Physics 101 OMG

[SN13]

I know that not all of us liked physics in highschool or even took it (physics was an elective science where I went to school) but can we go over some SIMPLE rules please?



#1. BULLETS ACCELERATE TOWARDS THE EARTH!

Bullets begin accelerating TOWARDS THE EARTH the moment they leave the barrel. There's this thing called Gravity that pulls everything down at 9.8m/s^2.

In a Vacuum, we can calculate gravity's acceleration by a = Dv/Dt where a = acceleration, D(delta) = the change, v = velocity and t = time. In other words, if an object is moving at 9.8m/s after 1 second and it started at 0m/s at 0 seconds then the calculation a = (9.8m/s - 0m/s) / (1s - 0s) or 9.8m/s^2.

Air resistance does SLOW the acceleration towards the earth and there is a point at which air resistance and gravity are equal so that there is no more acceleration (Terminal Velocity) which is as fast as the object can fall, but we won't concern ourselves with that for now.

IF YOU SHOOT A BULLET WITH A TRAJECTORY THAT PARALLELS THE EARTH, IT WILL NOT RISE. Not for the first 25yards, 100yards or EVER. When you THINK a bullet is rising, it's because you shot it at an upward angle. It's beginning to accelerate DOWN instantly.



#2 - RECOIL BEGINS THE MOMENT THE BULLET MOVES.

FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION!

The bullet CANNOT move unless there is an equal and opposite reaction occurring. Bullets ACCELERATE until they exit the barrel and then begin to decelerate.

A 150 grain bullet weighs 9.72 grams which is 2/100ths of a lb. (0.009719837 kg)

A 150lb man weighs 68,038.86 grams which is 1,050,000.07 grains. (68.038860036 kg)

There are Approximately 7000 Grains in a lb.

So lets say the average man (150lbs) weighs SEVEN THOUSAND TIMES the weight of the .308 projectile (150gr).


The acceleration of a 150gr bullet in a 2 foot (24") barrel up to 3000 ft/s is an acceleration of 2,307,692 ft/s^2

The equal and opposite acceleration of a 150lb (1,050,000gr) shooter would be 329.7 ft/s^2

The bullet accelerates 7000 times faster than the person so it SEEMS like the bullet is out of the barrel before the shooter and gun recoil. .0013 seconds is all the time spent in a gun for a .308 going 3000ft/s at the end of a 24" barrel.

In that same time, a person accelerating at 329.7 ft/s^2 reaches a whopping .42 ft/s.

So you SEE the bullet leave, then SEE the gun and shooter recoil because .42 ft/s is hardly fast at all. After .0013 seconds, the acceleration force is over and momentum takes over and you decelerate thanks to friction.

OK GOT IT?

#3 - A 65 gr bullet and a 500gr bullet DROP AT THE SAME RATE!

If you fire a 65gr bullet at 100fps and a 500gr bullet at 100fps, they'll have the SAME drop rate.

So, if you fire a 150gr bullet with the same BC as a 180gr bullet, and they travel at the SAME velocity when exiting the barrel, which one will have more drop at 200yrds for a 100yrd zero?

If you said 180gr you're WRONG.

Gravity pulls objects towards the earth. If you set up 5 guns, 9.8 meters above the ground, with all barrels parallel to the earth, one shooting a 500gr bullet at 100fps, one shooting a 200gr bullet at 100fps, one shooting a 500gr bullet at 2000fps, one more shooting a 200gr bullet at 2000fps, and ONE last one shooting a 50gr bullet at 4000fps, and you fire ALL OF THEM AT THE SAME TIME, the bullets will contact earth at the EXACT same time... ONE second after firing.

Well why is there a difference in drop between 150gr .308 and a 168gr .308?

Simple really.... one's traveling at 3000fps and the other is traveling at 2700fps. The heavier bullet takes a bit longer to reach the target, thus it will fall a bit more due to gravity and gravity ALONE. A bullet moving at 3000fps (in a vacuum to eliminate drag and air friction) will travel 3000 feet in one second and fall 32.15 (9.8m) due to gravity. For a bullet traveling 2700fps to move 3000 feet, requires 1.11 seconds which results in 35.68ft of drop DUE TO GRAVITY AND TIME... Not WEIGHT.



Now I haven't had physics in 6 years so if anyone knows a reason for me being wrong, please tell me. I'm not a physicist but damn, bullets don't drop faster because they're heavier. They just take longer to reach their target thus allowing gravity to act on them longer before the hole in the paper.

 

[Rubber_Duck]

This was an interesting read. Thanks for posting.

So you SEE the bullet leave, then SEE the gun and shooter....

Maybe it's just me but I have never been able to SEE a bullet leave. I guess I should head over to the eye doctor!!!

 

[cracked butt]

I'm not a physicist but damn, bullets don't drop faster because they're heavier. They just take longer to reach their target thus allowing gravity to act on them longer before the hole in the paper.

Tsk Tsk.

Heavier bullets in the same caliber with a much higher BC will actually reach a long range target faster and with less drop than a lighter bullet that is less aerodynamic

 

[SN13]

Yah but they DON'T DROP FASTER because they're HEAVIER.

in my hypotheticals I used Vacuums or identical BCs when saying they'd drop at the same rate.

And besides, aweful BC's don't make them DROP FASTER they just SLOW the bullets speed down, causing it to take longer to reach the target and Thus giving gravity more time to act on it before it punches that hole.

 

[lions]

DUE TO GRAVITY AND TIME... Not WEIGHT.

The formula for gravitational pull has to do with weight. I don't know it off the top of my head but it takes into account the weight of the earth and the weight of the other object. Every object with mass has a gravitational pull. The more mass the more gravitational pull. You and I have a gravitational pull it is just minimal because of our relatively low mass. A bullet has that same pull, it is actually pulling the earth towards it with its own miniscule gravitational force. So if two bullets are exactly the same except for density the more dense bullet would actually hit the ground slightly before the less dense one. The difference of time would be so small you probably couldn't even measure it but my point is weight does figure into the equation. It has also been a long time since I have taken physics but weight does play a role.

EDIT:

The standard formula for gravity is:

Gravitational force = (G * m1 * m2) / (d2)
where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

 

[BigBlack]

I had a 270 friend be littling my 06 due to its extreme drop as compared to his 270. I was hitting a steel out a 300yrds from a 100yrd zero and he was going to show me how flat his 270 was. From a 100yrd zero he aim dead on the steel plate and missed (hit the dirt), he proceeded to explain how he probally bumped his scope while on his last hunting trip and decided to verify it. Hit bullseye at 100yrd and being perplexed blamed dud round and again drew down on the steel plate, missed again, I asked if I could try. Using the ballistic table on his box of ammo aimed high and hit steel. He tried again and missed. I told him to hold high according to the balistic table on his ammo box but he was sure something was wrong with the rifle, because hey my 270 is a flat shooter.

:banghead:

 

[younganddumb]

sn13 stop saying THUS, its a bit annoying

 

[BigBlack]

The formula for gravitational pull has to do with weight. I don't know it off the top of my head but it takes into account the weight of the earth and the weight of the other object. Every object with mass has a gravitational pull. The more mass the more gravitational pull. You and I have a gravitational pull it is just minimal because of our relatively low mass. A bullet has that same pull, it is actually pulling the earth towards it with its own miniscule gravitational force. So if two bullets are exactly the same except for density the more dense bullet would actually hit the ground slightly before the less dense one. The difference of time would be so small you probably couldn't even measure it but my point is weight does figure into the equation. It has also been a long time since I have taken physics but weight does play a role.

Nope a neat little trick

Take a book and a feather hold side by side and drop. Which one hits the floor first?

Now take the feather and lay it on top of the book and drop the book. The feather (wind resistance removed by book) will fall at the same rate.

A sheet of paper can be substituted for the feather. This is a classic Phyics classroom example of gravitation pull being the same no matter the weight. Wind and other variables can effect the rate of fall by gravity is still the same.

 

[wally]

Recoil basically doesn't happen until the bullet exits the muzzle because the gun/bullet is a closed system until the bullet exits. The force from momentum backward as the bullet moves forward is basically balanced by the friction force of the bullet dragging the gun forward with it.

Don't believe me? Get a medicine ball, put on roller skates and "throw" the ball without letting go. Note you will wobble but have no net "recoil' (more complications from gravitational torques generated as your center of mass varies) . Now do it again and let go to actually throw the ball -- you will roll backward in recoil this time.

The actually situation is more complicated still, but as a practical matter recoils happens when the bullet exits. Elastic waves in the barrel will outrun the bullet and can change POI relative to POA depending on the bullet weight/velocity and barrel stiffness -- which is why great pains are taken to make barrels as "stiff" as possible


Your exterior ballistics bullet drop calculations are way off as the velocity is continuously dropping from air resistance as it moves down range thus time of flight is a complicated integral not a simple velocity/distance calculation. Slower heavier bullets can drop less than lighter faster ones if they have better ballistic coefficients and end up being enough faster at longer ranges to make the integral smaller (win the race to the target).

A bullet moving at 3000fps (in a vacuum to eliminate drag and air friction) will travel 3000 feet in one second and fall 32.15 (9.8m) due to gravity

The distance traveled in free-fall is s=g*t*t/2 so the time to fall a given distance is t=SQRT(2*distance/g) or the square root of 2 (~1.41 seconds) to fall 9.8 meters. Not the one second you claim. But you are correct the downward component of velocity will be 9.8 m/s after one second of free-fall.

As Einstein said: "Things should be as simple as possible, but no simpler." You've oversimplified to the point of not being correct.

--wally.

 

[rcmodel]

You can see .45 ACP bullets in flight, and I have done it many times when I was younger.

I have even seen .22 bullets if the sun angle is just right to catch the glint off of them.

Even now, it is easy to do with a 20x spotting scope positioned behind the shooter.

And then there are tracers.

rcmodel

 

[lions]

Take a book and a feather hold side by side and drop. Which one hits the floor first?

Note that I am not talking about a book and a feather, I am talking about two objects that are exactly the same except for density. I also want to point out that the gravitational pull of two objects (lets say about bullet size) is so tiny that it is not something that would be observed with the naked eye. It would probably be somewhere in the fractions of a nanosecond range.

EDIT:
http://www.physicsforums.com/showthread.php?t=103269

 

[BigBlack]

lions,

made me go back and learn again, guess the book and feather thing just stayed in my mind as mind boggling and I forgot the details. I concur.

 

[wally]

I have even seen .22 bullets if the sun angle is just right to catch the glint off of them.

Off a rest, if the light is good you can watch the base of the bullet travel to the target thru the scope. Great fun when shooting steel plates at 75-100 yards. 4x scope is about right, much more and the recoil can make the field of view lose the bullet and target.

you can see .45 ACP bullets in flight, and I have done it many times when I was younger

Even at 57, its pretty easy to see the .45ACP slugs "rainbow" towards the target from my Thompson SMG when shooting at targets ~75 yards away -- having a steady stream helps find them

--wally.

 

[corncob]

Everything sounded good to me except:

the gun/bullet is a closed system until the bullet exits

What is the basis for this statement? The bullet has two forces acting on it: powder presseure and friction. One dwarfs the other. The rifle/shooter has the same two (assuming he has a good squeeze on the stock and they can be treated as a single mass), and one dwarfs the other. Both masses accelerate due to the net forces acting on them. Seems simple enough, but maybe I'm missing something.

Your medicine ball example is a closed system, but only if you watch both the pushing that accelerates it and the pulling that decelerates it. Pause video half-way through and you can't tell a difference between the time when you hold it back and the time when you let it go. If the rifle barrel could "squeeze" the bullet before it exited, like you sqeeze the medicine ball to prevent yourself from pushing away from it, it would be a closed system too.

 

[RonE]

SN13

Great post!

Great High School physics lesson!

A very hard concept for some because they have heard for so long about how bullets rise through the line of sight and then drop back down to zero.

Thank you!

 

[esmith]

How you HIGHLIGHT certain words for emphasis is rather annoying. Kool stuf tho.

 

[Art Eatman]

Sorry , Wally. Recoil begins when the bullet starts its journey. "Equal and opposite". Newton's Laws of Motion and all that good stuff...

cracked butt, in air you're correct when you're talking about Ma Bell distances. In vacuum, however, BC doesn't have anything to do with anything. Irrelevant. In vacuum, the bullet could be a flat-end cyclinder.

 

[fal 4 me]

I've taken high school physics, and to show this concept we rolled balls off a table at various speeds and timed the drop. Still whenever I hear someone say that a bullet rises, I just assume that they are talking about the bullet path, which does rise in relation to the line of sight because the barrel is tilted upwards at a slight angle.

Every once in a while someone will post a similar rant and I always think OMG WTBFD. Saying the bullet rise is just easier to say. Life's easier when you can let little things like this slide.

 

[fletcher]

The formula for gravitational pull has to do with weight.

Yes, but as Mass of Earth >>>>>>>>>>>>>>>>>>>>>>>>> Mass of anything on it, the latter can be neglected.

 

[lions]

While I realize that the disparity between the two masses involved is so drastic as to render my point scientifically insignificant, it doesn't change the fact that...

DUE TO GRAVITY AND TIME... Not WEIGHT.

...is an incorrect statement.

The fact that it can be neglected does not mean that it is not true, I merely intended to demonstrate that weight does have something to do with it.

After all, we did learn about the infinitesimal force of gravity everything enacts on the earth in Physics 101, remember?

 

[Husker1911]

My HS physics teacher, back in the day of stone tablets, told us, "If you fire a rifle and drop a bullet from the same height at the same time, they'll both strike the ground at the same instant."

Except for the curvature of the earth causing the fired bullet to drop slightly further, he was absolutely correct, and it proved quite a conceptual whack upside the head to a young HS physics student.

 

[lee n. field]

#1. BULLETS ACCELERATE TOWARDS THE EARTH!

Not only that, but the downward motion due to gravity is completely independent of the forward motion.

 

[Harvster]

Sorry , Wally. Recoil begins when the bullet starts its journey. "Equal and opposite". Newton's Laws of Motion and all that good stuff...

I'll have to take Wally's side on this. You see the equal and opposite in the case of a bullet being fired exist as pressure while in the chamber. To illustrate lets take two M-80 firecrackers (the fun ones from way back when), put one in a soda can and one in a can of the same size but with 1 inch thick walls and ends. Now we need a volunteer to hold both as we detonate the M-80s. In one case there is going to likely be some missing fingers and some shards of aluminum flying around. In the other hand....a steel cylinder(maybe a little warmer than it was). The force of the equal and opposite reaction is the same in each case but in the steel cylinder it exists as pressure. Without getting into minutia, the moment of firing the bullet is similar.

 

[AndyC]

Recoil starts as soon as the bullet starts to move - that's just physics.

 

[scythefwd]

sn, in a vaccum, you are 100% correct, that objects fall at the same speed. In the real world, where scientists still can't tell how a bumble bee is able to fly... aerodynamics play a massive role. Shape and drag coefficient of jacket material both play a role in how fast something drops. If the jacket is of the same material, and they are the exact same shape, then yes they fall at the same speed.