# Cars on TV

Why is it that when I see pictures of cars on TV or on videos, & they are obviously moving forward, that their wheels appear to be going in reverse?

Forgive the non-scientific reply. It has to do with the brain receiving signals from the eyes. The holes or spokes in a rim spinning at different rates of speed are a factor in this. How is that for armchair science?

This is not just on film. Next time the guy passes you on the highway and just hangs out next to you, you might notice it then.

It has to do with the frame rate of the camera vs the speed of rotation of the wheels. Say the camera records 30 frames per second. If the wheel is turning exactly thirty times per second, the wheel will appear motionless on camera. If it’s turning just under thirty times per second, it will appear to be turning backwards. How far or near the speed is from a multiple of the camera speed determines how you’ll see the wheels move.

You can get a similar effect from your naked eye, but it isn’t as consistent or pronounced.

I can’t offer a better explanation than the others have, but if you look at the propeller of an airplane or helicopter, you will notice a similar illusion.

OK That makes sense. Thanks.

OK, I hope this doesn’t get too long. Movies and television display a series of still pictures taken in rapid succession. For a TV its 60 per second, for movies it’s 25. The brain interpolates (fancy term meaning it fills in the gaps) these series of still pictures into smooth apparent motion. Now, suppose there is a big white dot on the side of a black tire, and the tire is rotating at a speed such that when the first picture is taken the dot is top center and the next it has turned 3/4 of a full turn. Your brain interprets that as having moved back 1/4 instead of forward 3/4 because it’s closer and that makes more sense to our poor brains which are geared to tracking moving game, etc.