv2
    Spring: 1/2kx2
    3. The attempt at a solution
    I attempted to use conservation of energy but I get stuck trying to figure out where d goes in to be able to solve for it. Also, other online resources use a conservation of energy eqn that has different terms than what I was taught, but I'm assuming they are all the same.
    System: Ball and Earth
    Initial time: just after ball leaves launcher
    final time: just before ball hit ground
    Einitial= 1/2mvo2+1/2kx2+mgH
    And then I'm stuck trying to figure out how the distance goes into this. I'm wondering whether I need to integrate the velocity with respect to time and relate that to the distance since the distance the ball travels is the velocity*time. Any help is appreciated!!
  2. jcsd
  3. May 2, 2016 #2


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    Hi phys, :welcome:

    Not bad for a first post. Yes, you need some projectile motion equation to complete this. From initial Einitial= 1/2mvo2+1/2kx2+mgH (where vo = 0 ?) you get v0. And your projectile trajectory is uniform motion horizontally (needing t) and uniformly accelerated vertically (which gives you a quadratic equation for t) .
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