The original version of this story appeared in Quanta Magazine. Imagine standing on a railway platform watching a trolley go past. A girl on the trolley drops a bright red ball. To her, the ball falls ...

This PSSC film made in 1960 is one of the finest physics films ever made. All students should see it at some point in their studies. See the article '"Frames of Reference" revisited' by Alistair Steyn ...