Opening my door by pushing vs. striking.

[wasteofo2]

My door was just opened a bit, and I needed to open it more to get in my room. For no particular reason, I struck the door with my fist relatively hard. The door is not that dense so I could see it bending as it went forward. It stopped opening before it was half way opened. When I push it open lightly, it opens much farther.

Anyone care to explain why?

 

[mathman]

I'll take a wild guess. Watch what happens at the hinges in these two cases. I suspect that by hitting the door hard, the vertical forces at the hinges are more uneven leading to increased friction.

 

[russ_watters]

Time.

Lets say to open a door it takes 10 pounds of force for 1 second. Impulse is force times time, so that's 10 pound-seconds. If you punch it, you may only be touching it for a 100th of a second. So to give it the same impulse, the force will have to be 1,000 pounds (ouch).

Another issue is the bending you saw. When you hit the door, it doesn't have time to evenly absorb the force and accelerate, so it bends. That's because of inertia. And if the force goes into bending it, not accelerating it, it won't move as far.

 

[LURCH]

Originally posted by russ_watters
Time.



Another issue is the bending you saw. When you hit the door, it doesn't have time to evenly absorb the force and accelerate, so it bends. That's because of inertia. And if the force goes into bending it, not accelerating it, it won't move as far.

Of the two factors Russ mentioned, I would say this is by far the greater. You imparted energy into the door and it flexed. Then it flexed back the other way, etc. By the time it got done vibrating, it probably traveled twicw the distance you needed it to, but did most of that travel back-and-forth while almost remaining in one place. You try a more extreme form of the same experiment by striking the door just as hard, but pulling your hand away as quickly as you can (this will minimse the time your hand stays in contact with the door). You will probably see the door vibrate even more, while making almost no forward progress at all.

 

[pallidin]

Ah, excellent comments... very informative.
Do I hear the word "shock wave"! Of course!

I recently read a series of articles with regards to shock absorption; in particular, one dealing with a scientist in New York was most interesting.
Shock waves do not behave or interact in the same way as linear pressure.
In fact, shock waves produce non-linear waves. More specifically, they are spherically radiated.
See here: http://physicsweb.org/article/news/5/10/15