Date | May 2019 | Marks available | 3 | Reference code | 19M.3.SL.TZ2.6 |
Level | Standard level | Paper | Paper 3 | Time zone | 2 |
Command term | Calculate | Question number | 6 | Adapted from | N/A |
Question
A train of proper length 85 m moves with speed 0.60c relative to a stationary observer on a platform.
Define proper length.
In the reference frame of the train a ball travels with speed 0.50c from the back to the front of the train, as the train passes the platform. Calculate the time taken for the ball to reach the front of the train in the reference frame of the train.
In the reference frame of the train a ball travels with speed 0.50c from the back to the front of the train, as the train passes the platform. Calculate the time taken for the ball to reach the front of the train in the reference frame of the platform.
Markscheme
the length measured «in a reference frame» where the object is at rest ✔
ALTERNATIVE 1:
ALTERNATIVE 2:
v of ball is 0.846c for platform ✔
length of train is 68m for platform ✔
ALTERNATIVE 3:
Examiners report
Proper length is quite well understood. A common mistake is to mention that it is the length measured by a reference frame at rest.
Because there were three frames of reference in this question many candidates struggled to find the simple value for the time of the ball’s travel down the train in the train’s frame of reference.
Almost no candidates could use a Lorentz transformation to find the time of the ball’s travel in the frame of reference of the platform. Most just applied some form of t=γt’. Elapsed time and instantaneous time in different frames were easily confused. Candidates rarely mention which reference frame is used when making calculations, however this is crucial in relativity.