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Question 18M.3.HL.TZ1.c

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Date May 2018 Marks available [Maximum mark: 2] Reference code 18M.3.HL.TZ1.c
Level HL Paper 3 Time zone TZ1
Command term Explain Question number c Adapted from N/A
c.
[Maximum mark: 2]
18M.3.HL.TZ1.c

A galaxy can be modelled as a sphere of radius R0. The distance of a star from the centre of the galaxy is r.

M18/4/PHYSI/HP3/ENG/TZ1/19

For this model the graph is a simplified representation of the variation with r of the mass of visible matter enclosed inside r.

Explain, using the equation in (a) and the graphs, why the presence of visible matter alone cannot account for the velocity of stars when r > R0.

[2]

Markscheme

for v to stay constant for r greater than R0, M has to be proportional to r

 

but this contradicts the information from the M-r graph

OR

if M is constant for r greater than R0, then we would expect v ∝ r − 1 2

 

but this contradicts the information from the v-r graph

 

[2 marks]

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