Okay, so you've read the frames of reference
explanation, right? If not, go do that, I can wait.|
Done? Okay. In the pictures on this page, the black cloth is an inertial
frame. It can be treated as standing still, and things will move in a
straight line as far as observers standing in it are concerned. The yellow
circle is a non-inertial rotating frame, and observers standing in it will
see things go a little wonky. The frame effects of Coriolis and centrifugal
forces will make motion seem strange to someone standing on the yellow
We start with our viewpoint character standing in the rotating frame, a
spinning disc. We're going to assume the missile is effectively weightless,
so we can ignore gravity and just keep things in a two-dimensional
The character has shifted around on the rotating platform. As far as he's
concerned, he shot his missile straight out, and it should still be in front
of his face, moving away. Instead, it looks like it has curved off to the
To someone watching from the black field, the missile actually drifted to the
left, since it had its outward tan speed plus the green spin speed, and you
need to add these together to find the total speed. But it didn't move to
the left as much as our hero's viewpoint (the green missile) did.
Same idea, but instead of shooting straight out, we're now trying to shoot
along the direction of the spin, to see if that changes anything.
In this case, an outside observer on the black surface would see the missile
going in a straight line in the direction it was fired, but a little faster
than the firer sees it. However, while the missile travels, the firer's idea
of what "forward" means changes, so the missile seems to be forced to the
Now let's try shooting backwards, and we'll shift to a little robot firing
balls, so we don't clutter the field up with huge missiles. The robot is on
the same rotating platform, with a speed at the edge shown by the green
arrow. He's going to fire against that direction. However, how fast his
shot travels is now important! You see, if you look at the overall frame
effect, it could be positive or negative depending on the launch speed.
If he shoots too slowly, the actual speed of the shot will be to the left,
even though he's facing the right in this picture. In other words, if the
green arrow is a speed of 100 meters per second to the left, and the ball
leaves the gun at 50 meters per second to the right with respect to the
gun, then to someone standing on the black field it will be going to the
left at 50 meters per second.
And here's where it gets really weird. From the perspective of the robot,
his shot "should" travel along the yellow line. Where the shot ends up,
however, depends on how fast he fired.|
The white ball is a shot fired at exactly the same speed as the turntable's
edge was moving, so the total speed from the point of view of an outside
observer is zero. The shot just hangs there if there's no gravity. To the
robot, though, it looks like the shot has curved to the right, hugging the
edge of the turntable.
The green ball was fired even faster than this, and it also appears to curve
to the right of the yellow "straight" trajectory. This works pretty much the
same way as in the case in figure 4.
But the orange ball was fired too slowly, or even just released at no speed
relative to the shooter. It has a total speed to the left, so it appears to
go forward and to the left...the slower its firing speed, the more
significantly it curves to the left.
So, if you put something in motion "backwards", it may curve to the right, or
to the left. You can even see a place where the yellow and gray lines cross,
so the shot appears to be on the straight course, if only for an instant
(eventually the shot will have to go off the yellow line, if only because
after half a rotation the yellow line won't intersect the gray one
So, in summary, there's no actual force making the missile
veer to the right. Instead, the shooter's idea of what "forward" means is
shifting to the left as the rotating frame of reference spins. The
only force involved is the one that makes the observer go around in a circle,
anything that seems to happen to the missile is just a result of the
observer's shift in perspective. This is pretty much the frame effect in a
nutshell, although once you go to higher dimensions (like, on a sphere), the
details can get tricky.
Back to the Main Coriolis force Page.