题目
GEOL0012_25-26 **********GEOL0012 Moodle Test (unassessed) 2025/26**********
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Question text1. The water volume of the Aral sea was 1093 [math: km3] in 1960, and decreased exponentially to 102 [math: km3] in 2009. The density of salty water is about 1020 [math: kg/m3]. a) Calculate the mass of water lost by the Aral sea from 1960 until 2009. Approximate your answer to 3 significant figures and give the answer in kg. Answer 1 Question 7[input] [math: ×1015kg] b) Assume that the mass of the lost water was [math: 9.09×1014] kg and that it was evaporated and redistributed over the surface of Earth as a homogeneous thin spherical shell. This change in mass distribution will result in a change in Earth’s moment of inertia. Assume that initially the lake had all its mass concentrated at a point with coordinates 45°N 60°E (a point mass approximation). Calculate the change in Earth’s moment of inertia due to the redistribution of Lake Aral’s water. Give the answer in [math: kgm2] to 3 significant figures (be careful with the sign of the final answer). You may use: -Moment of inertia of homogeneous thin spherical shell with mass M and radius R is: [math: I=(2/3)MR2] -Moment of inertia of point mass with mass M and distance r from axis of rotation is: [math: I=Mr2] -Radius of Earth=6371 km Answer 2 Question 7[input] [math: ×1027kgm2]c) Assume the moment of inertia of Earth in 1960 to be [math: 8.04×1037kgm2] and that the change in moment of inertia due to Aral's water evaporation was [math: 6.77×1027kgm2]. Conservation of angular momentum tells us that: [math: Iinitial/Tinitial=Ifinal/Tfinal] Where [math: I] and [math: T] are the moment of inertia and Earth’s period of rotation about its own axis (24 hours). Calculate in microseconds the expected change in Earth’s rotation period. Approximate to 2 significant figures.Answer 3 Question 7[input] [math: μs]

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思路分析
Question 7 involves three subparts (a, b, c) with numeric values provided, and the student must reason through each step carefully before arriving at the final numbers.
Option a) mass of water lost from 1960 to 2009
- The problem gives initial volume 1093 km^3 and final volume 102 km^3. The lost volume is 1093 − 102 = 991 km^3.
- To convert to cubic meters, use 1 km^3 = 1 × 10^9 m^3, so 991 km^3 = 991 × 10^9 m^3 = 9.91 × 10^11 m^3.
- With density ρ = 1020 kg/m^3, the lost mass is M = ρ × V = 1020 × 9.91 × 10^11 kg ≈ 1.011 × 10^15 kg.
- Rounding to 3 significant figures gives 1.01 × 10^15 kg. The numerical result aligns with the provided target format (1.01 × 10^15 kg).
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Question text1. The water volume of the Aral sea was 1093 km3[math] in 1960, and decreased exponentially to 102 km3[math] in 2009. The density of salty water is about 1020 kg/m3[math]. a) Calculate the mass of water lost by the Aral sea from 1960 until 2009. Approximate your answer to 3 significant figures and give the answer in kg. Answer 1 Question 7[input] ×1015kg[math] b) Assume that the mass of the lost water was 9.09×1014[math] kg and that it was evaporated and redistributed over the surface of Earth as a homogeneous thin spherical shell. This change in mass distribution will result in a change in Earth’s moment of inertia. Assume that initially the lake had all its mass concentrated at a point with coordinates 45°N 60°E (a point mass approximation). Calculate the change in Earth’s moment of inertia due to the redistribution of Lake Aral’s water. Give the answer in kgm2[math] to 3 significant figures (be careful with the sign of the final answer). You may use: -Moment of inertia of homogeneous thin spherical shell with mass M and radius R is: I=(2/3)MR2[math] -Moment of inertia of point mass with mass M and distance r from axis of rotation is: I=Mr2[math] -Radius of Earth=6371 km Answer 2 Question 7[input] ×1027kgm2[math]c) Assume the moment of inertia of Earth in 1960 to be 8.04×1037kgm2[math] and that the change in moment of inertia due to Aral's water evaporation was 6.77×1027kgm2[math]. Conservation of angular momentum tells us that: Iinitial/Tinitial=Ifinal/Tfinal[math] Where I[math] and T[math] are the moment of inertia and Earth’s period of rotation about its own axis (24 hours). Calculate in microseconds the expected change in Earth’s rotation period. Approximate to 2 significant figures.Answer 3 Question 7[input] μs[math]
Two uniform disks have the same mass but different radii: disk 1has a Radius R, disk 2 has a radius 2R. What is the ratio between the moment of inertia of the second disk and the first disk?
What does the moment of inertia describe?
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