From: Roy Mayeda (roy_mayeda@isd743.k12.mn.us)
Date: Wed Jan 22 2003 - 08:41:39 PST
Date: 22 Jan 2003 10:41:39 -0600 Message-ID: <978593249roy_mayeda@isd743.k12.mn.us> From: Roy Mayeda <roy_mayeda@isd743.k12.mn.us> Subject: Re: Objects Dropped from Tall Buildings
No, dropping a penny from the Empire State Building will not turn it into some kind of armor-piercing wonder, but "ignoring air resistance" it would get up a good head of steam. Dropping an object in a vacuum over the distance involved (369 m from observation deck) would give a velocity of about 85 m/s, and taking the mass as 0.00311 kg, that would give an energy of just over 11 J. In order to have this same energy though, a BB gun would have to fire the projectile at about 245 m/s (800 ft/s), which some of the more serious adult versions can do. Gee, Al. Even the ol' "Red Ryder" is rated at about 85 m/s! What were you guys using? :-) (A BB only has a mass of about 0.375 g -- the lead pellets in that caliber can have nearly twice that mass.) Just for comparison, a .22 caliber bullet will have 150-200 J of kinetic energy leaving the barrel.
As far as penetration, I'm not sure what's going on. It's more complicated than just the amount of kinetic energy. Otherwise a very large object at a low speed would penetrate just as well as a small object at high speed. I'm not sure how to combine the aspects of energy, momentum, and projectile shape/orientation, but I'm sure it has to do with all of these.
In reality, air resistance has a great deal to do with the actual event. I'm not sure what the terminal velocity range is for rifle/pistol bullets, but I know that birdshot falls to the ground at a low velocity (having been the landing location of quite a bit while hunting and shooting clays). The real world is so much more complicated than the simple equations that we often like to employ.
Roy Mayeda
Sauk Centre HS
Sauk Centre, MN
This archive was generated by hypermail 2.1.3 : Mon Aug 04 2003 - 16:18:11 PDT