OrbitalMotionSE

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Barbara Goleman Senior High *

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Physics

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Jun 1, 2024

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2019 Name: ______________________________________ Date: ________________________ Student Exploration: Orbital Motion – Kepler’s Laws Vocabulary: astronomical unit, eccentricity, ellipse, force, gravity, Kepler’s first law, Kepler’s second law, Kepler’s third law, orbit, orbital radius, period, vector, velocity Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 1. The orbit of Halley’s Comet, shown at right, has an oval shape. In which part of its orbit do you think Halley’s Comet travels fastest? Slowest? Mark these points on the diagram at right. 2. How might a collision between Neptune and Halley’s Comet affect Neptune’s orbit? mass Gizmo Warm-up The path of each planet around the Sun is determined by two factors: its current velocity (speed and direction) and the force of gravity on the planet. You can manipulate both of these factors as you investigate planetary orbits in the Orbital Motion – Kepler’s Laws Gizmo. On the CONTROLS pane of the Gizmo, turn on Show trails and check that Show vectors is on. Click Play ( ). 1. What is the shape of the planet’s orbit? circular 2. Watch the orbit over time. Does the orbit ever change, or is it stable? stable 3. Click Reset ( ). Drag the tip of the purple arrow to shorten it and reduce the planet’s initial velocity. Click Play . How does this affect the shape of the orbit? Gets smaller and turns to an oval
2019 Activity A: Shape of orbits Get the Gizmo ready : Click Reset . Turn on Show grid . Introduction: The velocity of a planet is represented by an arrow called a vector . The vector is described by two components: the i component represents east-west speed and the j component represents north-south speed. The unit of speed is kilometers per second (km/s). Question: How do we describe the shape of an orbit? 1. Sketch : The distance unit used here is the astronomical unit (AU), equal to the average Earth-Sun distance. Place the planet on the i axis at r = –3.00 i AU. Move the velocity vector so that v = -8.0 j km/s (| v | = 8.00 km/s). The resulting vectors should look like the vectors in the image at right. (Vectors do not have to be exact.) Click Play , and then click Pause ( ) after one revolution. Sketch the resulting orbit on the grid. 2. Identify : The shape of the orbit is an ellipse , a type of flattened circle. An ellipse has a center (C) and two points called foci (F 1 and F 2 ). If you picked any point on the ellipse, the sum of the distances to the foci is constant. For example, in the ellipse at left: a 1 + a 2 = b 1 + b 2 Turn on Show foci and center . The center is represented by a red dot, and the foci are shown by two blue dots. What do you notice about the position of the Sun? It’s a foci point_ 3. Experiment : Try several other combinations of initial position and velocity. A. What do you notice about the orbits? There slight ovals B. What do you notice about the position of the Sun? always a foci point You have just demonstrated Kepler’s first law , one of three laws discovered by the German astronomer Johannes Kepler (1571–1630). Kepler’s first law states that planets travel around the Sun in elliptical orbits with the Sun at one focus of the ellipse. (Activity A continued on next page)
2019 Activity A (continued from previous page) 4. Observe : Use the Gizmo to create an orbit that is nearly circular. Then create an orbit that is flattened. Observe the foci in each ellipse. A. What do you notice about the spacing of the foci when the ellipse is very round? They are close B. What do you notice about the spacing of the foci when the ellipse is very flat? They are far 5. Calculate : The eccentricity of an ellipse is a number that describes the flatness of the ellipse. Eccentricity is equal to the distance between foci divided by the total width of the ellipse. There are no units for eccentricity. Click Reset . Move the planet to r = –5.00 i AU (does not have to be exact) and drag the velocity vector to set the velocity close to –8.0 j km/s. Click Play , and then click Pause after one full revolution. A. What is the distance between the foci? _1.84 AU B. What is the approximate width of the ellipse?8 AU C. What is the eccentricity of the ellipse? .23 AU D. Click Reset , and change the initial velocity to –4.0 j km/s. Click Play . What is the eccentricity of this ellipse? Distance between foci:4.47 au Width:5.5 Au Eccentricity: .81 AU 6. Draw conclusions : Think about the eccentricity and shape of each ellipse. A. What is the relationship between the eccentricity of an ellipse and its shape? Both start to become B. What is the eccentricity of a circle? Explain. The number that represents how flat the circle is C. What is the eccentricity of a completely flat ellipse? Explain. 1 D. The eccentricity of Earth’s orbit is 0.017. What can you infer about the shape of Earth’s orbit? Earths shape is Slightly flat Activity B: Velocity and area Get the Gizmo ready : Click Reset . Turn off Show foci and center .
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