While in physics today we were coming up with stupid ideas that could barely be related to physics. and someone brought up the idea of a door, and that sprung an idea. Some doors like screen doors or doors in older public places is springs to make sure the door closes. Using physics I am able to calculate the amount of force I need to apply to open the door and the amount of force the door will be closing, if I am given the spring constant. however if I know how much force I apply to open the door, stretching the spring a specific distance I can calculate the spring constant. The higher the spring constant the more force the door will close with. So lets say in a hypothetical world a door with a springed latch is opened and the spring has a spring constant of 200. if someone were to get by that door when it flies back would be in a world of hurt. And that's how the cookie crumbles.