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Date	May 2019	Marks available	1	Reference code	19M.2.SL.TZ1.6
Level	Standard level	Paper	Paper 2	Time zone	1
Command term	Outline	Question number	6	Adapted from	N/A

Question

The Moon has no atmosphere and orbits the Earth. The diagram shows the Moon with rays of light from the Sun that are incident at 90° to the axis of rotation of the Moon.

A black body is on the Moon’s surface at point A. Show that the maximum temperature that this body can reach is 400 K. Assume that the Earth and the Moon are the same distance from the Sun.

[2]

a.i.

Another black body is on the Moon’s surface at point B.

Outline, without calculation, why the aximum temperature of the black body at point B is less than at point A.

[2]

a.ii.

The albedo of the Earth’s atmosphere is 0.28. Outline why the maximum temperature of a black body on the Earth when the Sun is overhead is less than that at point A on the Moon.

[1]

Outline why a force acts on the Moon.

[1]

c.i.

Outline why this force does no work on the Moon.

[1]

c.ii.

Markscheme

T = ${\left( {\frac{{1360}}{\sigma }} \right)^{0.25}}$ ✔

390 «K» ✔

Must see 1360 (from data booklet) used for MP1.

Must see at least 2 s.f.

a.i.

energy/Power/Intensity lower at B ✔

connection made between energy/power/intensity and temperature of blackbody ✔

a.ii.

(28 %) of sun’s energy is scattered/reflected by earth’s atmosphere OR only 72 % of incident energy gets absorbed by blackbody ✔

Must be clear that the energy is being scattered by the atmosphere.

Award [0] for simple definition of “albedo”.

gravitational attraction/force/field «of the planet/Moon» ✔

Do not accept “gravity”.

c.i.

the force/field and the velocity/displacement are at 90° to each other OR there is no change in GPE of the moon ✔

Award [0] for any mention of no net force on the satellite.

Do not accept acceleration is perpendicular to velocity.

c.ii.

Examiners report

Many candidates struggled with this question. A significant portion attempted to apply Wein’s Law and simply stated that a particular wavelength was the peak and then used that to determine the temperature. Some did use the solar constant from the data booklet and were able to calculate the correct temperature. As part of their preparation for the exam candidates should thoroughly review the data booklet and be aware of what constants are given there. As with all “show that” questions candidates should be reminded to include an unrounded answer.

a.i.

This is question is another example of candidates not thinking beyond the obvious in the question. Many simply said that point B is farther away, or that it is at an angle. Some used vague terms like “the sunlight is more spread out” rather than using proper physics terms. Few candidates connected the lower intensity at B with the lower temperature of the blackbody.

a.ii.

This question was assessing the understanding of the concept of albedo. Many candidates were able to connect that an albedo of 0.28 meant that 28 % of the incident energy from the sun was being reflected or scattered by the atmosphere before reaching the black body.

This was generally well answered, although some candidates simply used the vague term “gravity” rather than specifying that it is a gravitational force or a gravitational field. Candidates need to be reminded about using proper physics terms and not more general, “every day” terms on the exam.

c.i.

Some candidates connected the idea that the gravitational force is perpendicular to the velocity (and hence the displacement) for the mark. It was also allowed to discuss that there is no change in gravitational potential energy, so therefore no work was being done. It was not acceptable to simply state that the net displacement over one full orbit is zero. Unfortunately, some candidates suggested that there is no net force on the moon so there is no work done, or that the moon is so much smaller so no work could be done on it.

c.ii.

Syllabus sections

Core » Topic 6: Circular motion and gravitation » 6.1 – Circular motion

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Markscheme

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