Terminal velocity
 

Heard a guy on the radio (during the chaos of Arafat's funereal) talking about all the shooting up in the air and where all those bullets go and how dangerous they are. He said that according to the law of conservation of energy the bullet would be traveling the same speed when it hit the ground or whatever that it was traveling at when it left the muzzle. If this was true, a bullet would travel forever in a straight line, as long as nothing got in the way, right? Impossible, right? Say a bullet is fired perfectly straight up. It stops at the "top" and falls back to earth. At what speed will it be traveling when it strikes the ground?

RE: Terminal velocity
 

Thats a physics question. I can't tell you the answer but I know that the air resistance and gravity are what slows the bullet down. As for the speed on the way down after being shot straight up has to do with the mass of the bullet, the distance trveled (down) and the acceleration of gravity. The bullet would travel in a staight line forever in a vacuum like outer space, but gravity and air resistance are forces acting on it to slow it down.

RE: Terminal velocity
 

Yeah, I agree with everything you said except the"mass of the bullet" helping determine the speed of the falling bullet. Doesn't that go back to Newton's Law about dropping a marble and a cannon ball at the same time and they both hit the ground at the same time? Always kinda bothered me that a bullet dropped at the same time as a bullet shot LEVEL will hit at the same time. That's right, isn't it? Even tho they may be a mile apart.

RE: Terminal velocity
 

The bullet fired vertically will often return base down, which limits its' terminal velocity even more. IT DOES NOT RETURN WITH ITS' MUZZLE VELOCITY! According to Hatcher's Notebook, (In an entire chapter entitled BULLETS FROM THE SKY) returning bullets come down at unpredictable locations, and their return velocity cannot be accurately chronographed (obviously). However, impact dents made by returning 150-grain .30 caliber bullets which were fired vertically with a MV of 2700 FPS were about 1/16 of an inch in soft pine. These dents indicate that the velocity at impact was in the vicinity of 300 FPS.

While an 150 grain .308" bullet landing on you at 300 FPS could cause some damage and pain, 1/16" of an inch dent in soft pine is not indicative of a dangerous wound - it takes 1" of penetration in soft pine to equal a dangerous wound. BUT, a .50 BMG bullet of 718 grains would fall with close to 500 FPS, carrying around 400 Ft/Lb of energy - no joke!

RE: Terminal velocity
 

Now, by golly THAT'S an answer. Is this Hatcher's Notebook available somewhere? Sounds like interesting reading. I've heard a lot of cussin' and discussin' on this topic. Like to be the guy with the answers next time.

RE: Terminal velocity
 

Hatcher's should be readiliy available from sources that stock this type of publication. Midway used to have it and certainly Amazon should have it. It's the best treatment of this topic I've stumbled into. It is also a wealth of information on other subjects - some of it is dated but a lot of it is as true today as it was then.

Despite what my kids tell me I don't think physics has changed all that much since my time or even Newton's - as long as you don't approach the speed of light or want to talk about the "speed of time" (see Stephen Hwkings "A Brief History of Time" - he's got Sir Isaac's job today).

With regards to the assertions regarding gravity, feathers, cannon balls etc. - Galileo actually started that line even though Newton shared in some later credits (try here - http://www.astro.utoronto.ca/~philf/...at1/sld017.htm). As I recall Galileo was even older than Newton and I - although once again my kids dispute this.

RE: Terminal velocity
 

In the absense of of all those trillions of pesky drag producing air molecules (atmosphere), a bullet would indeed land with a velocity identical to that which it was launched. So if you fired that bullet straight up on the moon, you could be in for a very bad (albeit BRIEF) headache in few seconds!

(Un)Fortunately, Earth's atmosphere literally "gets in the way" of the bullet, causing it to slow down dramatically once it leaves the muzzle. The terminal velocity depends on the aerodynamic efficiency (BC) of the bullet based on how it's falling (point v. base down, or tumbling).

Let's just put it this way, if bullets fired into the sky were dangerous when they landed, the Muslims would have wiped themselves out by now.

Mike

RE: Terminal velocity
 

Blahahah lol no joke!!!!

RE: Terminal velocity
 

So, how high would a 150 gr. .308 boattail go before it stopped and fell back to earth (fired straight up)?

RE: Terminal velocity
 

Hey Driftrider: Only air drag? Gravity doesn't slow it down as well?

RE: Terminal velocity
 

In the vertical plane while the bullet is moving upward away from the center of the Earth, yes, technically.

But a better way of thinking about gravity is that it is a constant force that acts to change the direction of a projectile in flight by accellerating it toward the ground. Drag, on the other hand, varies based on a number of variables, and effects a projectile no regardless of the projectiles relative velocity to the ground.

This means that if a fighter jet were to fire its cannon straight down toward the ground, gravity would provide an additional accellerative force that would tend to make the bullet fall faster. However, the force of drag created as the bullet passes through the air is of a much greater magnitude that the accellerative force of gravity and would cause the bullet to slow down considerably before the bullet hit the ground. Now, if we were to assume that the bullet was fired from an extremely high altitude, and that the density of the atmosphere was constant (which it's not, but this is for simplification and visualization purposes only), the bullet would eventually slow down to the point were the accelerative force of gravity exactly equaled the force of aerodnamic drag, and the bullet would settle into an equilibrium, or terminal, velocity.

RE: Terminal velocity
 

Wow! OK, That means that gravity's affect diminishes over time until it becomes negligible and air drag becomes the controlling factor?

RE: Terminal velocity
 

No, Gravity's effect is (almost) constant. It accellerates any body near the Earth toward the planet at 9.8m/s^2, all the time no matter what.

The air resistance (drag) is the variable force, which provided the shape and orientation of the object doesn't change relative to the airflow, increases exponentially with the velocity of the projectile. In other words, the faster the bullet goes, the more resistance the bullet encounters. When the bullet is going faster than terminal velocity, the drag force the bullet is encountering is greater than the force of gravity, so the NET FORCE, causes the bullet to slow down even though it's falling toward the ground (think: parachute). However, as the bullet slows down, the drag force it encounters decreases as well, until eventually the force of gravity and the force of drag exactly match each other in equilibrium. At that point the net force on the bullet is ZERO, and the bullet continues to fall at a constant (terminal) velocity. Stated differently, in equilibrium (terminal velocity) the force of gravity and the force of drag exactly cancel each other out. Since an object in motions tends to remain in straight line motion at a constant velocity unless acted on by a net force not equal to zero, the bullet, in the absence of a net force, continues to fall at its constant terminal rate unless there is a change in the system that causes the net force to be something other than zero.

Mike