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Tennis Ball Launcher Project Calculations

  1. Nov 28, 2004 #1
    I am building a tennis ball launcher for my physics class, and it needs to be no bigger than 1m X 1m, it must reach to a target that is 2 - 7m away from the launch site, and the target may or may not be at a certain height. No fire, explosives, gasses may be used. It must run on pure mechanical physics As of now, my model goes as such:
    3 wood panels making 45-45-90 triangle.
    A plastic tube with inside the tennis ball resting on a small 'cup' which it can rest it, but can not be stuck in. The 'cup' is attached to the top edge of the tube with rubber bands, and the bottom of the cup has a rope attached, which runs out of the tube. To launch the ball, i pull on the rope, which tightens the rubber bands, and letting go, launches the ball.

    Building the model is not the hard part (although my dad has taken it upon himself and it is now a huge, over the top thing which i am not happy about), my problem is when i come to class, and i am given the distance and the height, how can i calculate how far to pull back my rope? as of now, i have determined that i can make marks on the wood on how far back to the pull the rope, which can tell me how far it will launch.
    Should i test it out, and see what velocity it flys with how far back i am pulling it? does this involve force? can i measure the force with which i am pulling the tennis ball back?

    we have learned about projectile motion and newtons laws, so i am guessing we must use some of that in our calculations.

    Thanks to anyone who helps! :shy:
  2. jcsd
  3. Nov 28, 2004 #2
    So let me get this straight, you basically built a cannon, but with rubber bands instead of explosives? ...One suggestion i might make is a spray paint can lid, the tennis ball rests perfectly in the cap and has a good launch with an easy release. if i were to suggest a rubberband type, some sort of clothing elastic that is self limiting would be the best. But you would have to probably use four strands per side to get the force pushing the tennis ball you need. If you do that, it allows you to know an exact stretch length of your rubber band, so you can know how much rubber band you want to pull back ( so if you have a shorter distance, you shorten the amount of band you pull back with some kind of device that allows you to slide the band forward and backward easily). For calculations i would do the following, if you have a program on your computer (which you should, its standard on all pc's) called Microsoft Excel , you can use the program to plug in numbers and create graphs. So for example you do some test launches, and get that if you pull back 1 yd of band, the ball launches 3.4 meters into a height of 1 ft. , and when you pull back 2 yards of band the ball launches 9.8 meters into a height of 1 ft. on the graph, you plot two cordinates, the distance of rope used, and the distance of the ball. I would suggest that you do 10 launches for each possible height. This way, on you launch day, your teacher says height 2 feet, distance 7 meters, you look on your 2 ft graph and find the distance of band for 7 meters. you measure that out, and pull back and launch. If that made any sense, it is a really good way of using your information, one which i have used in Science Olympiad competitions for years, and also in my personal physics classes. I hope that helped. Have fun with your project :smile:
  4. Nov 28, 2004 #3
    I'm a little confused on how you suggest in measuring the height ("into a height of 1ft??"), and how you suggest we have this built. we can't measure how much of band is being stretched out, because it is inside the tube, but we are measuring with a rope that is attached to what we could use as the 'spray can lid' (good idea, thanks), that is going through the tube, and comes out the other end. we were thinking having knots, that like you said, a certain knot is a certain distance. When we test it, how would we measure heights that it passes? are there any projectile motion calculations, or newton law calcualtions (with force and acceleration, the force or tension that the bands are being pulled back with) that could be used?
    Thank you so much for your help and great ideas! sorry if my explanations are murky...
  5. Nov 29, 2004 #4
    I think you should calibrate your cannon by empirical (trial and error) means so you know what horizontal distance it will fire for a given amount of string pull. You need to pull the string back to a mark, fire the cannon, measure the distance shot, and repeat this a few times and take the average. Then repeat with a different mark, and so on.

    You could have a table of values for the results, or better, actually mark a scale on the device where you have to pull a knot in the string to, for given distances. The scale needs to have the numbers 2, 3, 4, 5 etc., showing where the string has to be pulled to to fire that many metres.

    For the vertical part, I think you are expected to use some math. So if you are asked to fire 5 metres horizontally, to a point 2 metres high, you would do your math and say, "That needs the same launch that would fire 6.5 metres horizontally (or whatever)", and then pull back the string to the 6.5 mark.

    Let me know if you want some hints on the formulae you need to turn height and distance into an equivalent horizontal distance.

    Edit to add: Having read hawKMX2004's post now, I see that he said much the same, though I think he said it better.
    Last edited: Nov 29, 2004
  6. Nov 29, 2004 #5
    :smile: Yes, and what i meant by height, is try launching the ball into a container 1 ft high and fairly small around, like a coffee can or something. As for the band length, make the band adjustable at the end so that depending on the distance and height, you can change how much of the band you want to pull back. So for example ( these values arent correct) lets say you know if you pull back 1 meter of the band ( all the way untill it stops due to the self limitation of clothing elastic ) and release it, the tennis ball fires 6.3 meters and at a height of 1 ft. You plot the points, after trial and error and have something like this (once again incorrect values)

    Table for 1 ft Height

    Length of Band Distance Launched
    2 ft 3 meters
    2.5 ft 3.76 meters
    3 ft 4 meters
    etc.. etc..

    then, on graphs, you can plot the points, using the length of band as the x-axis, and the distance launched as the y-axis. This way, if your teacher says 6.3 meters, with a height of 2 ft. you look on your 2ft height graph, and find the point where y is equal to 6.3 , then take the x-value, measure out that much of your band, tie it tight, and launch. Your almost guaranteed to make it. I have one other suggestion for you. Before each launch, no matter what elastic you use, stretch it ALOT and i mean ALOT like 50 or 60 full stretches before your tests and launches...If you do this, your band is a consistant temperature, and is less likely to vary. ( If you launch the elastic cold, your ball wont go as far as if it was warmed up, and so if you launch warm on your trials, and then launch cold on your test, you wont do so well.) Just something to keep in mind. If you would like more suggestions, or are still confused, let me know, and I'd be glad to help, I'm also in the process of designing a trebuchet for a competition in the spring. :smile: Good luck with your project :smile:

    P.S. By The Way for the best results on your project, I suggest doing multiple launches with every band length, and multiple height's. When I made my physics catapult, I did 20 launches per height, and did 15 heights (giving me 15 graphs worth of data). I only missed one shot ( out of 10 random height and lengths) and it was due to a miss fire. If you want other ideas on catapults I'd be glad to give you a few of my best
    Last edited: Nov 29, 2004
  7. Dec 5, 2004 #6
    Bissli. Do you attend DMHS and have Mr. Johnson. I do. If so what period. I'm still looking for designs. If anyone has any less complicated designs i would like to hear them. I feel that if I over complicate the catapult it will be too difficult to build. My partner and I are not extremely handy but we can build somehting that is fairly simple. Please help me. Thank you.
  8. Dec 6, 2004 #7


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  9. Feb 25, 2010 #8
    hey, i am building a similar device, however after doing many trials, I sometimes get an outlier as a result. I was wondering if anyone had any ideas for a more consistent elastic.

    I am already using medical tubing elastic, however the temperature seems to be effecting its consistency..

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