Coriolis Force in Two Dimensions
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 circle.

 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 plane. 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 right.     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 right again. 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 anymore).

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.