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Homework Help: Water ballon launcher

  1. Nov 29, 2008 #1
    It is a Project:

    I have constructed a waterballon launcher with a group. It looks similar to this, http://www.engr.trinity.edu/images/balloonlaunchers/3balloonlaunch2002.jpg
    We are to launch a waterballon an unknown distance between 10-50 meters. We need to determine horizontal range the waterballon will travel. Through measurements we can dermine the angle of launch, time, and also we can measure how much our surgical tubing has been stretched.

    2. Relevant equations: Maybe hook's law 1/2(k)(x^2)
    We do not know how to find spring constant k

    3. The attempt at a solution
  2. jcsd
  3. Nov 29, 2008 #2
    The only way I can think of, is to measure the k value is by a physics experiment

    You need a device to measure the force in newton, such as a spring scale, and a meter stick, or anything that will measure the horizontal distance. Since you know F=-k(x^2), by plotting the square root of F against the distance, x can get you a linear graph.

    Get your spring scale, and take a measurement of the force required to strectch your spring some distance, which you should measure with respect to some point, and repeat the procedure until you have gathered enough data.

    Then, plot F^(1/2) against the x on a graph, and find your slope, and that's your k value.

    (Plotting F against x^2 will also work in this case.)
  4. Nov 29, 2008 #3


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    You aren't going to be able to say much about this analytically - the air resistance of a water balloon cant really be ignored.
    You are going to have to do some experiments. The idea of an experiment is to only vary one thing at a time, so you want to make sure that all your water balloons are the same size (contain the same amount of water), then try the same angle and different amounts of spring stretch, then try the same amount of spring stretch and different angles.
    Assuming things like wind stay the same you should be able to plot what stretch and what angle you need for each distance.
  5. Nov 29, 2008 #4
    Fun times.
    I remember doing this in CEGEP :)

    What you can do is just remember that there are no horizontal acceleration, only vertical. So you can find the initial horizontal V with distance / time right?
    And since you can measure your angle, you can find your actual initial velocity.

    The rest is just conservation of energy.

    So you can find k.
  6. Nov 29, 2008 #5


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    For a crude calculation you can hang known weights and measure displacements. That's all k is anyway. How many Newtons to displace it a known distance.

    With water balloons a lower k is preferable, because, depending on the balloon, a rapid acceleration and the weight of the water may cause the balloon to break at launch. Double stuffing a balloon in a balloon can often resolve the problem, but that makes for a harder impact at the target with the potential for injury or damage. (I've seen a double stuffed go through a window, not just crack it, for instance.)

    Hence a lower k and a longer pull and distance to accelerate the balloon to Initial speed is preferable.
  7. Dec 2, 2008 #6
    Thanks very helpful.

    We kept popping our balloon at launch. We had fast fast acceleration over short distance. Now we have loosened the tension and pulled back further the water balloon.
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