# The effect of gravity on a speeding bullet

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

1. ### Magical RealistValued Senior Member

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Suppose I shot a bullet towards earth from a high flying airplane. Would the bullet:

A. Keep speeding towards earth at the same speed it left the barrel?

B. Speed up as it approaches earth?

C, Slow down as it approaches earth?

D. Stop suddenly at 10,000 feet?

3. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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C. It would slow until it reached terminal velocity and then the speed would remain constant.

5. ### Magical RealistValued Senior Member

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What force is causing it to decelerate?

7. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Friction from the air.

8. ### Magical RealistValued Senior Member

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What else? I mean supposedly it would slow down in a vacuum too right?

9. ### KittamaruAshes to ashes, dust to dust. Adieu, Sciforums.Valued Senior Member

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An object in motion stays in motion unless acted upon by an outside force.

Thus, in a true vacuum, no, it would not slow down unless gravity (or some other force) acted upon it.

In space, you have gravity, various sources of various types of radiation, et al to contend with... but generally, even in space, it will continue on forever until it hits something or other (either by being pulled in due to gravity, or random happenstance)

10. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Nope, if this scenario was in a vacuum the bullet would speed up. Not sure how the plane would be able to fly though.....

11. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Well, I guess I will have to wait until early March to see if Magical Realist understands what I was saying here...

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Yes.

13. ### sculptorValued Senior Member

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It seems likely that you would have to factor in the effect of the initial supersonic flight of the bullet?
then also the shape of the bullet?

14. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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No it would make no difference the best answer would still be C.
Regardless of the shape the best answer would be C.

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15. ### sculptorValued Senior Member

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Agreed. Perhaps, I was unclear. I was thinking of the rate of change of speed.

16. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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The shape of an object moving through the air has a big effect. A piece of paper will fall more slowly than a dense sphere of the same mass.
Friction increases with speed of the bullet. So in general the faster the bullet the faster the rate of change of speed (rate of slowing). Friction is rather difficult to calculate since you end up with a nonlinear differential equation, hence the often seen statement in introductory physics of 'ignoring the effects of friction'...

17. ### TheFroggerValued Senior Member

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Hello.

Astronauts practice in zero-gravity by boarding a plane and flying it straight towards the Earth at or faster than the speed of gravity. This produces zero-gravity.

Surely throwing something at the ground would take longer to travel to the ground than an object simply dropped, because it is approaching the speed of gravity.

Firing a bullet towards the Earth should have the same effect. It should take longer to arrive than a bullet dropped.

Now, bullet trains are interesting!

If a bullet were fired from a gun on the front of a train travelling at the speed the bullet is fired, would the gun back-fire??

Oh incidentally it IS possible to travel the speed of light. I did it in two-thousand-and-four. I disappeared.

18. ### Michael 345Home just over a week still jet lag sleepyValued Senior Member

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Nope it will keep the same speed as when it left the barrel

19. ### Q-reeusValued Senior Member

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No and no. The plane flies in a parabolic arc that on average is ~ horizontal, so as to satisfactorily approximate gravitational free-fall. Which is an acceleration, not a velocity. Check out 'Einstein's elevator'. Also, the only meaningful definition of 'speed of gravity' is that of gravitational waves which is c - the speed of light.
Given the rest of your post I doubt above comment will have much impact on you but maybe worth a try....

20. ### Michael 345Home just over a week still jet lag sleepyValued Senior Member

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Free fall

Speed of gravity?

Longer?

Again speed of gravity?

Whattttt?

No

Depending on the speeds involved the train may impact on the train at a lower level than the barrel or pass over the bullet on the track

21. ### Michael 345Home just over a week still jet lag sleepyValued Senior Member

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Is it April 1?

22. ### Q-reeusValued Senior Member

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Inside some folk's skulls it probably is in a way always April 1. Regrettably.

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