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Does a RB travel with constant rotation, or accelerate?

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bisleyjohn

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Mathematics aside. ALL projectiles, lead or jacketed, by whatever means propelled and despite the rate of spin/twist/velocity (all irrelevant) will slow down their spin rate due to air resistance. If you don't beleive me, spend about twenty years as I have, in Stickledown butts when firers are at 1000 yards plus and listen to the bullets tubmle, see the oblong, square, oval mis-shaped holes in the target.

Balls won't tumble so much I admit but they are either part cylindrical or flat bottomed when they leave the barrel and they will all slow down/reducespin rate and tubmle. Fact! :dunno:
 

tenngun

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I'll take your word for it, I never timed a shot, myself, ever.
Drawn from Lyman black powder hand book. A .495 ball at 1600 fps 0.250894 seconds at one hundred yard0.618131 seconds for two hundred.
 

Carbon 6

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Balls won't tumble so much I admit but they are either part cylindrical or flat bottomed when they leave the barrel and they will all slow down/reducespin rate and tubmle. Fact!
Right you are John.
If one closely examines a fired ball from a rifle you will discover that it is more cylindrical after firing and not perfectly spherical. Loading and firing both obturate the ball to the tube.
 

MrMackc

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I think the question could be spin rate
Does a ball continue at one turn in four feet ( or 56”60”66”72, or what ever) or does it’s spin rate increase as the ball slows.
I THINK it keeps one/forty eight, but gyroscopic action can suggest an increased rate of spin
Everything slows but how much. Will 24000 RPM out last 1600 fps?
Here is my take on it, It takes cut spiral rifeling gripping the patch to give the bullet moving down the barrel it's rotation air can't grip the ball and make it spin faster air acting as friction is acting as a brake and brakes only slow down, rotation, velocity, gravity any other stuff, period. Gravity accelerates a falling body at 32 fet per second per second, when a body falls into the earth's atmosphere the air friction slows and heats up the falling object, whether it is a metiorite, space craft, bullet, or a human body When you shoot an object straight up gravity and air friction slows and stops the bullet and gravity starts it falling, and air keeps the bullet from reaching a constant acceleration of 32 feet per second per second. Is it spinning or not ? that depends.... Have you ever put spiral fletchings on an arrow to give it spin, or ever seen spiral fins on an aerial bomb?
 
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Carbon 6

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Everything slows but how much. Will 24000 RPM out last 1600 fps?
Typically in real world applications, spin comes to a stop before the velocity does. Though the reverse could happen, I have never witnessed it.
 

Tanglefoot

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Gentlemen.. and Ladies, if any are present .. The physics of this question are very simple, especially once the ball (or bullet) leaves the muzzle. All of the force that is driving the projectile is effectively expended once the ball leaves the barrel. The forces that are still acting on the ball/bullet are gravity and friction. Both are effectively constant over the time and distance of travel unless the projectile hits something denser than the air --- which is not what we started out discussing. Gravity tends to pull the bullet downward and friction tends to cause it to lose both forward velocity and spin speed. Nothing acts on it to make it spin faster.
The forward velocity and the spin velocity are greatest at the instant the projectile leaves the muzzle. After that they degrade through it's entire path.
 

Carbon 6

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Have you ever put spiral fletchings on an arrow to give it spin, or ever seen spiral fins on an aerial bomb?
Never seen spiral fins on an aerial bomb, only straight ones.
An arrow and a bullet or ball both have to start out stabilized in line with a center axis for either rifling or spiral fletching to work. Take a slingshot to the range and launch some spent bullets, you'll never get them to stabilize.
 

bisleyjohn

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Here’s a thought. Spin or not. Practice and load testing will help you reduce the POI (Point of Impact) circle. Knowing if, or how fast your projie is spinning when it gets there doesn’t make a hoot of difference - does it?


:cool:
 

tenngun

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Typically in real world applications, spin comes to a stop before the velocity does. Though the reverse could happen, I have never witnessed it.
Well let’s imagine a ball shot level at the Grand Canyon. The ball will drop as soon as it leaves the barrel, but it will remain going in a straight line relative the barrel. So seen from above it goes in a straight line, seen from the side we see a curve till air resistance robs the ball of forward movement maybe six hundred yards?
Now as it drops it will accelerate near one g until terminal velocity, about 1100 fps.
Of course there is cross winds going in every which direction on the way down, but let’s ignore that for a minute.
Will the ball stay with it’s original 24000 RPM? Or close to?
 

Carbon 6

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Well let’s imagine a ball shot level at the Grand Canyon. The ball will drop as soon as it leaves the barrel, but it will remain going in a straight line relative the barrel. So seen from above it goes in a straight line, seen from the side we see a curve till air resistance robs the ball of forward movement maybe six hundred yards?
Now as it drops it will accelerate near one g until terminal velocity, about 1100 fps.
Of course there is cross winds going in every which direction on the way down, but let’s ignore that for a minute.
Will the ball stay with it’s original 24000 RPM? Or close to?
I know what you are getting at but you made some errors.
Looking at it from above the flight path is not straight in line with the bore either.
You also didn't factor in air density which slows the acceleration of gravity because the closer you get to the ground the greater the density becomes.
Now if you fired a very low velocity ball straight down from a high enough altitude, then yes, you could see acceleration of velocity but not acceleration of spin.
 
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Bob McBride

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If we did find that a RB's spin accelerated when fired from a non-gain twist barrel all the world's energy problems would be solved....

The spin is under the thumb of Newton's first law of motion, just as the speed is.
 

tenngun

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I think I did factor in air density since I said it would accelerate at ‘near one g’ up to terminal velocity.
Sans air resistance it would accelerate downward at one g and have no terminal velocity.
 

Carbon 6

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I think I did factor in air density since I said it would accelerate at ‘near one g’ up to terminal velocity.
Sans air resistance it would accelerate downward at one g and have no terminal velocity.
Touche' :thumb:

However, the ball would have to start out slower than the force of G (terminal velocity) to experience any acceleration. Or it would simply decelerate to the terminal velocity of G and experience no acceleration at all.
Either way, it's going to decelerate completely when it hits the bottom of the canyon.

It's also worth noting that regardless of the acceleration of G, spin is independent of velocity, once it leaves the barrel, and it can only slow down.
 
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tenngun

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If we did find that a RB's spin accelerated when fired from a non-gain twist barrel all the world's energy problems would be solved....

The spin is under the thumb of Newton's first law of motion, just as the speed is.
Right on I Think. But....( damn them buts)
If it starts at 24000 RPM and it stays at 24000 as the ball slows it forward velocity. It hasn’t accelerated its spin rate, its only changed in relation to the to the distance the ball moved in time.
A spinning top, even spin on ice isn’t perpetually spinning, the low friction even with ice steals RPM, while in space it would spin nearly with out end as gravity and solar wind would ultimately slow it. In this case it might be billions of years.
I’m so confused. It seemed soooo simple when I first thought about the question
 

Bob McBride

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Right on I Think. But....( damn them buts)
If it starts at 24000 RPM and it stays at 24000 as the ball slows it forward velocity. It hasn’t accelerated its spin rate, its only changed in relation to the to the distance the ball moved in time.
A spinning top, even spin on ice isn’t perpetually spinning, the low friction even with ice steals RPM, while in space it would spin nearly with out end as gravity and solar wind would ultimately slow it. In this case it might be billions of years.
I’m so confused. It seemed soooo simple when I first thought about the question
It's been a minute since I read any Einstein or Hawking and keep in mind I'm just answering the OP and haven't followed the discussion. I think the atmospheric forces acting to slow the forward movement of the ball will also be acting to slow the rotation (in space or on the range) though the rotation will resist the forces acting against the ball better than the forward momentum will, causing the spin to slow at a slower rate than the forward momentum since the major force is acting on the poles of the rotation. Like spinning a basketball, with a vertical axis, between your two fingers and adding wind to the mix. I think if the wind is horizontal it will slow the ball much faster than if it's coming from directly overhead, such as it would be with a spinning RB. I definitely can't see a situation in which the rotation would accelerate, outside of the ball shrinking in flight (which would be adding another force to the mix). I could be wrong. Probably am. This is above my pay grade.
 
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Carbon 6

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I’m so confused. It seemed soooo simple when I first thought about the question
Just use that which is useful to you and ignore the rest. It's not like we can change physics anyway. None of this will help you hit your target.
 

Carbon 6

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It's been a minute since I read Hawking's 'A Brief History of Time' and keep in mind I'm just answering the OP and haven't followed the discussion. I think the atmospheric forces acting to slow the forward movement of the ball will also be acting to slow the rotation (in space or on the range) though the rotation will resist the forces acting against the ball better than the forward momentum will, causing the spin to slow at a slower rate than the forward momentum since the major force is acting on the poles of the rotation. Like spinning a basketball, with a vertical axis, between your two fingers and adding wind to the mix. I think if the wind is horizontal it will slow the ball much faster than if it's coming from directly overhead, such as it would be with a spinning RB. I definitely can't see a situation in which it would accelerate, outside of the ball shrinking in flight. I could be wrong...
The thing I haven't reconciled yet is;
If a ball is fired at long range does the axis of the spin stay in line with the bore or does it change with the arc of the trajectory.
My theory is that it stays in line with the bore and resists a change in direction.
This means that as the trajectory becomes more vertical, spin becomes more perpendicular to the flight path. The result is increased drag resulting in increased deceleration of both velocity and spin.

Now, an elongated projectile is a slightly different matter.

Time to oil the gears. :)
 

Bob McBride

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The thing I haven't reconciled yet is;
If a ball is fired at long range does the axis of the spin stay in line with the bore or does it change with the arc of the trajectory.
My theory is that it stays in line with the bore and resists a change in direction.
This means that as the trajectory becomes more vertical, spin becomes more perpendicular to the flight path. The result is increased drag resulting in increased deceleration of both velocity and spin.

Now, an elongated projectile is a slightly different matter.

Time to oil the gears. :)
I think that the stabilization of the ball imparted by the spin is the greatest force that's acting on the ball in relation to it's arc, then adding the friction being slightly greater on the bottom of the ball (forward+downward) would tend to keep the axis of the spin in line with the direction of it's flight while at the same time adding to the slowing of the spin on it's downward trajectory.

Once again, no warranty implied or given...
 
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