Week 6 - Kepler's Laws and Retrograde Motion

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Mt San Antonio College *

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Physics

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Dec 6, 2023

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docx

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Name: Key Kepler’s Laws and Retrograde Motion 1 st Law 1. Consider the 4 ellipses above. Just by looking at them, which of the ellipses has the lowest eccentricity? Explain how you know. Ellipse A because it is the least squashed (closest to a circle). Just by looking at them, which of the ellipses has the highest eccentricity? Explain how you know. Ellipse D because it is the most squashed/stretched out. 2. Now we will calculate the eccentricity of the 4 ellipses above. For each ellipse, use a ruler to measure the length of the major axis and the minor axis. The major axis is the longer direction and the minor axis is the shorter direction. Make your measurements in centimeters and record in the table on the next page. Once you have measured the length of the major and minor axis, calculate the eccentricity for each ellipse using the following equation: Eccentricity = 1 Minor Axis 2 Major Axis 2 A B D C
Ellipse Major Axis (in cm) Minor Axis (in cm) Eccentricity A 4 4 0.00 B 4.2 4 0.30 C 5 4 0.60 D 10 4 0.92 3. a) Using the eccentricities you just calculated for the four ellipses, which ellipse has the lowest eccentricity? Ellipse A b) Which ellipse has the highest eccentricity? Ellipse D c) Do your answers match your answers in Question 1? Yes 4. a) The Earth has an eccentricity of 0.016. Which of the four ellipses on the first page best matches the orbit of the Earth around the Sun? Ellipse A b) Halley’s Comet has an eccentricity of 0.967. Draw an ellipse that looks like the orbit of Halley’s Comet. The ellipse will look a lot like Ellipse D which has an eccentricity of 0.92, but just a little more squished.
5. Since planets do not orbit the Sun in perfect circles, their distance from the Sun is constantly changing. We can calculate the closest and farthest distances that an object gets from the Sun using the following equations: Closest Distance = ( 1 eccentricity ) × ( Semimajor axis ) Farthest Distance = ( 1 + eccentricity ) × ( Semimajor axis ) The semi-major axis is just the average distance of the planet from the Sun. For instance, the semi-major axis of the Earth is 1 AU. Calculate the closest and farthest distance that each of these objects gets from the Sun: Object Average Distance (in AU) Eccentricity Closest Distance (in AU) Farthest Distance (in AU) Earth 1.00 AU 0.016 0.984 1.016 Mars 1.52 AU 0.093 1.379 1.661 Neptune 30.1 AU 0.008 29.859 30.341 Pluto 39.5 AU 0.248 29.704 49.296 Halley’s Comet 17.8 AU 0.967 0.587 35.013 Comet Hyakutake 1700 AU 0.9999 0.170 3399.830 a) Your friend claims that there are times in its orbit that Mars gets just as close to the Earth as the Moon does. The Moon never gets farther away from the Earth than 0.003 AU. Using the table above, explain whether you agree or disagree with your friend. I disagree. The distance from Earth to Mars will be the smallest when Mars is the closest it gets to the Sun and the Earth is the farthest it gets from the Sun. The closest Mars ever gets to the Sun is 1.379 AU. The farthest the Earth gets from the Sun is 1.016 AU. They are still 0.363 AU apart, which is more than 100x bigger than the distance from the Earth to the Moon. b) A different friend claims that there are times when Pluto (the old 9 th planet) is actually closer to the Sun than Neptune (the 8 th planet). Based on the table above, could this be a possibility? Explain. Yes it could. The closest Pluto gets to the Sun is 29.704 AU which is closer than Neptune ever gets to the Sun. So Pluto will be closer to the Sun than Neptune for part of its orbit (it turns out that Pluto is closer than Neptune for about 20 years in its 240 year orbit).
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