Chapter-2 Q006 - Physics For Engineers And Scientists - Solutions


Amateur Radio and Astronomy website. JO10UX

Question 6 - Problems - Chapter 2


A galaxy beyond the constellation Corona Borealis is moving directly away from our Galaxy at the rate of 21600 km/s.

This galaxy is now at a distance of $1.4x10^9$ light-years from our galaxy.

We assume that the Galaxy is been moving always at a constant speed.


How many years ago was it right on top of our Galaxy?


Distance $s = 1.4x10^9 $ ly

Average speed $v = 21600 $ km/s

Time $t = \frac{s}{v}$

1 light-year = $9460730472580.8$ km

In [1]:
Distance = 1.4e9 # in ly
Ly = 9460730472580.8 #km
Distance_km = Distance * Ly
Speed_km_s = 21600 # in km/s
Speed_km_y = Speed_km_s * 3600 * 24 * 365.24
Time = Distance_km / Speed_km_y
# limit to 2 significant figures as distance is given in 2 significants too
print(f'The distance in km is {Distance_km:.1e}.')
print(f'The distance traveled in one year is {Speed_km_y:.1e}.')
print(f'The time needed to travel the distance is {Time:.1e} year.') 
The distance in km is 1.3e+22.
The distance traveled in one year is 6.8e+11.
The time needed to travel the distance is 1.9e+10 year.

سُوۡرَةُ حٰمٓ السجدة / فُصّلَت
ثُمَّ اسْتَوَىٰ إِلَى السَّمَاءِ وَهِيَ دُخَانٌ فَقَالَ لَهَا وَلِلْأَرْضِ ائْتِيَا طَوْعًا أَوْ كَرْهًا قَالَتَا أَتَيْنَا طَائِعِينَ ﴿١١
فَقَضَاهُنَّ سَبْعَ سَمَاوَاتٍ فِي يَوْمَيْنِ وَأَوْحَىٰ فِي كُلِّ سَمَاءٍ أَمْرَهَا ۚ وَزَيَّنَّا السَّمَاءَ الدُّنْيَا بِمَصَابِيحَ وَحِفْظًا ۚ ذَٰلِكَ تَقْدِيرُ الْعَزِيزِ الْعَلِيمِ ﴿١٢

Surah Fussilat: Thereafter turned He to the heaven and it was as smoke, and said Unto it and Unto the earth: come ye twain, willingly or loth. They said: we come willingly. (11) Then He decreed them as seven heavens in two days, and revealed Unto each heaven the command thereof; and We bedecked the nether heaven with lamps and placed therein a guard. That is the ordinance of the Mighty, the Knower. (12)

De hemelen en aarde (waren) een samenhangende massa. Wij hebben ze toen van elkaar gescheiden… Wij hebben de hemel tot een beschermend dak gemaakt … en de dag en de nacht, de zon en de maan geschapen. (Koran 41 11:12)

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