gizmos student exploration orbital motion – kepler’s laws
gizmos student exploration orbital motion – kepler’s laws answer key
student exploration orbital motion – kepler’s laws
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Student Exploration: Orbital Motion – Kepler’s Laws
Directions: Follow the instructions to go through the simulation. Respond to the questions and
prompts in the orange boxes.
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?
Halley's Comet does not have the size or mass to affect
Neptune's orbit at all
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 an ellipse
orbit?
2. Watch the orbit over time. Does the orbit ever change, or is it stable? yes
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?
, Activity A: Get the Gizmo ready:
● Click Reset.
Shape of orbits ● 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 = –3.00i AU. Move the velocity vector so that v = -8.0j 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. Draw the
resulting orbit on the grid to the right.
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 (F1 and
F2). 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:
a1 + a2 = b1 + b2
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?
its on the foci to the right
3. Experiment: Try several other combinations of initial position and velocity.
A. What do you notice about the orbits?
the sun is never the center
B. What do you notice about the position of the Sun?
its always on the furthest side from the initial position
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.
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