Having this debate at work.

[extendor]

Right, you've all had your fun, now let's put this one to bed with some boring maths

 

First off, let's set some parameters which reflect what the OP was getting at: "Is it actualy possible to dodge a bullet after you see the barrel flash?"

 

Assumption 1: You can see the muzzle flash. There are no suppressors, he isn't looking the wrong way. His guidedog is awake, etc, etc.

 

Assumption 2: "Dodge" means "avoid being physically hit". Ignore shockwaves, vorticies, etc, etc.

 

Assumption 3: Muzzle velocity = just supersonic (350m/s)

 

Assumption 4: Distance required to move to avoid being hit = 2m in any direction.

 

Assumption 5: The target is a healthy male weighing 90kg (14 stone) In a life-threatening situation his leg muscles can exert a force of 775N, the same as a sprinter in the starting blocks (source: http://www.elitetrack.com/article_files/newtonian-sprinting.pdf)

 

Assumption 6: His reaction time is 0.5s from seeing the flash to starting to move.

 

Assumption 7: For reasons which will become obvious later on, the shooter is 414m away

 

Right, easy part first. At 350m/s, the bullet will take 1.18s to cover the 414m from muzzle to target. This is neglecting air resistance, so in reality it will be longer.

 

The light from the muzzle flash travels at 300,000,000m/s and takes 1.38 microseconds to reach the target's eyes. Given the very rough calculations this is small enough to ignore.

 

So, with a 0.5s reaction time, the target has 1.18 - 0.5 = 0.68s in which to move 2m to avoid the shot.

 

Now on to the person on the receiving end:

 

Remember he weighs 90kg and can exert 775N of force. Newton's laws of motion tells us that he can therefore accelerate at 775 / 90 = 8.6m/s^2 This is assumed to be linear, although it certainly won't be in real life.

 

He has to cover 2m to avoid the bullet. By taking one of the o-level physics linear motion equations and rearranging:

 

s = u.t. + 0.5.a.t^2

 

Where:

s = distance covered

u = initial speed

t = time

a = acceleration

 

In this case, we are starting from rest, so u = 0 so the equation becomes:

 

s = 0.5.a.t^2

 

Now all we have to do is rearrange so that t becomes the subject:

 

t = (2.s / a)^0.5

 

So now given s (2m) and a (8.6m.s^2) we can work out that covering 2m will take 0.68s

 

Well, waddya know? It takes exactly the same time for the bullet to reach the target as it takes to move 2m out of the way, so in this case, given all the assumptions above (most importantly the muzzle velocity and range) yes, the target will get out of the way in time.

 

If you want to play "what if", (or more likely find an error in my maths) I've attached the spreadsheet to this post.

 

 

Yeah yeah yeah, unless he is as wide as a barn door he does not need to move 2m in any direction. At worst he only needs to to move 0.3m so he has loads of time. Anyone who has shot a rifle knows this anyway.

[Scott]

Right, you've all had your fun, now let's put this one to bed with some boring maths

 

First off, let's set some parameters which reflect what the OP was getting at: "Is it actualy possible to dodge a bullet after you see the barrel flash?"

 

Assumption 1: You can see the muzzle flash. There are no suppressors, he isn't looking the wrong way. His guidedog is awake, etc, etc.

 

Assumption 2: "Dodge" means "avoid being physically hit". Ignore shockwaves, vorticies, etc, etc.

 

Assumption 3: Muzzle velocity = just supersonic (350m/s)

 

Assumption 4: Distance required to move to avoid being hit = 2m in any direction.

 

Assumption 5: The target is a healthy male weighing 90kg (14 stone) In a life-threatening situation his leg muscles can exert a force of 775N, the same as a sprinter in the starting blocks (source: http://www.elitetrack.com/article_files/newtonian-sprinting.pdf)

 

Assumption 6: His reaction time is 0.5s from seeing the flash to starting to move.

 

Assumption 7: For reasons which will become obvious later on, the shooter is 414m away

 

Right, easy part first. At 350m/s, the bullet will take 1.18s to cover the 414m from muzzle to target. This is neglecting air resistance, so in reality it will be longer.

 

The light from the muzzle flash travels at 300,000,000m/s and takes 1.38 microseconds to reach the target's eyes. Given the very rough calculations this is small enough to ignore.

 

So, with a 0.5s reaction time, the target has 1.18 - 0.5 = 0.68s in which to move 2m to avoid the shot.

 

Now on to the person on the receiving end:

 

Remember he weighs 90kg and can exert 775N of force. Newton's laws of motion tells us that he can therefore accelerate at 775 / 90 = 8.6m/s^2 This is assumed to be linear, although it certainly won't be in real life.

 

He has to cover 2m to avoid the bullet. By taking one of the o-level physics linear motion equations and rearranging:

 

s = u.t. + 0.5.a.t^2

 

Where:

s = distance covered

u = initial speed

t = time

a = acceleration

 

In this case, we are starting from rest, so u = 0 so the equation becomes:

 

s = 0.5.a.t^2

 

Now all we have to do is rearrange so that t becomes the subject:

 

t = (2.s / a)^0.5

 

So now given s (2m) and a (8.6m.s^2) we can work out that covering 2m will take 0.68s

 

Well, waddya know? It takes exactly the same time for the bullet to reach the target as it takes to move 2m out of the way, so in this case, given all the assumptions above (most importantly the muzzle velocity and range) yes, the target will get out of the way in time.

 

If you want to play "what if", (or more likely find an error in my maths) I've attached the spreadsheet to this post.

 

You missed the most important assumption.

 

The guy should really be warned that he is being shot at. Otherwise the reaction time would increase greatly as he figured out what the flash was.

[bigbloodyturbo]

Right point number one.

The round will travel at those speeds and will only slightly go slower.

but only slightly

how do i know this i hear you ask.

Because we rifle the barrels which makes the round spin and cut through air

The round will tail towards the ground due to gravity so it wont stay at a constant speed for miles and miles as it eventually hit the ground

 

Point number 2.

If i had 2 rifles pointing at the ground say 100 feet in the air.

And i dropped one bullet and fired another.

They will hit the ground at different times

I SAY AGAIN they will hit at different times.

Why? I hear you ask.

yes gravity will pull all mass the same but one bullet is being pushed through a barrel with high pressure so it will hit the ground sooner than the other.

 

And there is one way you can test this method yourself.

Get 2 tennis balls and stand on top of your sofa/table.

With one hand just release the ball and the other throw it at the ground and tell me which one hits first.

 

 

 

Point no.1 rifling has nothing to do with velocity AT ALL, it only gives the bullet gyroscopic stability to stop it tumbling in the air after leaving the barrel, other than that rifling just stops the barrel clogging with unburnt powder and cellulose residue. So NO it WILL NOT be travelling at the same speed at 300m as it is at the muzzle, this is FACT so stop trying to refute it.

 

Point no.2 I dont know why you've decided to use an analogy of pointing 2 rifles at the ground then dropping a bullet while firing the second, the outcome is obvious, your point is not. READ my post again, with the rifle parallel to the ground fire a bullet, as you do this drop a bullet at exactly the same time from the same height as the muzzle. Physics tell us that both will hit the ground at EXACTLY the same time just the fired bullet will be somewhere in the distance. This is also fact.

[Conrad]

Michael would have a field day in this thread with his http://www.youre.co.uk link!

 

As for the question not a clue, but all this talk of snipers got me thinking about a book I read when in Thailand.

 

Sniper One, excellent read!

[Al Massey]

Point no.2 I dont know why you've decided to use an analogy of pointing 2 rifles at the ground then dropping a bullet while firing the second, the outcome is obvious, your point is not. READ my post again, with the rifle parallel to the ground fire a bullet, as you do this drop a bullet at exactly the same time from the same height as the muzzle. Physics tell us that both will hit the ground at EXACTLY the same time just the fired bullet will be somewhere in the distance. This is also fact.

 

So are you saying if we had a rifle 5 ft of the ground and parallel.

And then fired a round aswell as drop a round they will both hit the ground at the same time?

[Scott]

So are you saying if we had a rifle 5 ft of the ground and parallel.

And then fired a round aswell as drop a round they will both hit the ground at the same time?

 

Exactly!

 

Gravity is constant.

[bigbloodyturbo]

So are you saying if we had a rifle 5 ft of the ground and parallel.

And then fired a round aswell as drop a round they will both hit the ground at the same time?

 

If the barrel is parallel to the ground yes. This means none of the kinetic energy imparted on the projectile is being used to counter gravity i.e. gain height. The bullet will immediately start to drop as it leaves the barrel, a .220swift will fire a bullet through the same hole at @100yrds however it will have dropped very very slightly, take the same shot a 300yrds and it will have dropped a huge amount as although its fast, its only a 57 grain bullet. Point is bullet drop happens the second the bullet exits the barrel at exactly the same rate as it would whether it was stationary or super sonic.

[Al Massey]

Exactly!

 

Gravity is constant.

 

So how long do you think it will take for a round to hit the ground when dropped from 5 ft?

[SimonB]

So are you saying if we had a rifle 5 ft of the ground and parallel.

And then fired a round aswell as drop a round they will both hit the ground at the same time?

 

That's basically correct. The only force making both bullets head for the ground is gravity, and it acts on both the same. Being mega picky it may not be quite correct though, because if the bullet travels far enough the curvature of the earth will effectively make the ground fall away underneath it, so it will take very slightly longer to hit the ground.

[bigbloodyturbo]

So how long do you think it will take for a round to hit the ground when dropped from 5 ft?

 

10m/s2, figure out the acceleration of that, I haven't done thise since school so I dont know.

[Gaz6002]

Is there a comedy fail image?!

 

It's next to the orange car fail image.

[DJButler]

Right, you've all had your fun, now let's put this one to bed with some boring maths

 

First off, let's set some parameters which reflect what the OP was getting at: "Is it actualy possible to dodge a bullet after you see the barrel flash?"

 

Assumption 1: You can see the muzzle flash. There are no suppressors, he isn't looking the wrong way. His guidedog is awake, etc, etc.

 

Assumption 2: "Dodge" means "avoid being physically hit". Ignore shockwaves, vorticies, etc, etc.

 

Assumption 3: Muzzle velocity = just supersonic (350m/s)

 

Assumption 4: Distance required to move to avoid being hit = 2m in any direction.

 

Assumption 5: The target is a healthy male weighing 90kg (14 stone) In a life-threatening situation his leg muscles can exert a force of 775N, the same as a sprinter in the starting blocks (source: http://www.elitetrack.com/article_files/newtonian-sprinting.pdf)

 

Assumption 6: His reaction time is 0.5s from seeing the flash to starting to move.

 

Assumption 7: For reasons which will become obvious later on, the shooter is 414m away

 

Right, easy part first. At 350m/s, the bullet will take 1.18s to cover the 414m from muzzle to target. This is neglecting air resistance, so in reality it will be longer.

 

The light from the muzzle flash travels at 300,000,000m/s and takes 1.38 microseconds to reach the target's eyes. Given the very rough calculations this is small enough to ignore.

 

So, with a 0.5s reaction time, the target has 1.18 - 0.5 = 0.68s in which to move 2m to avoid the shot.

 

Now on to the person on the receiving end:

 

Remember he weighs 90kg and can exert 775N of force. Newton's laws of motion tells us that he can therefore accelerate at 775 / 90 = 8.6m/s^2 This is assumed to be linear, although it certainly won't be in real life.

 

He has to cover 2m to avoid the bullet. By taking one of the o-level physics linear motion equations and rearranging:

 

s = u.t. + 0.5.a.t^2

 

Where:

s = distance covered

u = initial speed

t = time

a = acceleration

 

In this case, we are starting from rest, so u = 0 so the equation becomes:

 

s = 0.5.a.t^2

 

Now all we have to do is rearrange so that t becomes the subject:

 

t = (2.s / a)^0.5

 

So now given s (2m) and a (8.6m.s^2) we can work out that covering 2m will take 0.68s

 

Well, waddya know? It takes exactly the same time for the bullet to reach the target as it takes to move 2m out of the way, so in this case, given all the assumptions above (most importantly the muzzle velocity and range) yes, the target will get out of the way in time.

 

If you want to play "what if", (or more likely find an error in my maths) I've attached the spreadsheet to this post.

 

 

These calculations are great, now tell me has the rifling got a common right hand twist or the more unusual left hand and secondly is the target going to move to his right or left, if the common right hand twist is used i would advise the target to move to his right when facing the shooter. This will ensure he obtains every last bit of advantage he can get, there are reasons for this if you study ballistics like the SASC used to.

 

( SASC = Small Arms School Corps , used to be based near Hythe iirc)

[Al Massey]

Well you are wrong.

The time it would take for the round to hit the ground when dropped is a lot less than the time it will take the round fired.

Why because the round fired horrizontally has a huge force behind it proppelling it forward.

The aerodynamics and the twisting motion(through rifling) will keep it straight longer and maintain a speed.

[Al Massey]

( SASC = Small Arms School Corps , used to be based near Hythe iirc)

 

We still study it at great lenght and even have to teach a lesson on it

[Kranz]

10m/s2, figure out the acceleration of that, I haven't done thise since school so I dont know.

 

Oooh as we're geting into complex maths lets use 9.807m/s^2 lol

 

Speaking of Hythe (or Lydd) lets bin the maths and put it to the test.... any volunteers

[bigbloodyturbo]

Well you are wrong.

The time it would take for the round to hit the ground when dropped is a lot less than the time it will take the round fired.

Why because the round fired horrizontally has a huge force behind it proppelling it forward.

The aerodynamics and the twisting motion(through rifling) will keep it straight longer and maintain a speed.

 

 

Why would a force pushing the bullet FORWARD cause it to oppose gravity when it is fired parallel to the ground? Your statement doesn't make any sense.

 

And why do you think rifling makes a bullet stay in the air longer? If I was to spin a stationary bullet up to the same rpm as a fired bullet would it float in mid air also?

[DJButler]

C'mon Whitefeather lets hear your opinion

[Al Massey]

10m/s2, figure out the acceleration of that, I haven't done thise since school so I dont know.

 

The rate of acceleration of a dropped object is 32 feet per second.

So a round dropped from 5 foot will take 0.5 seconds to hit the floor.

A round fired from a rifle will take considerably longer than 0.5 seconds to hit the floor

[Scott]

The rate of acceleration of a dropped object is 32 feet per second.

So a round dropped from 5 foot will take 0.5 seconds to hit the floor.

A round fired from a rifle will take considerably longer than 0.5 seconds to hit the floor

 

Only if fired upwards, paralell to the ground gravity is still constant.

 

Gravity isn't something that changes with velocity. The only reason that a bullet may take VERY slightly longer is the curvature of the earth. This wouldn't be considerably longer though.

 

Edit

 

That's basically correct. The only force making both bullets head for the ground is gravity, and it acts on both the same. Being mega picky it may not be quite correct though, because if the bullet travels far enough the curvature of the earth will effectively make the ground fall away underneath it, so it will take very slightly longer to hit the ground.

 

[bigbloodyturbo]

The rate of acceleration of a dropped object is 32 feet per second.

So a round dropped from 5 foot will take 0.5 seconds to hit the floor.

A round fired from a rifle will take considerably longer than 0.5 seconds to hit the floor

 

Seeing as how your trying to re-write the laws of physics would you care to explain why you think a bullet fired from a rifle defies the laws of physics?

[Al Massey]

Why would a force pushing the bullet FORWARD cause it to oppose gravity when it is fired parallel to the ground? Your statement doesn't make any sense.

 

And why do you think rifling makes a bullet stay in the air longer? If I was to spin a stationary bullet up to the same rpm as a fired bullet would it float in mid air also?

 

Rifling causes the bullet to spin rapidly.

Depending on the rifling will depend on the RPM of the bullet.

The shape and spin of the round causes the round to cut through the air easily using aerodynamics.So the air resistance or drag is greatly reduced.

 

The force pushing the object will make it travel further before hitting the ground.its common sense.

Maybe you should use it.

 

Heres an exapmle for you.

Get a piece of paper and a pen.

 

Now draw a box.

 

Now draw a line from the top left corner to the bottom right corner.

 

Now go get a ruler.

 

Now measure the line you just drew.

Now measure the left hand line of the box.

Which is longer?

 

Its the diagonal one right?

Yeah course it is.

 

Now if you had 2 objects travelling down both lines, which one will get to the end first?

[Al Massey]

Only if fired upwards, paralell to the ground gravity is still constant.

 

Gravity isn't something that changes with velocity. The only reason that a bullet may take VERY slightly longer is the curvature of the earth. This wouldn't be considerably longer though.

 

Edit

 

 

 

 

Seeing as how your trying to re-write the laws of physics would you care to explain why you think a bullet fired from a rifle defies the laws of physics?

 

 

Ok lets start again.

We are on a range and my target is 400 meters away.

I am on a box 5ft in the air holding my rifle with a round in the chamber.

you are sat next to me holding a round.

As you drop the round i pull the trigger.

In the 0.5 seconds it takes for your round to hit the floor my round is still in the air

[Scott]

Rifling causes the bullet to spin rapidly.

Depending on the rifling will depend on the RPM of the bullet.

The shape and spin of the round causes the round to cut through the air easily using aerodynamics.So the air resistance or drag is greatly reduced.

 

The force pushing the object will make it travel further before hitting the ground.its common sense.

Maybe you should use it.

 

Heres an exapmle for you.

Get a piece of paper and a pen.

 

Now draw a box.

 

Now draw a line from the top left corner to the bottom right corner.

 

Now go get a ruler.

 

Now measure the line you just drew.

Now measure the left hand line of the box.

Which is longer?

 

Its the diagonal one right?

Yeah course it is.

 

Now if you had 2 objects travelling down both lines, which one will get to the end first?

 

No offence intended here but what you are writing now is a load of bull and is making you look very silly and ignorant.

 

Have you ever done physics? Do you understand equations that use gravity? Gravity is constant (at a given altitude or distance from source), a constant is something that doesn't change. This means that no matter what speed, kinetic energy, will power, hope or pride is involved the force is still the same.

 

The only thing that combats gravity is lift. A bullet doesn't have lift as its constantly spinning. Your theory of a bullet "cutting" through the air is a load of crap too. A bullet spins to keep its trajectory, nothing more nothing less. If spinning meant less friction then the noses of airplanes would do it.

 

You are stating your opinions on all of this, we are giving you facts and laws.

[bigbloodyturbo]

Rifling causes the bullet to spin rapidly.

Depending on the rifling will depend on the RPM of the bullet.

The shape and spin of the round causes the round to cut through the air easily using aerodynamics.So the air resistance or drag is greatly reduced.

 

The force pushing the object will make it travel further before hitting the ground.its common sense.

Maybe you should use it.

 

Heres an exapmle for you.

Get a piece of paper and a pen.

 

Now draw a box.

 

Now draw a line from the top left corner to the bottom right corner.

 

Now go get a ruler.

 

Now measure the line you just drew.

Now measure the left hand line of the box.

Which is longer?

 

Its the diagonal one right?

Yeah course it is.

 

Now if you had 2 objects travelling down both lines, which one will get to the end first?

 

 

Your starting to sound very silly. First of all your the one not using common sense. Since your a military man tell me this, what ballistics table have you EVER seen with a straight line on it. Your analogy using the box assumes the bullet is travelling at the same speed the whole way through its journey which I alread yproved to you it doesn't.

 

Second of all no-one is saying the fired bullet wont travel further than the dropped one, of course it is. What I (and most others) are saying is they will hit the ground at the same time because the both weigh the same and they both started of at the same height. And rifling of the barrel does not improve aerodynamics other than stopping the bullet tumbling, the bullet is conical and therefore looks the same from all sides, its profile dictates its drag through the air.

[Conrad]

Rifling causes the bullet to spin rapidly.

Depending on the rifling will depend on the RPM of the bullet.

The shape and spin of the round causes the round to cut through the air easily using aerodynamics.So the air resistance or drag is greatly reduced.

 

The force pushing the object will make it travel further before hitting the ground.its common sense.

Maybe you should use it.

 

Heres an exapmle for you.

Get a piece of paper and a pen.

 

Now draw a box.

 

Now draw a line from the top left corner to the bottom right corner.

 

Now go get a ruler.

 

Now measure the line you just drew.

Now measure the left hand line of the box.

Which is longer?

 

Its the diagonal one right?

Yeah course it is.

 

Now if you had 2 objects travelling down both lines, which one will get to the end first?

 

Wouldn't that just mean the line/bullet travelled a further distance, nothing to do with time?

 

That said, I haven't a clue which of you guys are correct.

 

Just out of curiosity though, what's the range of a standard bullet you fire in the army? Then what's the speed of that bullet? Surely we can work out how long it takes it to hit the ground from that?