A 160m long train is traveling towards a 100m long tunnel A

A 160-m long train is traveling towards a 100-m long tunnel. A trainspotter (me) is standing beside the track and watching.

160 m is the proper length of the train. In which frame of reference is the proper length of the train measured?

Question 1 options:

The train\'s frame of reference.

My frame of reference.

Both my frame of reference and the train\'s frame of reference.

The proper length cannot be measured in this problem.

Please explain reasoning, so I can learn.

Solution

The length of train is contracted relative to the observer standing on the track. Because relative to the observer the train is in motion.

The train is at rest according to observer on the tran\'s reference frame. Hence the proper length of train is measured by the train\'s frame of reference.