Shooting a slingshot with the slingshot

Discussion in 'New project ideas' started by JoergS, Nov 8, 2013.

  1. JoergS

    JoergS Administrator

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    This is one of the most common "challenges", or, better, requests my fans come up with.

    The idea is to cascade two or more slingshots so that the speed is accumulated.

    While I am waiting for the parts of the "barreled" slingshots to come in (the huge PVC tube already showed up), I am also thinking about tackling this challenge.

    My current idea is to set up a rail system based in two round rods, 2 meters long, that guide a 1 meter long lightweight slingshot crossbow. The crossbow is shot and glides along the rails, then, with an automatic trigger (adjustable timing), the crossbow is fired while in full motion.

    The trigger position has to be tuned just right, so the ball would leave the pouch just at the end of the "ride".

    The crossbow would be stopped by strong rubber, spear gun stuff.

    Ideally the speed of the crossbow (30 m/s?) and ball (50 m/s?) will add up to a final speed of 80 m/s.

    This could be a small science lesson more than anything else, plus of course one more "first timer".

    What do you think?
     
  2. Jeremy

    Jeremy Senior member who totally rocks a pink Scout! Pink

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    I can not wait to see how it all works.
     

  3. Will

    Will Thread Hijacker

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    I'm looking forward to seeing this plan come to life!
     
  4. JohnKrakatoa

    JohnKrakatoa Loudest boom on Earth

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    Wouldn't this cancel the transfer of the acceleration from the crossbow onto the ball? My common sense tells me you have to fire the ball before the crossbow is stopped in order to add the momentum.

    P.S.: just thougt about something... taking two sets of bands and attaching them in sequence (he first is tied to forks ends tied into a division of rope or something non elongating which then connects to the other bands which are then connected to te pouch. Wouldnt this increase speed? Or would it just act like a normal bandset of the same length as are the two sets combined? Man.... we need a physics professor ... :D
     
  5. JoergS

    JoergS Administrator

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    You have to accelerate it before the crossbow is stopped. But once the acceleration process is complete (when the ball has left the pouch), then the ball does not "care" what happens to the pouch and the rest of the weapon. Once it is on its way, everything else is history (from it's point of view).

    Why should this increase the speed? The rope would just be a passive piece that connects the two rubber bands.
     
  6. Withak

    Withak aka Whitehawk!

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    Okay, this sounds like an interesting challenge. The theory seems good. I'm looking forward to your creation on this one.
     
  7. Cadman

    Cadman New Member

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    A quick google search provided this explanation:
    If you go to the front of a train that is moving at 1,000 mph and shoot the gun forward, the bullet will move away from you and the train at 1,000 mph, just as it would if the train were stopped. But, relative to the ground, the bullet will travel at 2,000 mph, the speed of the bullet plus the speed of the train. So if the bullet hits something on the ground, it will hit it going 2,000 mph.

    I think this supports Joerg's theory, and I can't wait for the video.
    Regards,
    Brian
     
  8. JohnKrakatoa

    JohnKrakatoa Loudest boom on Earth

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    lmao I don't know.... :D just a thought...:p:eek:
     
  9. JoergS

    JoergS Administrator

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    Have already made the rail system! Now comes the crossbow.
     

    Attached Files:

  10. rock slinger

    rock slinger I rarely shoot rocks!

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    This is going to be a cool one.
     
  11. dolomite

    dolomite Banned

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    Is it a slingshot slingshot?, maybe sling sling shot shot. I've got it! Slingshot squared. Dig the concept, can't wait to see it.
     
  12. AttilaTheFun

    AttilaTheFun New Member

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    Hey there!

    Interesting idea, I can't wait to see it. I have only concern about the speed addition.
    I was always terrible at this stuff but considering Newton's third law of motion i think as the trigger of the moving crossbow is triggered and the rubber starts to speed up the ammo, the same force will slow down the moving crossbow as the rubber would like to move not just the ammo but the crossbow as well to the center of the rubber(Usually this force is stopped due to you holding the crossbow so all the force is able to move the ammo. Just think of recoil). I'm not stating it will stop the moving crossbow then let it move again once the ammo is fired but it will slow it down. So the addition isn't just a X+Y it has a -Z as well. My head starts to hurt if i try to imagine the forces in work but I suppose the rubber moving the crossbow should be very strong and you would need a heavy crossbow compared to the ammo you shoot. Of course the question comes whats the perfect reatio between the weight of the ammo, the weight of the corssbow and the force of the rubber moving the crossbow... but that is simply too much for me:D

    If my thought is wrong, please correct me, I always sucked at physics:D
     
  13. JohnKrakatoa

    JohnKrakatoa Loudest boom on Earth

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    nono I think you are correct... to expand on my silly idea about bands divided in half wih a non elongating object(metal..etc.)... I thought of it exactly like of a crossbow on a rail system... the object dividing the bands acting as the crossbow in Jörgs design... but as you say the ratios would have to be super tight for this to work. if it even can work on that small of a scale. But Jörgs is bulduing it in much bigger scale so I think he can pull it of.

    Regarding he weight of the xbow Q you posed... I would say it doesnt matter... what matters is the friction of the xbow on he rail system. I think.
     
    Last edited: Nov 9, 2013
  14. JoergS

    JoergS Administrator

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    You must think in terms of a system. A train in motion is such a system. That's why the gun shot from Brian's example is flying at 1,000 fps within the system, but at 2,000 fps when viewed from outside of the system.

    There is no "Z" factor.
     
  15. AttilaTheFun

    AttilaTheFun New Member

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    Well... to be honest I'm not exactly sure, the train is perfect to simulate this example. We are talking about forces and masses much closer to each other than in the train's example. I mean: the gun's recoil is NOTHING compared to the train's inertia and the force that makes the train moving so the gun doesn't affect the system it's in.

    But what if I had a gun that has recoil so big, it would move an unmoving train a bit? Would that affect the moving train by slowing it since it's working against the train's inertia and the force that makes the train moving? I think it would.
    By this i mean: the force of an object that is in a system WILL affect the system in it. Of course a simple gun wouldn't affect a moving train enough to matter, but this case we are talking about a pair of rubber bands pulling a crossbow that has rubber bands shooting a ball/arrow/bolt and has a recoil that MIGHT needed to be considered compared to the rubber's force that makes the crossbow move.

    I think there is a Z factor, I'm just not sure if its big enough to be considered.



    Lets play with our imagination, and think in a theoretical way :D
    Just imagine this
    First of all if i would happened to have a rubber band elongated and I released both ends at the exact same time, the band's both end would go to its mass center (In case there is no tapering, that would make it a bit more complex) of course, we all know that I just wrote it down to be easier to imagine the things I'll write.

    If we are in space and I have a crossbow that is VERY light, lets say it is 10g and I have a bullet in it armed that is 10Kg what would happen if I fired the crossbow without holding it? Would the ball move? Not really, but the crossbow would move towards the ball thanks to its mass which is 1000 times smaller.

    Now lets get back to Earth. I hold the mentioned 10g crossbow and I fire it, What would happen? The ball would move since I'm holding the crossbow and my Mass is much bigger than the Ball's + I have friction related to the ground and everything. In this case I am a system in which, there are a crossbow and a ball and if i would fire this from the train the forces would add up, yes. BUT if I would happen to be 5Kg and I stood on a very slippery surface or on a skateboard or something like that, when i fire the crossbow, what would happen? Would I be able to stand in place and the ball would fire at it's maximum force or I would move thanks to the recoil? I think the latter. Now, if I'm still 5Kg my crossbow is 10g and the ball is 10Kg and we are on a train that is a mere 10Kg yet moving, what would happen if I fire the Crossbow? The rubber would want to move the crossbow since the Ball is much harder, I would have problem holding the crossbow but I could since I have friction related to the train so the "recoil" (the rubber forcing the crossbow instead of the ball) would affect the train making it maybe slower enough to be recognized.
    Summarizing. If the crossbow is light and the ball is very heavy, the force of the crossbow's rubber would have a huge impact on the crossbow not just the ball. Now, if the crossbow's rubber has a huge impact on the crossbow that might had effect on the rubbers that would make the crossbow itself move.

    SO: If the crossbow's mass is much bigger than the Ammo's then this force is really not important, but if it's the other way round, I think it could affect the performance.

    I'm VERY sorry if I seem to taunt or something like that, I don't want. I have a huge respect towards you for 99% of the things you done (1% is that small "Untitled" prank:D) I just try to understand this as well and this is the best way to do it.
     
    Last edited: Nov 9, 2013
  16. Achso_42

    Achso_42 Senior Member

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    This only works as long as you can use inertial systems.
     
  17. JoergS

    JoergS Administrator

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    Finished the contraption... it shoots!

    It is now dark and I can't test it, that will happen tomorrow. But here are some pictures!

    The sled weighs 500 gramms plus the rubber and the steel ball... sled travels 107 cm, active draw of the slingshot is 87 cm.

    As you can see the notch that pushes the ball up and releases the shot can be attached further back or front - that will be optimized with the help of the hispeed cam tomorrow.

    The whole thing is fired with one simple pull on the trigger lever! The shock at the end is remarkable, but everything holds.
     

    Attached Files:

  18. AttilaTheFun

    AttilaTheFun New Member

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    Now THAT was fast:D I can't wait to see that beast shooting! While I was sweating by the keyboard to be able to translate what I'm trying to say into English you built this... :D Judging by the sizes and the rubbers and weights even if what i say is technically right it wouldn't slow it down a bit (like the case of train:D)!
    Please update a video soon! :)