Q265 · Practice questionSimple familiar3 marks
QUESTION 265 (3 marks)
A spacecraft travels toward a station at and emits light toward it in vacuum. Both observers are inertial.
a)[2 marks]
State the two postulates of special relativity.
b)[1 mark]
A student predicts a signal speed of in the station frame. Assess this prediction.
WORKED SOLUTION
3 marksPractice marking scheme
ANSWER
(a) The laws of physics have the same form in all inertial frames. The speed of light in vacuum is the same for all inertial observers, independent of source motion. (b) Incorrect: the station measures .
Worked solution
(a) The first postulate concerns all physical laws, not just mechanics. The second prevents adding the source velocity to the speed of a vacuum light signal.
(b) The invariant-light-speed postulate applies to the station as well as the spacecraft.
Equivalent physically justified methods, alternative valid sketches and consistent units accepted.
Accept sensible significant figures and consistent rounding from stated constants.
Part a: State the equivalence of physical laws in inertial frames.
Part a: State the invariant vacuum light speed, independent of source motion.
Part b: Reject the prediction and identify c using the second postulate.
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