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# dis5-slides - ASTR 100 Discussion 5 Oct 10 2011 1 The...

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ASTR 100 Discussion 5 Oct. 10, 2011

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1. The shortest wavelength of light that the Kepler spacecraft can observe is 4 x 10 -7 m. What is the frequency of this light? Show your work. Your answer must be in proper scientific notation – no fractions. The speed of light is 3 x 10 8 m/s.
Answer: We need to use the equation that relates the speed of light, wavelength, and frequency, and we need to re-arrange it to solve for frequency. c = f f = c In this case, =4 x 10 -7 m. f = 3 × 10 8 m / s 4 × 10 -7 m = 0.75 × 10 15 s -1 f = 7.5 × 10 14 Hz The frequency of the light is 7.5 x 10 14 Hz.

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Grading checklist: Did you write down the equation you were using? Did you put in the correct values? Did you remember to put your answer in proper scientific notation? Did you remember to put the correct units on your answer?
2. Suppose astronomers discover a planet with an average density of 4 kg/L. Use the table below to help you decide which of the following statements gives the best description of the composition of the planet. A. Completely rocky material B. Mostly rock material, with a small amount of metallic material C. Mostly gases, such as hydrogen, with about 10% heavy metals, such as lead D. Approximately an equal amount of rocky material and metallic material

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Material Density hydrogen gas 8.5 x 10 -5 kg/L water ice 0.9 kg/L water 1 kg/L sand 1.6 kg/L stone 2.5 kg/L iron 7.9 kg/L lead 11 kg/L
Correct: D. The sand and stone have densities around 2 kg/L. Iron has a density of about 8 kg/L. If each of these materials made up half the planet, then the average density would be about 4 kg/L. Wrong: A. If the planet was completely rocky, its density would be about 2 kg/L. Wrong: B. If the planet was mostly rocky with only a bit of metallic material, then its density would also be close to 2 kg/L. Wrong: C. This mixture would give a very low- density planet.

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