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Date May 2019 Marks available 1 Reference code 19M.3.HL.TZ1.7
Level Higher level Paper Paper 3 Time zone 1
Command term State Question number 7 Adapted from N/A

Question

A box is in free fall in a uniform gravitational field. Observer X is at rest inside the box. Observer Y is at rest relative to the gravitational field. A light source inside the box emits a light ray that is initially parallel to the floor of the box according to both observers.

State the equivalence principle.

[1]
a.

State and explain the path of the light ray according to observer X.

[2]
b.i.

State and explain the path of the light ray according to observer Y.

[2]
b.ii.

Markscheme

a freely falling frame in a gravitational field is equivalent to an inertial frame
OR
a frame accelerating in free space is equivalent to a frame at rest in a gravitational field ✔

a.

X is in an inertial frame ✔

so light will follow a straight line path «parallel to the floor of the box» ✔

b.i.

ALTERNATIVE 1
light must hit right wall of box at same place as determined by X ✔

«but box is accelerating» so path must be curved downward ✔

ALTERNATIVE 2
light is affected by gravity «for the observer at rest to the ground» ✔

so the path is curved downward/toward the ground ✔

b.ii.

Examiners report

Freefall in a gravitational field. Most of the candidates answered well.

a.

Freefall in a gravitational field. Most of the candidates answered well but in b) omitted to mention that X is inertial.

b.i.

Quite a high number of candidates, in b ii), stated incorrectly, that the light ray will be curved upward.

b.ii.

Syllabus sections

Option A: Relativity » Option A: Relativity (Additional higher level option topics) » A.5 – General relativity (HL only)
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Option A: Relativity » Option A: Relativity (Additional higher level option topics)
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