Brain teaser

[Bob_VT]

Re: Brain teaser

People people.... Yes I took physics too. My arguement is that the "fall" of a bullet is affected by gravity AND only when the forward motion is diminished.

I agree about two object falling at the exact same speed ...... only in a vacuum. The shape of the object has bearing on the fall rate too due to wind resistance.

Isn't that the reason behind a specific bullet and a specific load will have a varied drop at a specific distance? So.... if all rifles were set at zero at 100 yards then you are saying that all bullets would have the same exact drop (fall rate)? Regardless of forward velocity?....

 

[QC]

Re: Brain teaser

OK, I am struggling with two things. Doesn't the energy of the propellant count for anything? And what about the affect of aerodynamics on the fired slug? The dropped slug? Aren't there some measurable affects of both aero and the propellant? I am not buying that they are simply equal due to gravity's affect (yes I understand the curvature discussion, but put that aside for a second).

Is this a rifled barrel? Rifled slug? The original question says "rifle" and then "slug". IMHO both the spinning slug and the shape of the slug bring aerodynamics into the equation. If the dropped slug is held horizontally, it is initially less aerodynamic in relation to it's eventual pathway. Soooooo, let's say we are standing there like the problem says. The dropped slug ( I assume somewhat conical on one end and flatter on the other) would start falling sideways and then start to rotate in it's attempt find the most aerodynamic position (maybe cone down, maybe not). This condition will have some affect on the time it takes for it to fall . . .

Now to the horizontally fired one. Maybe the fact that is conical will make aerodynamics equal on the "top" and the "bottom" which makes the effect zero . . . But it is spinning, this could cause it to slightly accelerate downward towards the side of the spin, right? Wouldn't that make it fall even faster? So with those two issues to deal with; 1) horizontally held "slug" pitching to fall vertically and 2) spinning "slug" spiralling slightly toward the direction of the spin, don't we have the potential for the fired slug actually to hit the surface first? Is aero meaningless? I don't think so . . .

Edit: great minds ^^^^^

 

[Bob_VT]

Re: Brain teaser

I think the key is flight time. We are not taking flight time into the equation. A faster bullet will have a longer flight time than a slower one on the "horizontal" and then fall at an equal rate. It is impossible to free fall when being propeled foward.

 

[i386]

Re: Brain teaser

It is impossible to free fall when being propelled foward.

Not at all. It seems as if you're suggesting that gravity will not affect an object until it decelerates to some defined velocity. Such a point, if it existed, would have a name, and would be a key component in any equation used for calculating these simple vectors. I don't claim to be an expert at physics, but I'd like to think I have a good understanding of the basics.

I'm glad QC brought up the point about the shape of the bullet. A conical projectile (of equal mass) will move faster through (on the same amount of propellent) the air than a wadcutter because it's more aerodynamic. Another way to say that is it is more resistant to deceleration in the x axis due to wind resistance. The rifling helps the bullet not turn or tumble because of the gyroscopic forces applied to it. When something is spinning on an axis, there is resistance to rotating that axis (in the same plane). This is the same principle that's demonstrated by spinning a bicycle wheel while holding on to the axles. You feel resistance when trying to turn the wheel. The faster the wheel spins, the more resistance you feel. In fact, if you did this sitting on a bar stool, it would make the stool spin when you turn the wheel.

So yes, air is a factor but not for providing lift. When fired level, the bullet will begin to fall at 9.8m/sec^2 as soon as it leaves the muzzle. It does not matter if it's moving forward at 5 ft/sec or 5000 ft/sec. It may appear to not drop at first and then drop sharply later but that can be explained. Objects falling due to gravity are not falling at a constant speed. They accelerate exponentially. That's where the parabolic curve (or eliptical if you wanna get that technical) comes from. It accelerates at 9.8 meters per second per second. Start squaring stuff and those kooky parabolas start popping up. Lose the exponents and they become linear.

 

[Bob_VT]

Re: Brain teaser

Here we go..... let's get a treadmill involved!!

QC in the original post the bullets are equal shape etc.

Yes when they free fall they are equal. A bullet fired will not free fall until it hits a certain point. We are dealing with Newtons laws. The bullet fire from the gun has acceleration forward yes regardless of 5 fps or 5000 fps but the bullet at 5 fps will be deceralted enough to free fall faster than a bullet at 5000 fps ...... agree? Because the time in flight will be shorter. It will free fall at an earlier moment because the flight time will be shorter.

Newton's second law couples with frictional force (I will call it flight time).... which last longer than a free falling object.....

Wow If we could get that treadmill and a few beers in us ..... we would all be locked up in a padded room.

I'm going to stick to this...... more is involved with a fired bullet than a bullet dropped.

 

[QC]

Re: Brain teaser

QC in the original post the bullets are equal shape etc.

I understood that, but the dropped one is assumed (by me) to be held horizontally. There is an aero issue with the fact that the dropped one would be initially meeting air resistance from the side (horizontal object going down), then it would begin to rotate slightly before hitting the ground. The fact is, that should be measurable. The fired one begins it's journey facing the air resistance both head on and spinning. The spinning keeps the cone of the conical shape headed into this resistance and due to the fact that it will still be moving forward when it hits the ground, the cone will still be facing the resistance on the air, which is more aerodynamic than the "side" of the horizontally dropped one. This to me says that the dropped one will have a slightly slower rate of descent. Also, I still believe that the aerodynamics of the spinning, fired one, will cause it to follow a somewhat spiral path in the direction of the spin. This would have some affect, but it is not clear to me what the affect would be. In a vacuum, I would agree with the belief that they would meet the surface at the same time. But aero is not meaningless unless there is some margin at which we are supposed to stop contemplating this discussion. I was not made aware of any margin or factor to disregard when I was supplied the rules . . .

 

[Bob_VT]

Re: Brain teaser

I am wrong I stand corrected :redface:

'http://members.aye.net/~bspen/onetrajectory.html'
'http://hyperphysics.phy-astr.gsu.edu/Hbase/traj.html#tra2'

There are some interesting things.......

I still want a treadmill brought into the equation...... dropping bullets out of an airplane

 

[BoatBuoy]

Re: Brain teaser

Let's say the rifle mount (with rifle) is on a treadmill going backwards at the same speed as the muzzle velocity of the bullet. How's that?

 

[SgtMaj]

Re: Brain teaser

Forward velocity has no effect whatsoever in the gravitational pull or decent of the bullets. The only thing that makes the fired bullet stay airborne longer than the dropped one is the curvature of the Earth making it's decent distance greater than that of the dropped bullet.