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Can I use physics to solve this?

  1. Sep 19, 2005 #1
    Ok, I just want to know if this is solvable by physics. If it can, it would also be helpful if you could supply an equation or something, but you don't have to. I just want to know if I can figure it out with physics. So here's the question:
    Suppose there is a person that is 80 ft away. I have a sling shot and one water balloon. If I am supposed to hit the person with a water ballon from 80 ft away, how far would I have to pull back the sling shot and at what angle would the sling shot have to be tilted at.
    Although it may sound silly, it is a serious question. I would really appreciate it if you could help me out. Thx.
     
  2. jcsd
  3. Sep 19, 2005 #2
    There are too many unknown variables here. The only constant given in your description is the distance.
     
  4. Sep 19, 2005 #3
    Well, lets say you know the variables. Is still it solvable?
     
  5. Sep 19, 2005 #4

    Pengwuino

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    Actually there are no variables I can think of that will make this impossible. What you would need to find however, and I'm sure its certainly possible, is the equivalent spring constant of the sling shot and of course, the mass of the water balloon.
     
  6. Sep 19, 2005 #5
    Thx a lot guys! I really appreciate your help.
     
  7. Sep 20, 2005 #6

    pervect

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    It rather depends on what you mean by "solve it". You would have to calibrate the slingshot, there are several possible conceptual ways you could do this, one of them would be to get a spring scale and measure the force it took to pull the sling back a distance of 'x'.

    You would also have to know the aerodynamic properties of the water baloon. This would probably be harder to model accurately, though you could certainly crudely approximate it as a sphere, and knowing the density of water you could figure out it's weight from it's volume (or vica-versa).

    It is an open question as to how accurate your predictions would be. The drag on the water baloon probably depends on its exact angle if it has a baloon shape. The angle of attack of the water baloon probably varies in flight, meaning in practice that no two will land in the exact same spot.
     
  8. Sep 20, 2005 #7

    Chronos

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    Like pervect and others said, you need to quantify the variables. It is obviously doable because certain missiles [e.g. cruise missiles] can be launched from relatively great distances and delivered with amazing accuracy. A good exercise would be to quantify the accuracy required to qualify as a 'hit', itemize all relevant variables, and quantify your technological capability to limit the margin of error for each variable. You then have a valid basis to deduce the statistical probability of a 'hit'. Theory is easy compared to application [an engineering thing].
     
  9. Sep 20, 2005 #8
    40 degree angle
     
  10. Sep 20, 2005 #9

    Pengwuino

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    But cruise missiles have radar-enabled computerized terrain following systems :P
     
  11. Sep 20, 2005 #10

    Chronos

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    Of course, it's just a matter of some inconsequential engineering details to make that happen :smile:
     
  12. Sep 20, 2005 #11
    But artillery can still hit a target up to 40kms away. They use computers to plot and aim the trajectory, but they can't control the round once its been fired.
     
  13. Sep 20, 2005 #12
    The easiest thing to do would be to pull the slingshot back to a certain point, and then launch the ballon and note how far it went and the angle at which it was launched.

    Using this information, it is a relatively simple physics problem to find the angle which would lead to striking your target 80ft away, if you pull the slongshot back to the same point.
     
  14. Sep 20, 2005 #13

    Pengwuino

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    For a longer range... attach rocket.
     
  15. Sep 20, 2005 #14
    Well, im starting to think this isnt going to be possible. The reason i want to know is because my physics teacher is letting us try and hit him from 80 ft away using a sling shot and a water ballon. And the thing is, i really want to hit him :devil: (he's really boring and gives really hard tests, so i mean he deserves it). The problem is, i only get two balloons to try with and the sling shot is supplied by my teacher. So i dont think i can do any of these tests. Is there any way just to assume a lot of it?
     
  16. Sep 20, 2005 #15

    Pengwuino

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    No way to assume. The slingshot can be as ... springy as an atv's shocks or as springy as a slinky. If you don't know how much energy can be stored up in that sling shot, no chance of doing any calculations.

    I advise you to 'feel' out the slingshot and on the first shot, try to fire it farther then you want it to go. That provides a better calibration then going short the first time in my opinion.
     
  17. Sep 20, 2005 #16
    are you planning on making a mechanical device to hit someones house...
    learn some projectile motion and then calculate the force and angle needed to make that projectil motion.
     
  18. Sep 21, 2005 #17

    Dhl

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    I guess this teacher has either done some calculations himself, or, more likely, he has just tried it and found it impossible to throw a balloon 80 ft. with his slingshot (I mean, it's unlikely he would allow to do that if he were not sure he wouldn't get hit...). Thus, the solution is to bring a water hose along, and then tell him you calculated that the task of making him wet cannot be solved with a water balloon and a slingshot :)
     
  19. Sep 21, 2005 #18
    Not a bad idea :biggrin:
     
  20. Sep 21, 2005 #19
    The trick is to use a very small water balloon, say about 2cm dia, about 3gm. Tie the balloon and then trim of the excess rubber, drag you know. Most readily available slingshots can reach 80 feet with this weight with an angle of 30deg. above horizontal when streached to the elastic limit ( which is where you feel a sudden increase in resistance). I tried it with my slingshot and a 3gm rubber ball (don't have a balloon) and it worked. The objective is to hit him not get him wet!
    Have fun!
     
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