# Building a wall to reduce maximum collision force

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:

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:

So, imagine if the ramp from the first image was inclined at 60-70° 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

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!

– JMac
Commented Feb 1, 2017 at 11:35
• @JMac Why would that count as non-mechanical energy? Commented Feb 2, 2017 at 0:12
• I guess I had a brain fart when I wrote that. I meant your second rule.
– JMac
Commented Feb 2, 2017 at 1:21
• Well, I doubt that it would be the main slowing mechanism that air resistance can't be that much. I can definitely build it such that it can be taken on and off the cart easily, as well. Commented Feb 2, 2017 at 2:26
• The drag would be either substantial or a waste to include that.
– JMac
Commented Feb 2, 2017 at 2:44