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Date November 2020 Marks available 2 Reference code 20N.3.SL.TZ0.4
Level Standard level Paper Paper 3 Time zone 0 - no time zone
Command term Explain Question number 4 Adapted from N/A

Question

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

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

[1]
a.

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

[2]
b(i).

Explain which of the lengths is the proper length.

[2]
b(ii).

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

[2]
c.

Markscheme

constancy of time
OR
speed of light > c is possible

 

OWTTE

a.

γ=1.15

length = 6.9«m» 

 

Allow length in the range 6.7 to 7.0m.

Allow ECF from wrong γ

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

b(i).

8.0m / measurement made on the probe

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

b(ii).

u=0.5c+0.8c1+0.5c×0.8cc2 

u=0.93c 

 

Allow all negative signs for velocities

Award [2] marks for a bald correct answer

c.

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.

Very well answered.

b(i).

"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.

b(ii).

Very well answered.

c.

Syllabus sections

Option A: Relativity » Option A: Relativity (Core topics) » A.2 – Lorentz transformations
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Option A: Relativity » Option A: Relativity (Core topics)
Option A: Relativity

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