What makes us laugh here? While one may not readily intuit it, its the person's very rapid deceleration, which causes an almost instant decrease of velocity to zero. Take our simplest kinematic equation: v

^{f}= v_{o}+ at. We know that v^{f}equals zero at the moment of impact with the glass. Let’s assume v_{o}, or initial velocity, equals he average human walking pace, which the internet tells me is 5.0km/hr, or 1.3m/s. The final variable we are going to assume we know is t; granted, I don’t actually know how long it takes for the person to stop moving as a result of running foolishly into a wall, but I think a very rough estimate would be somewhere between a tenth and a hundredth of a second (t = 0.05s). Putting these variables together, we get 0m/s = 1.3m/s + a(0.05s) --> -1.3m/s = a(0.05s) --> a = -26m/s^{2}. That’s really fast deceleration for just walking. And thankfully really funny too.
What about this?

While this video might not be as funny as the previous one, I have a personal connection to it (sophomore year, my friends wedged a plank under my door handle and couldn't stop laughing...meanwhile I had to use the bathroom, felt claustrophobic, and panicked; Campo got involved). I thought this video ties in wonderfully with our analysis of different types of forces, notably the force of tension. Why can't the students open their doors? It's because of the force of tension acting in the direction opposite of the person opening the door. A free-body diagram of a situation when the door is closed and the rope taut could be represented with a circle as the doorhandle with the force exerted by you moving in one direction and the force tension moving in exactly the opposite direction. While only the x components of force here play a significant role, there would also be a downward force of gravity and an opposite normal force exerted by the frame of the door holding the doorknob in place. An interesting note, though, is that as a person exerts more pulling force on the door handle, the rope exerts an increasing force in the opposite direction. The person will only be able to open the door if the pulling force exerted by their arm exceeds the maximum amount of tension force the rope can exert back on the door, at which point it will break...or if their friends just let them out. Also important to note is that this problem is a little more complicated than the situation I've described; this is because the direction of force will be changing at every point the door swings open. The free-body diagram one would draw for this situation would have to be specific to an instant in time where the force exerted by you on the door would have a particular direction.

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