# Maximum speed of falling objects?

Discussion in 'Physics & Math' started by Dinosaur, Mar 2, 2004.

1. ### DinosaurRational SkepticValued Senior Member

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It is my guess, that mass has (little) or no effect on the downward force. This is surely true in a vacuum.

However, the retarding drag force would have more effect on a less dense object. The equivalence of inertial mass & gravitational mass results in cancelling of the mass in gravitational equations. This cancelation is not applicable to drag/lift equations of aerodynamics.

3. ### sifreak21Valued Senior Member

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everything has a terminal velocity.. abullet when fired is instanly WAYYY past that so the milisecond it leaves the barrel "maybe before" the bullet is slowing down

5. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Not quite.
A bullet is still accelerating as leaves the barrel, because it still has gas expanding behind it. (Although, of course, at a much-reduced pressure compared to when it was still in the barrel).

7. ### three-braneRegistered Senior Member

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so like I was saying earlier the density is the determining factor for an objects t.v. atmospheric drag, size, and shape, they effect less dense objects. where as the greater the density the greater the t.v.

8. ### SyzygysAs a mother, I am telling youValued Senior Member

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I always wondered about bullet speed accelerations. I would say it takes several feet before a bullet reaches its maximum speed after leaving the barrel. Why? Because it built momentum in the barrel and it takes a while before the momentum reaches its peak and start to get lost.

I can be wrong on this one but that's how I see it...

Well, how about a thrown rock? Does it reach its highest speed at the time when it is leaving the throwing hand????

9. ### James RJust this guy, you know?Staff Member

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A bullet reaches its maximum speed before leaving the barrel of the gun. And once it has left the barrel, air resistance is working on it all the time, slowing it down.

A thrown rock has its highest speed at the moment it leaves your hand, for the same reason.

Of course, for the rock, if you're throwing it downwards from a high place (plane, cliff etc.) then it can actually speed up after leaving your hand. The same is not true for a bullet, unless you're on the moon.

10. ### SyzygysAs a mother, I am telling youValued Senior Member

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Actually I have to side with Dyw here and I say the bullet still accelerating at least for a few inches after leaving the barrel. The exploding gas behind it still propels it until the pressure quickly can finally go sideways in the air.

When the bullet is in the gun the speed is zero. At one point in its flight let's say the speed is 500 m/s. But obviously from zero it doesn't go to 500 m/s in 1 nanosecond. I would say it needs time to get to its highest speed and it happens outside of the barrel...

Edit: since this is offtopic, I am going to start a new thread on it...

Last edited: Jun 10, 2010
11. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Don't you think the friction in the barrel might be higher than that from air resistance? After all a bullet is larger than the bore size, and while in the barrel has to have the rifling grooves forced into the metal - it's effectively being extruded.

But when you throw a rock there's no propellant gasses still escaping from the barrel and pushing against the rear of it. (Admittedly any extra push would be a minute increment - it also causes some instability until the spin of the bullet takes "control" and stabilises it).

12. ### sifreak21Valued Senior Member

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yeah after i posted this i thought about that so id say what like 3-4 feet out of the barrel it startes slowing down

13. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Far faster than that, I'd say. Any "extra" push from the gasses escaping the barrel will be gone well within a foot.
When I wrote "A bullet is still accelerating as leaves the barrel" I literally meant as it leaves.
I haven't ever found figures (probably because as Phlogistician said in the thread dedicated to the question - muzzle flash makes measurement difficult) but I'd guess that deceleration starts before the bullet's travelled much more than its own length.

14. ### James RJust this guy, you know?Staff Member

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Certainly.

The maximum speed will be before the bullet leaves the barrel and the propellant gases start to escape.

(I could be wrong.)

15. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Take a look at the thread on the subject.