Now, the tortoise on the train holds a rod horizontally, so that it is pointing along the carriage. He decided to measure the length of the rod by attaching his mirror to one end, and a torch to the other, then measuring the time it takes for a pulse of light to make the round trip. From the platform, the sparrow makes the same length measurement ... and gets a different answer again!
Moving objects appear to be shorter when measured from a different inertial frame :
Bear in mind that if the sparrow was also holding a rod, the tortoise would measure it as
being shorter, with the same 
factor! This is due to the
fact that it doesn't matter whether the train is moving or the partform is moving -
the net result is the same!
The amusing picture on the right shows the effect of different factors very strikingly!
Activity 3 |
Work out what speed the tortoise would have to be going at in order for its
factor to be 2. Express this both in metres per
second and as a fraction of c.
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