# The Naked Scientists Forum

### Author Topic: Does the Coriolis Effect Alter the Path of Bullets or Shells?  (Read 18397 times)

#### Nobody's Confidant

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##### Does the Coriolis Effect Alter the Path of Bullets or Shells?
« on: 28/02/2008 16:32:27 »
No guarantees on spelling, I heard this mentioned in a video game, when you are attempting to snipe someone from a very far distance with a .50 cal sniper.

I would like to know what it is, if it even exists.
« Last Edit: 04/03/2008 11:26:22 by chris »

#### another_someone

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #1 on: 28/02/2008 18:18:10 »
It exists, and is responsible for much of our weather (the cyclones and anticyclones); but I cannot see how it would effect a bullet (unless it is travelling over an appreciable fraction of the Earth's surface).

http://en.wikipedia.org/wiki/Coriolis_effect
Quote
The Coriolis effect is an apparent deflection of moving objects from a straight path when they are viewed from a rotating frame of reference. The effect is named after Gaspard-Gustave Coriolis, a French scientist who described it in 1835, though the mathematics appeared in the tidal equations of Pierre-Simon Laplace in 1778. The Coriolis effect is caused by the Coriolis force, which appears in the equation of motion of an object in a rotating frame of reference. Sometimes this force is called a fictitious force (or pseudo force), because it does not appear when the motion is expressed in an inertial frame of reference, in which the motion of an object is explained by the real impressed forces, together with inertia. In a rotating frame, the Coriolis force, which depends on the velocity of the moving object, and centrifugal force, which does not, are needed in the equation to correctly describe the motion.

Perhaps the most commonly encountered rotating reference frame is the Earth. Freely moving objects on the surface of the Earth experience a Coriolis force, and appear to veer to the right in the northern hemisphere, and to the left in the southern. Movements of air in the atmosphere and water in the ocean are notable examples of this behavior: rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right (left) of this direction north (south) of the equator. This effect is responsible for the rotation of large cyclones (see Coriolis in meteorology).

 In the inertial frame of reference (upper part of the picture), the black object moves in a straight line. However, the observer (red dot) who is standing in the rotating frame of reference (lower part of the picture) sees the object as following a curved path.

#### daveshorts

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #2 on: 28/02/2008 19:21:52 »
Essentially the corriolis effect is due to the fact that you are moving at a different speed to someone at a different latitude. The effect would be strongest on the north pole where if you fired from 1km one side of the pole to 1km on the other the difference in velocities would be by my calculation about 14cm a second. The bullet would take at least 3-4 seconds to travel this far so you would have a correction of at least 45cm to make due to the Coriolis force if you wanted to hit a person, although at this range hitting anything would be very impressive.
Of course at lower latitudes the Coriolis force will be less significant.

#### JimBob

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #3 on: 28/02/2008 19:33:40 »
It is also responsible for the way water moves when going down the drain. Clock wise in the northern hemisphere and counter-clockwise (anti-clockwise for our English friends.) It has to do with a change in vectors in the different hemispheres. I can't remember the exact formulas that explain it but I am sure the exist somewhere on the web.

#### daveshorts

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #4 on: 28/02/2008 19:40:50 »
Naah unfortunately it isn't big enough. Although the coriolis effect does have an effect it is only just measureable on a huge tank that has been left to stand for a fortnight. In a conventional plughole there will be much greater effects due to the way you poured the water in, how you got out of the bat etc.
Cyclones and anticyclones on the other hand are big enough for coriolis to have an effect. Air circles a low pressure area anticlockwise in the northern hemisphere and clockwise in the southern, for the same reason that if you look at a spinning object from one side it is going clockwise, and from the other anticlockwise.

#### Ophiolite

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #5 on: 29/02/2008 01:02:07 »
As daveshorts has noted the effect with small arms fire is generally so small as to be insignificant. However it must be accounted for in the case of long range artillery.

#### JimBob

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #6 on: 29/02/2008 02:04:58 »
Naah unfortunately it isn't big enough. Although the coriolis effect does have an effect it is only just measureable on a huge tank that has been left to stand for a fortnight. In a conventional plughole there will be much greater effects due to the way you poured the water in, how you got out of the bat etc.
Cyclones and anticyclones on the other hand are big enough for coriolis to have an effect. Air circles a low pressure area anticlockwise in the northern hemisphere and clockwise in the southern, for the same reason that if you look at a spinning object from one side it is going clockwise, and from the other anticlockwise.

Dave, before I posted my comment on drains, I couldn't remember which way it moved in the northern hemisphere. I stoppered a bathroom basin and filled it with water. The water was turned off and allowed to come to rest. Then the stopper was pulled. (Actually the drain was opened from below by the remote mechanical plugging mechanism.) It was a clockwise rotation of the water down the drain. I repeated this twice, with the same results. Try it. I'll bet you cannot get it to go anticlockwise.

#### daveshorts

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #7 on: 29/02/2008 09:41:14 »
You see according to the Coriolis effect it should go down anti=clockwise in the northern hemisphere. Try filling the basin with the other tap, or swirl the water slightly with your hands in the basin, or it could even be that your sink is slightly twisted so it has a preferred way of water flowing down it.

#### paul.fr

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #8 on: 29/02/2008 11:49:07 »
Meterologically speaking, the Coriolis has a noticeable effect on the upper atmosphere, the 300 and 500MB charts. Influencing the wind direction and jets.

Ground or lower level effect i think is pretty much unnoticeable. I think MIT 'did the experiment' a few years back using wide shallow satellite type dishes. The water, with a few corks, was left to settle for a number of weeks and after that a very small cork plug was removed.

It took a large number of hours for the water to drain, and the Coriolis was noticeable. I don't think you can repeat this with much, if any accuracy, by using you kitchen sink.

#### lightarrow

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #9 on: 29/02/2008 12:14:41 »
No guarantees on spelling, I heard this mentioned in a video game, when you are attempting to snipe someone from a very far distance with a .50 cal sniper.

I would like to know what it is, if it even exists.

If you shoot towards north, Coriolis force makes the bullet bend towards East (and towards west if you shoot towards south), if you are in the northern emisphere, the opposite in the southern emisphere. The formula is:

Fc = -2mω X Vr

Fc = Coriolis force
m = bullet's mass
ω = Eart's angular velocity (of spin)
X = vectorial product
Vr = bullet's velocity

At 45° latitude, considering a total bullet's straight trajectory of 8 km, travelling at an average speed of 800 m/s (just invented data, not sure how that distance and average speed could be realistic for such distance) then the total bullet's shift towards east is about 4m.

If the average speed is proportionally smaller or greater, the shift will be inversely proportional smaller or greater, that is the shift is doubled if the speed is halved (with the same distance travelled); if the total distance travelled is proportionally smaller or greater, then the shift will be square proportionally smaller or greater (with the same average speed); for example for half distance the shift will be (1/2)2 = 1/4 --> 1m; for 3 times the distance the shift will be (3)2 = 9 --> 36m; the effect becomes more and more significant for long distances.

You can enjoy evaluating the shift with different values of speeds and distances.

This computation is very approximate because the trajectory can't be straight at all in this case (and I'm not referring to Earth's curvature only, but mainly on bullet's parabola).

Note that it's not a real force but an "apparent" force, as centrifugal force, due to the fact Earth is a non-inertial frame of reference. Another effect of that force is the more consumption of the right rails for trains travelling towards North (in the northern emisphere, ecc.)
« Last Edit: 01/03/2008 04:08:49 by lightarrow »

#### Nobody's Confidant

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #10 on: 29/02/2008 17:32:03 »
Alright, thanks everyone.

#### lightarrow

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##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #11 on: 01/03/2008 04:10:48 »
I have corrected my previous post, there was a mistake. As I've written now, the bullet's shift is doubled if the average speed is halved (at the same distance).

#### The Naked Scientists Forum

##### Re: Does the Coriolis Effect Alter the Path of Bullets or Shells?
« Reply #11 on: 01/03/2008 04:10:48 »