Wait wait wait:
If a bullet fired up into the air can come down with enough velocity to kill a person, why not a penny dropped from a high building?
Physics majors please opine.
If a bullet fired up into the air can come down with enough velocity to kill a person
...
Pretty much not true. I learned that here on FR many years ago. And like the penny, the reason is terminal velocity.
Well, I've taught physics, so I'll take a shot at this. It has a lot to do with air resistance. And air resistance depends on two things, shape and mass. Do not neglect the mass effect! A bullet cuts through air more efficiently than does a penny, even if that penny is dropped on its edge.
On the moon, or Mars, both would kill you just as easily as they would if you were shot directly in a comparable location.
Once a bullet stops spinning and starts tumbling, it has a pretty low terminal velocity in air, because it presents a huge cross-section of resistance to the atmosphere. However, if the bullet were able to reacquire spin -- maybe by being driven in a wind current -- it would present a dramatically lower profile, would have a higher terminal velocity and, if massive enough, might still kill you.
Pretty Unlikely, though.
The claim that objects return to earth with the same speed they were thrown or fired upwards with is only true in a vacuum.
Mass (and aerodynamics).
2) A moving object's kinetic energy is a function of its mass and its velocity. EK = (m*v2)/2
3) A moving object's momentum is also a function of its mass and its mass and its velocity. P = m*v
4) Kinetic energy and momentum are different quantities, but both can answer the question "How much will it hurt when it hits me?"
5) What are the respective terminal velocities of a falling penny, bullet, and bowling ball?
6) What are the respective masses of a penny, bullet, and bowling ball?
7) What are the respective kinetic energy and momentum of a penny, bullet, and bowling ball?
8) Rate the three object is order of how much they will hurt you when they hit you.
The short version is the ratio between air resistance and mass. That ratio determines something’s terminal velocity. Pennies are not as dense as bullets and they are also flat. Hence they have a high surface area for their size/weight. So they fall slower than bullets. Some people claim bullets don’t kill coming back down but that is clearly false as people die to errant vertically fired bullets all the time.
mythbusters has an episode where they do the bullet in the air test, straight up and at an angle. Fired straight up the bullet comes down tumbling at less than lethal velocity this is due to the bullet reaching zero velocity at apex and lossing its gyroscopic spin stabilization. fired at a 45 angle the bullet never looses gyroscopic stabilization and comes down nose first still aerodynamic and at very much lethal velocity.
A bullet fired upwards at an angle will continue in a ballistic arc, and velocity lost and then gained to gravity will keep it lethal.
I forget what that angle is, and whether it varies by caliber or powder charge.
A bullet fired at a higher angle— or straight up— will come to a momentary stop before returning at terminal velocity, enough to cause pain and some damage, but not anywhere near just-fired-bullet speed.
This was tested on Mythbusters.
Note too that there are bullets that weigh more than pennies, and impact energy is a function of both speed and mass. To illustrate that latter point, a 50 pound child jumping on your chest from the back of the sofa (oof!) will hurt a lot less than a Chevy Nova falling the same distance off a wobbly jack, so use jack stands :)
Depends on the angle the bullet was fired. Straight up it doesn’t come down with enough velocity to kill, as it loses all that velocity going up and just comes down at terminal velocity. At at non-vertical angle though it’s a different matter, it loses much less velocity going up and (depending on the angle) still comes down at a “proper” bullet speed.
A bullet has an aerodynamic shape. And the penny doesn’t start at 3,000 feet per second.