DP Physics (last assessment 2024)

Question 20N.3.SL.TZ0.4

Date	November 2020	Marks available	[Maximum mark: 7]	Reference code	20N.3.SL.TZ0.4
Level	SL	Paper	3	Time zone	TZ0
Command term	Calculate, Deduce, Explain, Outline	Question number	4	Adapted from	N/A

[Maximum mark: 7]

20N.3.SL.TZ0.4

(a)

The Lorentz transformations assume that the speed of light is constant. Outline what the Galilean transformations assume.

[1]

Markscheme

constancy of time
OR
speed of light > c is possible ✓

OWTTE

Examiners report

Although the expected answers was the constancy of time, the markscheme allowed references to the speed of light not being constant, as this was a common answer, deriving from the stem used in the question.

A spaceship is travelling at $0.80 c$ , away from Earth. It launches a probe away from Earth, at $0.50 c$ relative to the spaceship. An observer on the probe measures the length of the probe to be $8.0 m$ .

(b(i))

Deduce the length of the probe as measured by an observer in the spaceship.

[2]

Markscheme

$γ = 1.15$ ✓

length = $6.9 « m »$ ✓

Allow length in the range $6.7$ to $7.0 m$ .

Allow ECF from wrong $γ$

Award [2] marks for a bald correct answer in the range indicated above.

Examiners report

Very well answered.

(b(ii))

Explain which of the lengths is the proper length.

[2]

Markscheme

$8.0 m$ / measurement made on the probe ✓

the measurement made by an observer at rest in the frame of the probe ✓

Examiners report

"In the same frame" does not highlight the need to be "at rest" in that frame, and was the most frequent wrong answer, although a vast majority scored full marks here.

(c)

Calculate the speed of the probe in terms of $c$ , relative to Earth.

[2]

Markscheme

$u = \frac{0.5 c + 0.8 c}{1 + \frac{0.5 c \times 0.8 c}{c^{2}}}$ ✓