Q: Why do the wagon wheels on stagecoaches that we see in the
movies sometimes look like they are turning backwards?
A: To see why this happens, assume that a wagon wheel with eight
identical spokes is turning so that every 24th of a second, the
spokes of the wheel line up in the same eight positions.
Q: Why do the wagon wheels on stagecoaches that we see in the movies sometimes look like they are turning backwards?
A: To see why this happens, assume that a wagon wheel with eight identical spokes is turning so that every 24th of a second, the spokes of the wheel line up in the same eight positions.
Now suppose that a movie camera is filming the turning wheel at a rate of 24 frames per second. At this rate, the camera will take a picture of the wheel every 24th of a second. But since the spokes of the wheel are in the same eight positions every 24th of a second, it will appear to the camera that the wheel is not moving. And the viewer will see a wagon rolling along on wheels that are not moving. Everyone who has watched old Westerns has seen this.
Assume now that the turning wheel is slowed down a bit so that the spokes do not quite reach the position indicated above by the time the camera takes the second picture, but are just to the left of it. When the camera takes the picture, the spokes are located just a little before where they were on the preceding picture. On all successive pictures, the spokes continue to move back little by little.
Since this sequence of pictures is all the camera sees, it appears to the camera – and viewers – that the wheel is turning backwards.
