Question 19M.3.HL.TZ1.7
| Date | May 2019 | Marks available | [Maximum mark: 5] | Reference code | 19M.3.HL.TZ1.7 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Explain, State | Question number | 7 | Adapted from | N/A |
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 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 ✔
Freefall in a gravitational field. Most of the candidates answered well.
State and explain the path of the light ray according to observer X.
[2]
X is in an inertial frame ✔
so light will follow a straight line path «parallel to the floor of the box» ✔
Freefall in a gravitational field. Most of the candidates answered well but in b) omitted to mention that X is inertial.
State and explain the path of the light ray according to observer Y.
[2]
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 ✔
Quite a high number of candidates, in b ii), stated incorrectly, that the light ray will be curved upward.