Why Do Car Wheels Rotate The Wrong Way In A Movie?
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
11 September 2005
Charles Maina has made an interesting observation: when a car is
accelerating in a movie or on TV, the wheels appear to slow down, stop
and then rotate in the reverse direction. He wants to know, “Why does
this happen? Are my eyes doing tricks on me?”
To understand this phenomenon, we begin by looking at how a motion
picture is created. The camera takes very many pictures in succession
and these are recorded a length of film or on a magnetic tape. When
watching the movie, the pictures are replayed at the same rate that they
were recorded. If the rate is high enough, an illusion of smooth motion
will be created. Movies are recoded at 24 pictures (or frames) per
second (fps) and TV at 30fps.
Suppose the wheels of the car are making one complete rotation every
second, then in a movie recorded at 24fps, each successive frame will
capture a 15 degrees advancement of the wheel. This is simply 360
degrees (full circle) divided by 24 – because there are 24 pictures in
every second.
As the car accelerates, the rate of rotation of the wheels increases.
When it reaches 12 rotations per second, the camera will capture two
frames in every rotation. That is, between any two frames, the wheels
will have made half a turn. When replayed, the tyres will appear to be
vibrating back and forth.
Suppose now that the car accelerates further and the wheels now make,
say 18 rotations per second in a clockwise direction. From one frame to
the next, the wheels will make three quarters of a rotation. When the
film is replayed, the first picture will show the wheel at, say, the 12
o’clock position.
In the second frame the wheel will have rotated three quarters of a
circle to the 9 o’clock position. The third shot will have the wheel at
6 o’clock and so on. That is, it will appear as if it rotating
anticlockwise – from 12 to 9 to 6…
When the car accelerates to 24 rotations of the wheel per second, every
frame of the movie will capture the wheel at the same position. That is,
it appears to be stationary. At a slightly faster speed, the wheels will
appear to be rotating slower than then car.
This apparent reversing of the tyres occurs when the wheels rotate at 12
to 24 cycles per second (for movies). After 24 it disappears and repeats
again between 36 and 48 rotations per second and so on.
In the case of television where the pictures are take at 30fps, the
first reversal occurs between 15 and 30 rotations per second, then from
45 to 60 and so on. But at which speed (in kilometres per hour) does
this illusion occur? Well, that is a story for another day.
