DP Physics (last assessment 2024)

Question 20N.3.SL.TZ0.5

Date	November 2020	Marks available	[Maximum mark: 7]	Reference code	20N.3.SL.TZ0.5
Level	SL	Paper	3	Time zone	TZ0
Command term	Calculate, Determine, Draw, Plot	Question number	5	Adapted from	N/A

[Maximum mark: 7]

20N.3.SL.TZ0.5

The spacetime diagram is in the reference frame of an observer O on Earth. Observer O and spaceship A are at the origin of the spacetime diagram when time $t = t' = 0$ . The worldline for spaceship A is shown.

(a(i))

Calculate in terms of $c$ the velocity of spaceship A relative to observer O.

[1]

Markscheme

$0.6 c$ ✓

Accept $1.8 \times 10^{8} m s^{- 1}$ if unit given.

Examiners report

Very well answered.

(a(ii))

Draw the $x'$ axis for the reference frame of spaceship A.

[1]

Markscheme

line through origin and through $(5, 3) \pm$ one small square at this coordinate ✓

Answers shown for 5(a)(ii) and (b)(i) and (b)(ii).

Examiners report

Most answers successfully drew the correct x' axis.

Event E is the emission of a flash of light. Observer O sees light from the flash when $t = 9$ years and calculates that event E is $4 ly$ away, in the positive $x$ direction.

(b(i))

Plot the event E on the spacetime diagram and label it E.

[2]

Markscheme

X value of E at $4$ « $ly$ » ✓

Y value of E at $5$ « $y$ » ✓

Examiners report

Event E is the event when the light is emitted (4,5). Some candidates missed that and placed it at (4,9).

To determine these coordinates candidates were expected to construct a light path from (0,9) which intercepts at x=4 and t=5. This is a very common mistake which needs careful explanation by teachers.

(b(ii))

Determine the time, according to spaceship A, when light from event E was observed on spaceship A.

[3]

Markscheme

light cone from E «crosses $c t$ at $9$ so» intersection on $c t' = 5.6 \pm 0.2$ $y$ «on $c t$ scale» ✓