I Have A Bet On.....

[Guest saintgeorge]

with a colleague - he says that a bullet shot straight up in the air lands at the same speed at which it is fired - i say that this is not the case - £5 rides on this - anyone help?

[Guest Bunty_hoven]

Bollocks

[blike]

I would bet against it. Everything has a "terminal velocity" at which the object will no longer accelerate because the force of air resistance equals the force of gravity. A bullet will quickly reach this velocity.

 

The army did some testing on this awhile back. They determined a falling bullet's velocity is around 300 feet/s. Typical muzzly velocity is somewhere near 3000 feet/s.

 

However, in a vacuum, it would return at the same speed.

[blike]

oh, see here: http://www.loadammo.com/Topics/March01.htm

[wolfson]

Initial and Final Velocity are different saint, and bunty what an idiot keep it up and you will be on my ignore list.

[Guest saintgeorge]

yeah, put him your ignore list - that'll learn 'im

[wolfson]

Ok IGNORE, why did you bother joining this group, we really dont need morons here, this is for science DISCUSSION, not for little children to use offensive word(s), so from now on i will not have to listen to your rubbish.

[Guest saintgeorge]

yes, this is a serious forum, bunty hoven, and you are ruining it for everyone - you've embarrassed me, you've embarrassed the members but - most importantly - you have embarrassed yourself.

 

so, the question remains - have i won my bet.

[Sayonara]

There's no reason why it would return at the same velocity.

 

The factors involved are different on the return journey.

[wolfson]

If a ball id throw to its max height, the ball will have a downward accelartion of g (gravity) 9.81 ms-2

 

So if the bullet would fired at 60ms-1

 

Then reached its max height it would traveel down at g.

 

So the inital velocity would be different from the free-fall (g).

[Guest saintgeorge]

so i am right and he is wrong - he won't pay, he says you are all talking rubbish - that's is the kind of guy he is (the kind that says a bullet never loses speed)

[wolfson]

Tell him to look up kinematics, and free-fall.

[Sayonara]

Explain it to him like this:

 

When you fire the gun you give the bullet a tremendous amount of energy, and it leaves the muzzle at high speed.

 

In order to come back down again, it's going to have to slow down. And slow down it does - due to gravity's attraction for the mass of the bullet. In fact it stops the bullet completely.

 

The bullet has now lost all of the energy the gun gave it (disregarding any heat energy that is still held in it).

 

Now it begins its downward journey. The only reason this happens is because the mass of the bullet is subject to the gravitational pull of the planet, which causes an acceleration of 9.81ms^-2. The only other factor on the way down is the resistance of the air, which retards the acceleration due to gravity until the bullet reaches terminal velocity.

 

If he disagrees with this, invite him to test it with a live gun. He might want to consider though that it's not exactly news to the rest of the world.

[fafalone]

It could easily be worked out through simple mechanics incorporating air resistance... but then again anyone who can do that really wouldn't be wondering..

[YT2095]

a bullet from a riffle fired upwards will be quite lethal depending on it`s grain weight and projectile weight. it will also spin as it travels.

whe it reaches it`s maximum height (zenith), it`s forwards momentum and spin will cease.

as it falls it will tumble, and lack the typical charecteristic that makes a bullet so deadly, as well lose the other charecteristic of speed.

it will have a terminal velocity (about 250 KMh) but also be in a tumble. you wouldn`t want one to land on your car roof, or in your eye! but it`s unlikely to kill you in normal circumstances

[swansont]

whe it reaches it`s maximum height (zenith)' date=' it`s forwards momentum and spin will cease.

[/quote']

 

Why would its vertical momentum be coupled with its spin?

[YT2095]

quite simple, it`s the forwards velocity in the barrel that gives the bullet it`s spin, maybe a 1.5 to a 3 twist barrel, it`s spin is dependant on it`s forwards, the grooves left on the projectile stop it`s spin long before it stops it`s forwards, any bullet before reaching target will lose spin before forwards momentum.

the majority of the time it hits something (even if only the ground after a secod or so) but fired at an arc, the loss of spin needed to be compensated for, as well as distance, grains, projectile weight and shape, wind factors, barrel length etc...

[Sayonara]

If the target is ten feet away I doubt the bullet would have stopped spinning.

[blike]

Tell him to throw a feather upwards and see if it falls as fast as he threw it. It's the same concept. The feather reaches its terminal velocity rather quickly (because of surface area and shape). The bullet has a much higher terminal velocity, but it does have one, and it is much lower than the muzzle velocity.

[Dudde]

there isn't any reason (nor reasoning) that the bullet would move the same speed, although the rest of the thread is proving this, so you should be fine

[YT2095]

If the target is ten feet away I doubt the bullet would have stopped spinning.

 

yeah I think I know what line you`re on about, it doesn`t read well on second veiw, the word target in that sentence should be replaced with something else I can`t think of at the moment.

 

in effect what I was TRYING to say that the spin will stop before the forwards when it stops, meaning exhausts all it`s forwards before encountering something.

the grooves will work against it`s spin as it`ll create drag on the spin, the forwards isn`t so bad as the spin will work a bit like a properely blade and "screw" it into the air. when the spin stops it will still have it`s forwards, but not for long, then it will tumble and become eratic, that will then slow it`s forwards even more. the spin like in a gyro will be used up also as soon as the bullet starts to drift creating torsional forces that will slow it.

 

phew, `nuff said for now, things to do

[aommaster]

yes, but they are trying to give a reason why it won't work. They just don't give the answer!

[blike]

What do you mean why? It's been stated several times: Terminal Velocity

[swansont]

quite simple' date=' it`s the forwards velocity in the barrel that gives the bullet it`s spin, maybe a 1.5 to a 3 twist barrel, it`s spin is dependant on it`s forwards, the grooves left on the projectile stop it`s spin long before it stops it`s forwards, any bullet before reaching target will lose spin before forwards momentum.

[/quote']

 

But because you need the spin to stabilize it, I'd expect the optimal spin to be such that it stopped spinning after it's designed range.

 

Anyway, you originally stated that the upward momentum and spin ceased simultaneously at the zenith of the vertical shot, and I wonder if you meant to imply that, since your answer here doesn't support it.

[swansont]

Tell him to throw a feather upwards and see if it falls as fast as he threw it. It's the same concept. The feather reaches its terminal velocity rather quickly (because of surface area and shape). The bullet has a much higher terminal velocity, but it does have one[/i'], and it is much lower than the muzzle velocity.

 

This is certainly an extreme example that demonstrates your point, but even if the bullet were launched at below terminal speed, it would come down slower due to air resistance. Some of the original KE is lost to the air on the way up, and then even more is lost on the way down, so

KE_{ground, initial}>PE_zenith>KE_{ground, final}

 

The final speed has to be smaller than the launch speed, assuming no change in mass.