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Date	May 2019	Marks available	2	Reference code	19M.3.HL.TZ1.7
Level	Higher level	Paper	Paper 3	Time zone	1
Command term	State and Explain	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]

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 ✔

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.