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Building a wall to reduce maximum collision force

  1. Feb 1, 2017 #1
    1. The problem statement, all variables and given/known data
    I was recently assigned a project of, tl;dr, building a wall to reduce the maximum force during a collision. Let me start by describing the setup as best as I can. First, here's an image:
    pSiP5.jpg

    These aren't the exact track and carts we're using, but they provide a great overview. These, however, are the exact type of carts being used:
    4HKSl.jpg

    So, imagine if the ramp from the first image was inclined at 50-60° and then the a force sensor was placed on the lower edge, such that the cart would be released from a length of about 80 cm on the angled track away from the force sensor to crash into the force sensor. The carts being used, by the way, are 0.5 kg and there is very minimal friction between the cart and track.

    The project is essentially to build a wall in front of the force sensor - which can be up to 10cm in length and of any width/height as long as it does not touch the ramp and therefore use friction to slow the cart down. The goal is to release the cart in the aforementioned conditions and then to achieve the _lowest maximum force reading_ during the collision possible. The total impulse itself does not matter (which means, to my understanding, that elastic collisions would be no worse than inelastic collisions) as long as the lowest possible maximum force is achieved - my personal goal is a force under 8 newtons.

    There are a few caveats, and I'm going to try to explain all the rules in better detail here:

    1. the wall is not allowed to touch the track itself, to prevent the intentional use of friction to slow the car down
    2. I am allowed to attach things to the cart itself to a very limited extent; it has to be something that can be taken on and off easily and is not the main slowing mechanism - this cannot come into contact with the track either
    3. use of any non-mechanical energy is not allowed - nothing electronic or chemical

    2. Relevant equations

    The main equations relevant to this question are, as far as I can tell, the general momentum equations, such as J = FΔt, p=mv, and in addition the kinematic equations for further calculations. However, this question is mostly conceptual, so whereas equations can be used to check work and test ideas later on, at the moment, I am seeking ideas.

    3. The attempt at a solution

    So, here are some of the things I have already come up with:
    1. I am thinking that a wall made entirely of somewhat soft springs that go the full 10cm would be ideal, as it would create a collision that would last for the longest time, and therefore would have the lowest force
    2. I am also thinking of attaching a magnet to the rear end of the cart. I already discussed doing this to slow down the cart as the track is aluminum with someone else, but I was told to consider Eddy currents as a more important and stronger force - I have done some research into these, and while I can't say I really understand them, I do get the gist.
    3. I am also thinking of attaching a thin but huge in length and width piece of cardboard above the cart, to use the air resistance against the cardboard to slow the cart down. I am unsure of how effective this would be.

    I would appreciate any feedback any of you have to give, and I'm really sorry if this question is off-topic! Please let me know so I can copy it to somewhere where it might be more acceptable. Thanks again!
     
  2. jcsd
  3. Feb 1, 2017 #2

    haruspex

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    So what would the wall be attached to? What if that had a frictional connection to the wall?
    The trouble with springs is that they do not provide a constant force. The force increases as the spring compresses. That is why it is generally preferred to use a buffer that crumples (car body each end of the passenger section) or friction.
     
  4. Feb 1, 2017 #3
    There's a board on top of the force sensor, and the wall is attached to that.

    I get what you mean with springs. Could you expand a bit on using friction?
     
  5. Feb 1, 2017 #4

    haruspex

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    If the wall is mounted on rails parallel to the track, but with some grip, the retarding force will equal the kinetic frictional force, so remain more or less constant during the motion.
     
  6. Feb 2, 2017 #5

    CWatters

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    One possibility might be to build a wall containing a dash pot (aka damper). This could be a simple EPS "piston" in cardboard tube arrangement. If you provide a hole for air to escape then the size of the hole could be adjusted to optimise the system for minimum impact force.
     
  7. Feb 2, 2017 #6

    CWatters

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    If you have info about the track can you calculate the velocity at impact. Then if you were to assume the wall sets the maximum stopping distance you could estimate the likely minimum impact force to see if your target is feasible.
     
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