Download My Solar System Lab

Physics
HS/Science
Unit: 05 Lesson: 01
My Solar System Lab
Introduction:
Every physics student has had experience with the force of gravity. For most, this experience is
limited to the interaction between a very large object, the Earth, and much smaller objects that are
very close to it. This is a very limited range of possibilities. Software simulations of gravity allow
physics students to explore a variety of other gravitational interactions between objects. This lab
utilizes the simulation “My Solar System” developed by the Physics Education Technology (PhET)
group at the University of Colorado at Boulder. The URL is: http://phet.colorado.edu/sims/my-solarsystem/my-solar-system.swf.
This simulation does not use conventional units for distance, velocity, or G (the universal
gravitational constant), but is realistic in the behavior of gravity. In this simulation, the value of G can
be taken to be 10,000, and the units should be disregarded in calculations. For familiarity, we will
refer to the force unit as a Newton and the time units in seconds, but the units of the simulation do
not justify this convention.
Procedure:
Go to the “My Solar System” simulation on the PhET website, and carefully follow the instructions
below for each activity. Answer the questions, and record your results before going on to the next
activity.
Activity 1: Get familiar with the simulation
Look over the start screen. The simulation controls and settings are on the right, and the simulation
inputs are at the bottom.
 Click on Start to see the outputs in the center and lower right. The paths of the objects in
simulation are displayed along with elapsed time.
 Click Stop, and move the cursor over each object. Its current position and velocity are displayed
under the time display.
 With the trace and grid displayed, determine the grid distance, satellite velocity, time for one
period, and the radius of the orbit.
Distance for grid block _____________ units
Time for one period _______________ s
Radius of orbit ___________________ units
Speed of orbiting object ____________ units
Activity 2: Free Fall Observations
 Click Reset, and then move the Sun to -200 units and planet to +200 units. Set the speed of the
planet to zero.
 Find the time for the planet to fall to the Sun and final location of the Sun after the collision.
(Remember, you place the hand on the Sun, and its values appear under the time.) M = 200,
m = 10
Time for fall _________ s
Location of combined mass _________ units
Comment on the movement of the larger mass __________________
©2012, TESCCC
07/16/12
page 1 of 4
Physics
HS/Science
Unit: 05 Lesson: 01

Change the mass of the Sun to 2000, and determine the values again. M = 2000, m = 10
Time for fall __________ s
Location of combined mass _________ units
Comment on the movement of the larger mass__________________

Change the mass of the Sun to 2000 and the mass of the planet to .001. Run again. M = 2000,
m = .001
Time for fall _________ s
Location of combined mass _________ units
Summarize how the smaller mass accelerates toward the larger mass depending upon the two
masses involved. Include the movement of the larger mass.
Activity 3: Types of Orbits
 Reset the simulation to the original situation by selecting the preset 2-body simulation.
 Set the velocity of the planet to zero, and observe its fall into the Sun. Reset and increase its
speed until it misses the Sun. This is an elliptical orbit.
Value of lowest speed producing an elliptical orbit _________ units


Set the velocity of the planet to 50, and give the time for a complete orbit ___________ s.
Comment on the speed of the planet as it gets nearer and farther away from the Sun:

Set the velocity of the planet to 100. Describe this orbit:
Activity 4: Circular Orbits and the Mass of the Planet
While the original setup for two bodies presents a nearly circular orbit for the planet, a close
examination of the orbit parameters with the tape measure will verify that it is slightly elliptical. In
addition, the relatively equal masses create a significant wobble in the Sun-like inner body. In this
portion of the lab, we want to minimize the wobble and get orbits as circular as possible within the
limits of the simulation- two significant figure calculations.
 Reset the simulation to the original situation by selecting the preset 2-body simulation.
 Change the mass of the orbiting planet to 0.1, and set the velocity to 116. Verify that the orbit is
very close to circular using the tape measure. Find the time for one rotation.
Period for one orbit = _________ s with mass of 0.1 units
©2012, TESCCC
07/16/12
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Physics
HS/Science
Unit: 05 Lesson: 01

Change the mass of the orbiting planet to .001mass units, and find the period.
Period for one orbit = _________ s with mass of 0.001units

Kepler’s laws predict that the period depends only on the mass of the Sun and satellite distance.
Do you agree?__________ (yes/no)
Activity 5: Orbital Radius and Velocity Correlation
Kepler’s laws indicate that a lower orbit (smaller radius) satellite will move faster and have a smaller
period. Newton’s gravity law indicates that the inner planets will have larger gravity acceleration and,
thus, a larger centripetal acceleration. The purpose of this activity is to check out these ideas using
the simulation. The masses of the orbiting planets are set to be very small, so that they do not
influence each other to any significant degree. The orbiting masses will be 0.001, and the Sun’s
mass will be held at 200 units.
 Reset the simulation to three bodies, and alter the radii and speeds in the following manner.
o Set the inner planet radius to 75 units and its speed to 164 units. Set the mass to 0.001.
o Set the outer planet radius to 150 units and its speed to 116 units. Set the mass to 0.001.
 Verify that both planets travel in circular orbits, and measure the periods of revolution for each.
Period for inner planet = ___________ s
Period for outer planet = ____________ s
Do you agree with Kepler that the inner planets move faster and take less time to orbit?
_______ (yes/no)

Newton’s laws give an equation for the gravitational acceleration as g = GM/r2, where M is the
mass of the inner body and r is the radius of the orbit. Newton also gave an expression for the
centripetal acceleration as ac = v2/r for objects moving in circular orbits. Since gravity is the
source of the centripetal acceleration, these two numbers should be the same. In the spaces
below, calculate the values of the gravitational acceleration and centripetal acceleration for the
two planets. Recall that the value of G for this simulation is G=10,000 units and that value should
be used in the calculations.
Calculate the centripetal and gravitational acceleration for the outer planet. Use the equations above.
Show all of your work.
Calculate the centripetal and gravitational acceleration for the inner planet. Use the equations above.
Show all of your work.
©2012, TESCCC
07/16/12
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Physics
HS/Science
Unit: 05 Lesson: 01
Activity 6: View of Planetary Orbits from Different Locations
Early astronomers viewed the motions of the heavens from the Earth and thought that it was the
center of the universe. The motion of the planets was complicated with planets, sometimes moving
in one direction and then looping back on the path. This is known as retrograde motion. From the
perspective of the Sun, this apparent motion is a result of planets moving at different rates and
sometimes passing each other. In this activity, the motion of the Moon is viewed from the Sun’s
perspective.
 Reset the motion, select the preset Sun, planet, and Moon simulation, and play it. Observe the
path of the Moon, and imagine its path as viewed from the Earth. Describe the motion of the
Moon from both perspectives.
Activity 7: Interaction of Satellites – Slingshot and other** (as time permits)
In sending a probe to Saturn, it was necessary to slingshot the probe off planets to gain energy.
View the preselected simulation Slingshot. As time permits, view other simulations and try to imagine
what is happening.
©2012, TESCCC
07/16/12
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