Answer
The proper time interval is measured in a reference frame in which the two events occur at the same place. Therefore, an observer in the sprinter's reference frame would measure the proper time interval. The proper length is measured in a reference frame in which the object is at rest. An observer a rest with respect to the track would measure the proper length.
Work Step by Step
The proper time interval is measured in a reference frame in which the two events occur at the same place. Therefore, an observer in the sprinter's reference frame would measure the proper time interval. The proper length is measured in a reference frame in which the object is at rest. An observer a rest with respect to the track would measure the proper length.
