Dear Car Talk:
Why is it that sometimes, when you see a car in a movie, the car is moving forward but its wheels are rotating backward? -- Stan
Good question, Stan. It has to do with the "frame rate" of movie film.
Imagine you're in a room that's pitch black. And there's a single car wheel in the room at the end of an axle, so you can spin it. Now imagine you start spinning the wheel, and you turn on a strobe light. The first time the strobe light flashes, you catch a frozen glimpse of that wheel, wherever it happens to be the moment the light flashes on, right?
Now let's say the speed of the wheel is perfectly matched up to the speed of the strobe light. So every time the light flashes on, the wheel has made one complete rotation. Since you only see the wheel in the same position each time, the strobe light would fool you into thinking the wheel is not moving at all -- when it actually is!
Now let's say the wheel is turning a little slower than the speed of the strobe light. So the wheel makes a little less than one full rotation between flashes. Each time the light flashed on, the wheel would appear to be slightly "behind" where it was last time you saw it. That would effectively trick your eye into thinking the wheel was going backward, even though it's going forward, just more slowly.
Movies work kind of like that strobe light. Most film is shot at a rate of 24 frames per second. Normally, that's fast enough so your eye doesn't notice that it's seeing 24 individual pictures every second. What you see is smooth motion. But when you get something that's moving very quickly, like the spokes of a car wheel, you can get that "strobe" effect. And depending on how fast the wheel is turning, compared with the 24 frames per second the movie camera is taking pictures, the wheel can appear to be moving forward, backward or standing still.
We've tried to take advantage of this phenomenon at the garage. When we fail to fix a customer's car, and they call up to complain that it won't move, we'll sometimes ask them if it could be an optical illusion. Nobody's fallen for it yet, Stan.