# The effect of gravity on a speeding bullet

Discussion in 'Physics & Math' started by Magical Realist, Jan 31, 2017.

1. ### RajeshTrivediValued Senior Member

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The ideal definition of terminaal speed is when the object acquires steady state speed against the drag, that means the net force (and thus the net acceleration) on the object becomes zero. What you and sarkus are proposing is a small tinkering in definition wherein you are claiming that vertical compoenet can stabilise and can be called as terminal speed while still maintaining a variable horizontal velocity component. I am not sure if the vertical component can stabilise in presence of a horizontal velocity component, as the drag force is quite complex and dynamic. The drag force in vertical direction for (100, 500) system is not same as (0, 500) system [where (100, 500) denotes 100 m/sec as horizontal speed while 500 m/sec as vertical speed].

3. ### exchemistValued Senior Member

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I see what you are saying. I am relying here on what Wiki for example says about the meaning of terminal velocity: https://en.wikipedia.org/wiki/Terminal_velocity

However if you use it to mean the velocity - in any direction - at which drag counterbalances whatever force would otherwise cause an acceleration, then you are of course quite right. In this sense, you could speak of the terminal velocity of a car or a train, meaning its maximum speed. I don't believe I have ever heard the term used that way, myself.

5. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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First, I'm not sure I argued that it could maintain a variable horizontal velocity, only that it need not be zero, as you claimed.
Secondly, terminal velocity is usually in reference to the vertical speed under the effect of gravity. The "ideal definition" you speak of is not the widely understood one.
Indeed, but that complexity is what aeronautical engineers deal with day in and day out in their design of aircraft, aerofoils, etc.

7. ### RajeshTrivediValued Senior Member

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Not much dispute here.

The bullet has a horizontal velocity (aircraft speed) at blast along with vertical shoot velocity. You are right that the terminal velocity is usually in reference to the vertical speed under the effect of gravity, what it means is that the acceleration should be in the direction of motion, then only drag deceleration and accelreation can nullify resulting into steady terminal speed. But for the case in hand this situation will not arise till the horizontal component becomes zero from the intial aircraft speed of around 200 meter per seconds. Only when this becomes zero due to drag, the possibility of terminal speed can be seen, as now it is the true vertical drop.

Yours and exchhemist post gave an impression that even if we have non zero horizontal speed component, the vertical component will stabilize to terminal speed, no it will not.

8. ### sculptorValued Senior Member

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original question:
Suppose I shot a bullet towards earth from a high flying airplane. Would the bullet: ...
(by "towards" I had assumed "straight down")

How did horizontal speed enter this fray?

9. ### KittamaruNever cruel nor cowardly...Staff Member

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Rajesh brought it up, seemingly in an attempt to discredit arguments by making it appear we are "lacking insight" into how a flying plane works.

sculptor likes this.
10. ### sculptorValued Senior Member

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OK
Is the horizontal component based on the speed of the aircraft a significant factor?

11. ### RajeshTrivediValued Senior Member

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Your anti trump posts are quite involved and insightful and I must complement you on your decent understanding of US politics.

12. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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I have been discussing vertical terminal velocity, not speed.
Further, such a terminal velocity will be achieved at the same moment that horizontal motion has zero net force effect on the vertical motion. As explained, with aerodynamic design you can achieve this with there still being a horizontal velocity. You will then end with a terminal vertical velocity, a terminal horizontal velocity, and an overall net terminal velocity (in the direction of travel).

13. ### exchemistValued Senior Member

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Yes it will, UNLESS the bullet has an aerodynamic shape that gives it lift due its horizontal component of motion. If we assume a shape that gets no lift, then the vertical component of motion will be entirely independent of horizontal motion, as there will be no force with a vertical component acting, apart from gravity.

If all this palaver of yours is a roundabout way of finally getting aerodynamic lift into the discussion then - wearily - yes OK fine, but this is now miles from the original subject of the thread.

14. ### originTrump is the best argument against a democracy.Valued Senior Member

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You are wasting your time. It certainly seems that Rajesh just likes to argue (troll).
He seems to have only an infinitesimal understanding of physics with no desire to further his understanding and simply likes to rile people up by obfuscations and goal post moving.
His MO is like 'the gods'; a whole lot like 'the god' come to think of it.

That is why I have them 'both' (tee-hee) on ignore. Putting them on ignore has raised the quality of the discussions on the site immeasurably.

15. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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If perceived similar to a "Bomb" dropped from an airplane, then Horizontal Motion should be considered as a factor.
Although, if memory serves, said Horizontal Motion - as the "Bullet/Round" is initially fired it carries the forward speed of the Airplane with it - is affected only by 'Drag'.
In the case of "Bombs", I am fairly certain that that forward speed never actually slows to zero, until after impact of course.
Again, if I remember that "Period of Instruction" correctly, figuring the Horizontal and Vertical aspects of the "Bombs" trajectory may be simplified if considered separately.
For that reason, it would seem, to me at least, that the Horizontal Motion would act accordingly on a "Bullet/Round", would it not?

Last edited: Feb 10, 2017
16. ### RajeshTrivediValued Senior Member

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Ok, consider following two scenarios

1. when the bullet is shot downwards (800 m/sec) it has no horizontal component (hovering aricraft).
2. When the bullet is shot downwards (800 m/sec) it has 200 meter sec as horizontal component (flying aircraft at 200 m/sec).

Now as per you in both the cases bullet will acquire same terminal speed, at the same altitude and after the same time. This is what you are saying ? My answer is No.

17. ### RajeshTrivediValued Senior Member

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No, you should admit that horizontal speed did not even come to your mind, you were just referring to drop like scenario. Why dont you respond to the poser I have for Exchemist. Lets see about your profound knowledge of Physics.

18. ### RajeshTrivediValued Senior Member

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Overall net terminal velocity ?? You seem to be adding some new variable here.

In horizontal direction in present case there is no acceleration,only drag force, so the non zero horizontal speed can only stabilise when it becomes zero, there is no horizontal terminal speed or no net terminal velocity......

19. ### exchemistValued Senior Member

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Yes. And I have now had enough of this red herring.

20. ### exchemistValued Senior Member

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Ah, wise words glasshopper.

The [click] is strong in me.......

21. ### RajeshTrivediValued Senior Member

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Sarkus,

I think this will help.

The drag force on a velocity vector (200i + 800 j) is not same as vector sum of drag forces on 200i and 800j separately. Thats the mistake you are making. So, if there is horizontal component it is bound to impact the terminal speed, rather the bullet cannot stabilize till this horizontal component becomes zero.

PS : The drag force is so complex that even the drag coefficient is object speed dependent, shape dependent, air flow dependent etc.

22. ### RajeshTrivediValued Senior Member

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This is Physics, not red herring. Do yourself a favor, refer to basics on drag force etc and I am sure your response would change to 'NO" from "Yes" on these scenarios. Choice is yours.

23. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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RajeshTrivedi, as I opined in my ^above^ Post #112, I am fairly certain that that "horizontal component" never actually slows to zero, until after impact of course.
You may find this over a century old article : "Dropping Bombs From Flying Machines" by Riley E. Scott, an insightful read.
- https://www.scientificamerican.com/article/dropping-bombs-from-flying-machines/

edit : came across the following, in .pdf format : "Bomb Trajectories" , it contains numerous formulas that may assist in answering your query.