hi all you geniuses out there....(is it geniuses? i don`t know the plural form for that paricular word....geni-ii perhaps?)

this is a post regarding two of newtons laws (one and three i think). the subject is a question about wether a projectile launched from a standing point will reach its terminal velocity and at azimuth on its return equal or exceed its original speed. <i`ve been reading this again and felt i should re-word a little of this particular sentence/question.> "the projectile launched from a standing point will achieve its terminal velocity after reaching azimuth and is on its downward trajectory."

the projectile in question is a round fired from a Ak-47 (7.62x39mm) with a weight of 8 grams fired at a muzzle velocity of 710 m/s.

the argument my friend and i are having is that the projectile will not be travelling at the same speed on its return to earth as it was on its way to its azimuth. my side of the argument is bolstered in the logic that although what goes up must come down, the initial speed of the projectile leaving the barrel is magnified by the barrel and concussive force of the explosion from the powder in the casing, when the actual projectile reaches its natural velocity due to its mass after the initial propulsion it will settle at its terminal velocity and return to earth at this speed plus or minus natural occuring resistance i.e: gravity, wind resistance etc. my friends argument is that these factors taken into account, the projectile will still return to earth at the same speed it left it.

the origin of this topic came about after watching yet another middle eastern machine gun toting fellow blasting off rounds with total disregard for the fact that these rounds can and will come back down, sometimes and most likely causing another casualty somewhere in the vicinity of the shooter.

this argument has been going for a week now and only now have i decided to end it once and for all.

please solve this conundrum for us as everyone is getting over the same running debate. cheers and hello from australia to you all. McNuggy

Curlyben

I like where you are coming form with this, please bear with me as its been quite a few years since I studied Physics.
OK so this is really a two part question.
1/ How high does the bullet get.
After a quick search I found This.
2/ What is it's terminal velocity coming down.
So if we assume that it gets to a mile (for ease), that's 1609 meters.
After another quick search I found an online Calculater.

So let's throw our numbers into the equation;
Mass is 0.08 Kg
For this I used Cross section to be 0.01197 M^2
Left the drag as is in the calculater
Altitude we already have.
So from these figures we get a Terminal velocity of 13.379 m/s, lots lower than the original velocity.

Bear in mind this is a very quick and dirty calculation, but with some more time a better answer could be achieved.

Hope this helps a little.

Even with a drag of Zero Terminal velocity is only 111.94 m/s

McNuggy@LMC

thanks ben, the actual specifics of speed and height is a mott point here, some would say otherwise but objectively speaking the answer to the question indeed is that i am right on this one....as the majority of my friends and i thought.

that the object is being propelled by force external to its relative position and inertial value should have been enough for my friend to accept the newtonian law of inertia on its own but as usual he is the most stubborn person i know.

indeed now he is asking "well why is it that if i peg a tennis ball into the air, it basically comes down at the same speed i threw it at...?". my answer to this is that it is not travelling at the same speed, you are exerting force on an object as it leaves your hand, the mass of the tennis ball does not change and will still be of a uniform speed consistent to its trajectory and weight.

bear in mind i am also an amateur if not a totally unschooled observer on these issues.

thanks for your research curlyben, it has silenced him a little but we do look forward to more answers to this as i`m not totally satisfied until he is undeniably proven incorrect....cheers all....mcnuggy

dmatos

In a vacuum, with a constant acceleration due to gravity, the bullet would hit the ground at a slightly higher speed than it left the gun from, under the assumption that the gun is higher than the ground.

The velocity of the bullet can be divided into two components - the horizontal component, and the vertical component. With no force acting horizontally on the bullet, the horizontal velocity will remain unchanged. The only force acting on the bullet vertically is gravity. However, if you do the math, you'll find that the object will slow a certain amount (down to zero) as it's going up, but then speed up back to the same speed coming back down. Plug any numbers into the formulas, and that's what you'll see.

Of course, that is based on the assumption that the bullet is in a vacuum. Since it's travelling through air, there is air resistance. It is a force opposite to the direction of motion, and equal to some constant times the cross-sectional area of the object times the speed of the object. The important thing here is that the force is always in the opposite direction to the motion of the object. It will always be slowing the object down.

So, in air (or water, or cow dung, or anything but a vacuum) a bullet fired from a gun will be slower when it hits the ground than when it leaves the gun. I still think the bullets travel fast enough to kill, though . . .