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Date November 2014 Marks available 3 Reference code 14N.3.SL.TZ0.11
Level Standard level Paper Paper 3 Time zone Time zone 0
Command term Explain and State Question number 11 Adapted from N/A

Question

This question is about simultaneity.

Daniela is standing in the middle of a train that is moving at a constant velocity relative to Jaime, who is standing on the platform. At the moment the train passes Jaime, two beams of light, X and Y, are emitted simultaneously from a device held by Daniela. Both beams are reflected by mirrors at the end of the train and then return to Daniela.

N14/4/PHYSI/SP3/ENG/TZ0/11

State and explain the order of arrival of X and Y at the mirrors according to Jaime.

[3]
a.

Outline whether the return of X and Y to Daniela are simultaneous according to Jaime.

[2]
b.

Markscheme

beam X will reach the mirror first;

the speed of light of each beam is constant for all inertial observers;

the left mirror moves towards the beam X while the right mirror moves away from the beam Y;

a.

the beams returning to Daniela occur at one point in space;

if this is simultaneous to Daniela, the event will also be simultaneous to Jaime;

or

beam X has less to go to the mirror and then longer to Daniela, whilst beam Y has longer to the mirror and less to Daniela;

the sum of the times are the same because Daniela is in the middle so they arrive at the same time;

b.

Examiners report

Usually candidates gave acceptable answers to (a) but forgot that the speed of light must be constant for their statements to be true.

a.

Very few answered (b) well.

b.

Syllabus sections

Option A: Relativity » Option A: Relativity (Core topics) » A.2 – Lorentz transformations
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