TE Activity: The Great Gravity Escape
Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Summary |
Engineering Connection |
| Engineers and scientists make amazingly precise calculations so that a spacecraft's journey is timed exactly to reach the location where Mars will be at that time. Since Earth and Mars are always orbiting in their own paths, an elliptical transfer orbit from Earth to Mars might be compared to standing on a moving platform and shooting for a basket into a hoop that is also in motion. This requires that engineers think logically and use their math skills to forecast exactly where the planet will be located many months in the future. |
Contents
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| Grade Level: 7 (6-8) | Group Size: 4 |
| Time Required: 50 minutes |
Activity Dependency  :None |
Expendable Cost Per Group
:
US$ 1 |
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| Keywords: orbit, rocket, gravity, Mars, elliptical transfer | |
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Related Curriculum
:
| subject areas | Earth and Space Science and Technology |
| curricular units | Mission to Mars Solar System! |
| lessons | Get Me Off This Planet Our Big Blue Marble |
Learning Objectives (Return to Contents)
After this activity, students should be able to:
- Understand that an orbit is the balancing of object's velocity with the gravitational force.
- Realize that as the velocity of an orbiting object increases, gravity has a harder time keeping the object close.
- Understand that engineers must design and build huge rockets to escape the Earth's gravity.
- Understand that gravity is still acting on an object that is in orbit even though it is a weightless environment.
Materials List (Return to Contents)
Each group should have:
- 1 water balloon
- 5 ft (1.5m) length of twine
- 1 clothes pin (the type with metal springs)
- One stop watch
- One copy of the Orbiting Water Balloons Worksheet
Introduction/Motivation (Return to Contents)
Start off by asking the students if they think gravity is acting on an astronaut orbiting around the Earth? (Answer: Yes, there is gravity present; but, you just cannot see or feel it.) The students might not think gravity is acting on the astronauts because when we see video from space, everything is floating around; however, this does not mean gravity is not acting on the people in a spaceship. There is still gravity, but in an orbit, the tendency of an object to move toward the center is perfectly balanced by the spacecraft's tendency to continue in a straight line away from the planet.
Ask the students what they think would happen if a spacecraft orbiting the Earth kept speeding up? Would the spacecraft get closer or further from Earth? (Answer: further) They should realize as the velocity of the spacecraft speeds up it wants to keep going past and away from the Earth, which means gravity has a harder time keeping the spacecraft close and therefore the spacecraft enters an orbit that is further from the Earth. If the spacecraft continues to speed up, it will eventually escape Earth's gravity. This is exactly what happens when we send a spacecraft to another planet, such as Mars. It takes a lot of energy to get enough velocity to escape the Earth's gravity; however, once a spacecraft has escaped gravity, it can coast to Mars and only have to fire its rockets one more time to slow down as it approaches Mars. In reality, the spacecraft has not actually escaped the Earth's gravity, but it has gotten far enough away that it is not the largest gravitational force acting on our spacecraft any more. Ask the students if they know which gravitational force becomes the largest force once a spacecraft has gotten far enough away from the Earth? (Answer: the Sun's gravity takes over once the spacecraft has left the Earth's gravity.) Figure 1 illustrates the path of a spacecraft traveling from Earth to Mars. Demonstrated is that the path from the Earth to Mars is not a straight line since the spacecraft is actually orbiting around the Sun.
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Figure 1. An Elliptical Transfer Orbit from Earth to Mars. click for copyright |
In today's activity, we will be using water balloons to demonstrate how an orbit is the balance between gravity and the velocity of the spacecraft. We will also see that once an object is traveling fast enough, the orbiting object will escape from the gravitational pull of the planet.
Procedure (Return to Contents)
Before the Activity
- Cut one 5-foot length of the twine for each group.
- Thread one end of the twine through the metal spring on the clothes pin and tie a double knot in it so that the clothes pin hangs from the end of the twine.
- Fill water balloons with approximately 100 grams of water, making the balloons about 2 inches in diameter. Fill two balloons, or more, per group (one for the activity and one for a spare). Temporarily store balloons in a plastic bucket.
- Make enough copies of the Orbiting Water Balloons Worksheet, one per group.
- Find a place outside where each team has at least 20 feet (6 m) clear in all directions around them. The more space they are given, the safer the activity is. (Note: a football/practice field or large lawn area works well.)
With the Students
- Tell the students that today they will use water balloons to learn about orbits.
- Explain to them that they will be the planet and a water balloon — connected to a string that they will use to swing the balloon around them — represents an orbit.
Remind them that they learned that an object that is moving wants to travel in a straight line. For an object to turn, a force must act on it. To create the elliptical (curved) path of an orbiting object the gravitational force is the force that turns the object. To change the path of their water balloons they have to apply a force to the balloon. In this case, the tension in the string will represent the gravity that keeps an object in orbit. Figure 2 shows a basic drawing of what the experiment will look like.
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Figure 2. Activity setup. click for copyright |
- Hold up one length of twine, that already has attached the clothes pin. Demonstrate how to carefully clip the balloon onto the clothespin.
- Ask the students what will happen if you were to begin spinning the balloon around yourself — faster and faster? (Answer: The clothes pin will eventually let go of the balloon.) Ask them why this happens? (Answer: As the balloon spins faster the clothes pin cannot apply the force necessary to keep the balloon in orbit and it will let go. Once this happens the balloon will travel in a straight line according to Newton's First Law of Motion.)
- Pass out the Orbiting Water Balloons Worksheet, a stop watch and a string with clothes pin attached to each group.
- Move students outside to a pre-selected space with adequate room for students to spread out and do the activity.
- Now give each group one water-filled balloon and tell them to securely attach the end of the balloon to their clothes pin.
- Have one group member stand in the middle of a designated open area while the remainder of the group stands back at least 20 feet.
- The student with the string and balloon should start swinging the balloon slowly around their body so the balloon is moving just fast enough to keep the balloon a few feet above the ground.
- Have one of the students that is not swinging the balloon use the stop watch to time 10 seconds. While this person is timing, the other students should count how many times the balloon goes around in those 10 seconds. They should record this number on their worksheets.
- Have the student that is swinging the balloon speed up their balloon slightly.
- Have the other students repeat counting the number of rotations in 10 seconds intervals.
- Repeat steps 10 and 11 until the balloon comes off the clothes pin.
- Now have the students rotate so that each student has a chance to be the planet with the balloon orbiting around them.
- Once all the teams have finished with their activity, have them come back inside and calculate the escape velocity according to the worksheet.
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Make sure students stand a sufficient distance away from the swinging water balloon and that they pay attention to the other students as they perform the experiment.
If students swing their balloons using their whole body (that is, rotating their entire body with each swing), they might get dizzy and could fall. Instead, have them swing with a lasso motion over their heads.
Troubleshooting Tips (Return to Contents)
Make sure the balloon is well secured by the clothes pin. This will ensure that the balloon does not come off the string too early. If the students cannot handle water balloons, it may be a good idea to find another object that is soft, but also weighs around 100 grams, such as a wiffle- or sponge-type ball.
It is better if students use a lasso motion over their heads to swing their balloons as opposed to using their body to swing the balloons, as they will eventually get dizzy.
Assessment (Return to Contents)
Pre-Lesson Assessment
Discussion Question: Solicit, integrate and summarize student responses.
- Ask the students if there is gravity in space? (Answer: They might think there is no gravity in space since it is a weightless environment, but there actually is gravity in space. This is due to the fact that when a spaceship is in orbit, gravity and the velocity of the spaceship are exactly balanced.)
Post-Introduction Assessment
Discussion Question: Solicit, integrate and summarize student responses.
- Ask the students what happens as the spacecraft speeds up? (Answer: the spacecraft will escape the gravitational pull of the Earth.)
Lesson Summary Assessment
Numbered Heads: Divide the class into teams of three to five students each. Have the students on each team number off so each member has a different number Ask the students a question and give them a short time frame for solving it (~1 minute). The members of each team should work together on the question. Everyone on the team must know the answer. Call a number at random. Students with that number should raise their hands to answer the question. If not all the students with that number raise their hands, allow the teams to work on the question a little longer. Example questions:
- What is the force that pulls objects towards the center of the Earth? (Answer: gravity)
- Which of Newton's Laws tells us that an object in motion wants to stay in motion and that the only way for an object to slow down, speed up, or turn is if there is a force acting on the object? (Answer: Newton's 1st Law of Motion)
- True or False: There is no gravity in space? (Answer: false)
- An orbit is the balance between the velocity of the object and what force? (Answer: gravitational)
- As velocity decreases, the object will get closer/further from Earth? (Answer: closer)
Activity Extensions (Return to Contents)
Have the students weigh different balloons and see if there is a relationship between the mass of the balloon and the escape velocity. The students should see that a heavier balloon will release at a lower velocity. (Note: the mass of the larger balloon must be significantly heavier — at least 50% — to see this result.)
References (Return to Contents)
Wertz, James R. and Larson, Wiley J. Space Mission Analysis and Design, 3rd Edition, Space Technology Library, Volume 8, New York, NY: Publishing Company, 1999.
Contributors
Geoffrey Hill, Daria Kotys-Schwartz, Chris Yakacki, Malinda Schaefer Zarske, Janet YowellCopyright
© 2004 by Regents of the University of Colorado.The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.
Supporting Program (Return to Contents)
Integrated Teaching and Learning Program, College of Engineering, University of Colorado at BoulderLast Modified: September 26, 2008