Thinking like an Engineer

I was walking down a side street, minding my own business as much as that's actually possible in Manhattan, when I saw a young woman struggling to open the door to an office building.* The door was one of a pair of heavy glass doors with no frame, just hidden hinges; she was quite small and - important to note - wearing spike heels.

A lot of commercial building doors have stiff hinges. It's a combination of design (the doors won't accidentally swing open from wind or the difference in pressure between inside and out) and accident (the hinges are never cleaned). The woman pulled normally at first (standing vertically with her arm parallel to the ground), the way one would with an interior door, and nothing happened. Then she simultaneously pulled and leaned back slightly, and still nothing happened.

This is where the narrative switches to geek-speak because I switched to geek-think. She was approaching the problem as one of applying a force to the door, but that wasn't the problem. She needed to balance the force she was trying to apply with her arm and (by leaning back) upper body with a reaction to the ground. Her feet needed to be pushing towards the door with as much force as her arm was pulling, and her shoes made it nearly impossible: the configuration of the human body is such that we push forward with our heels, as in "digging your heels in." There was no way for her to do so on shoe heels that were probably under 1/4 square inch in area. On the other hand, if the door was set up to swing in, she could have pushed back off the balls of her feet and her toes to push forward with her arm.

I suspect that all women who regularly wear spike-heeled shoes understand this from experience, but I've never heard it discussed. As someone with large and partially flat feet, I take the reactive force for granted almost all the time. And, it can be endlessly fun demonstrating Newton's Third Law to people while ice skating.

*I might have been inspired to play Galahad, but a woman leaving the building pushed it open from inside before I was near enough to consider breaking stride to help.