This activity discusses the concept of gravity, how it works, and its all important role in the formation and maintenance of the universe.
To teach the fundamentals of astronomy as it relates to gravity.
Students will be able to:
- Describe how the size, distance, and relative speed of objects affect their gravitational attraction.
- List five common objects in the universe and describe then in terms of gravity.
Set up: 10 min.
Activity: 30 min.
- 1-2 Cardboard cards (3 x 5) per student
- A large piece of poster board
- An auditorium
|escape velocity||neutron star||supernova|
Anything that takes up space is said to have mass. All forms of matter, including plants, animals, rocks, liquids, and gases, have mass. A mountain has much more mass than a mouse but, mass is different than size because a marble has more mass than a inflated balloon. Even forms of energy like light waves have a tiny amount of mass. For more discussion on mass and matter refer to Do we need to Diet on Mars? activity.
Mass and energy are closely related. Matter stores energy. When matter is put under stress it releases energy. For example, when wood is burned it releases energy in the form of heat and light. Albert Einstein's formula:
E = energy, m = mass, c = the speed of light 186,282 mi / sec (300,000 km / sec)
which suggests that not only can a small amount of matter create a huge amount of energy, but also that a huge amount of energy can create a small amount of matter.
All matter in the universe is expanding outward in all directions at once, as if long ago everything exploded from a single point. Astronomers explain this phenomena with the Big Bang Theory. The Big Bang Theory suggests that everything began 13.7 billion years ago in a colossal explosion. Initially the universe only existed as energy, but as it began to cool and expand itself across the void of space this huge amount of energy quickly created the simplest forms of matter: hydrogen and helium gases. With matter came the mysterious force of gravity.
All matter has gravity, and gravity affects all matter and even some forms of energy, like light. Gravity is the mutual pull or attraction of objects towards each other. The English scientist Isaac Newton measured gravity using the following formula:
Gravitational Attraction =
(K) x (Mass of object 1) x (Mass of object 2) / Distance^2)
where K = Gravitational Constant
More massive objects have more gravity than smaller objects, but the most important variable that determines the strength of gravity is distance. As objects move further apart, their gravitational attraction to one another dramatically decreases.
The escape velocity for Earth is about 25,000 mph (40,320 km per hour), therefore any object traveling that speed or faster has the ability to leave the gravitational pull of Earth. Objects that move very fast but do not approach the speed of escape velocity are said to be in orbit. Instead of flying off into space they resist the force of gravity by revolving around the source of gravitational pull. This is why our Moon never crashes into Earth. Moon's velocity and Earth's gravitational pull balance each other out so that Moon neither escapes nor collides with Earth. However this balance is not perfect, our Moon is slowly drifting farther and farther, slowly increasing the size of its orbit.
Gravity is thought to be the main force that shapes and organizes all matter in the universe. Because of gravity's consistent and predictable influence the largest objects in the universe are similar to the smallest objects. For example, galaxies form in a similar manner as our solar system did.
After the Big Bang, gravity began to pull gases like hydrogen and helium closer and closer together, forming billions of enormous clouds called proto-galaxies. As each proto-galaxy continued to condense, some of the radial motion from the Big Bang was converted into angular momentum causing the cloud to spin. The more the cloud condensed the faster it rotated, just as a figure skater spins faster when she pulls her arms and legs in towards her. Within the proto-galaxy billions of much more dense clouds called nebulas began to form and also spin. Most of the matter in these nebulas eventually concentrated into individual zones. At these centers, the hydrogen gases were under so much gravitational pressure that they condensed and ignited to form stars where hydrogen is first converted into helium gas, and later into other heavier elements. This conversion process known as nuclear fusion creates phenomenal amounts of energy, which is why stars burn so hot and brightly.
These first-generation stars continued burning for millions and billions of years until they ran out of hydrogen. Without hydrogen they began to fuse helium into carbon, oxygen, and iron causing the star to swell in size. The largest of these stars, called supergiants, died in catastrophic and blinding explosions called supernovas. Supernovas are so bright that they can sometimes be seen on Earth in the day time! During these powerful explosions all the more complex and heavy forms of matter including silver, gold, lead, uranium, etc. are formed and blasted into deep space forming new smaller nebula.
These nebulas will again contract under the power of gravity and began to spin, eventually spawning second-generation stars and also something new: planets. It is thought that planet formation is only possible in nebulas with heavy minerals. Because heavy minerals have more concentrated mass and therefore more gravity, that makes it possible for multiple gravitational centers to form in addition to the proto-star at the center. The local gravitational pull of these proto-planets were stronger than the larger but much more distance influence of the central star. The more matter these early planets absorbed, the stronger their gravitational attraction became. Increasing mass also caused the planets to accelerate along their orbital paths, thus better resisting the pull of the growing star.
As these maturing planets raced through the nebula, the larger planets captured smaller ones, which became their moons. For about a billion years the young planets endured countless impacts from smaller asteroids and comets. At last, the free floating material that once made the nebula was either all absorbed into the star and planets, or blasted away into deep space by the diverse mix of energy, called solar wind, emitted by the young star. It is in this manner that our solar system formed.
Therefore, gravity is more than just what makes things fall on Earth. Gravity is the great glue that shapes and maintains the universe.
Note: Recent discoveries involving distant supernovas suggest, as Albert Einstein once speculated, that an anti-gravitational force also exists. It had always been assumed that gravity slows down the universe's expansion, eventually reversing the motion so that the universe might end in a Big Crunch. However, the analysis of light from distant supernovas suggests that the universe is NOT slowing down, but continuing to accelerate! Initial speculation is that a property of nothingness is repulsion. Perhaps the more nothing there is between objects, the more powerful this anti-gravity force is? If this is true, the universe is getting bigger at a faster and faster rate. All of the galaxies are growing farther apart from each other.
- Present background information as age appropriate.
- Spread children out in a large open space. Tell them that they each represent different particles of matter floating in empty space.
- Have children count the # of letters in their combined first and last name and hold up that # of fingers (If > 10 hold up a fist to represent 10 + # fingers on other hand. If >15 then round down to 15) Alternative have kids write their number of letters on a piece of paper and hold up for all to see. Explain that this number represents their mass.
- Have children find a neighbor that has the biggest # close to them.
- In a turn based manner have each child take the number of heal-to-toe ("baby steps") toward their closest neighbor that their neighbor is holding up. Note: when played correctly 1 child may walk toward another child who is walking in another direction because of the "pull" of a closer and/or more massive child. It may be good to explain that this is how the game works in advance because younger kids can easily feel bad when they are playing chase because nobody is walking toward them.
- When kids approach within arms reach they "collide" and stop taking steps. They join (add up) their masses together and look for a new child (particle) or group of children (combined particle) to move toward.
- Continue until all students form one or two giant balls. Note: binary stars are the most common type of star systems. It's surprising how many times this game quickly results in two big balls and therefor serves as a good model of gravity.
As the game progresses, ask the students to speculate which students are most and least likely to be absorbed by larger objects. Ask them how many turns they think it will take before all students are joined as one object. As the game is played periodically stop and encourage the students to discuss what they see as analogous to the formation of a solar system. Are there planets? with moons? How many stars? Etc.
After the game ends in one or two huge balls (stars) simulate a star exploding (supernova) by having all the kids now take as many gigantic steps away from the center as they have letters in their combined first and last names (their individual mass). Explain the big steps by saying that the power of a supernova is much stronger than the gravity that once held the star together. Next turn have them take the same # of regular sized steps (bigger than baby smaller than gigantic) away from exploded stars center, explaining that gravity though weak is persistent and will eventually slow the explosion. Now you can replay the game all over again!
The Solar System!
After the supernova variation have the children who's masses are less than 10 (number of combined letters) instantly "gravitate" back into a big stellar ball. The remaining children with masses > 9 now become proto-planets. Let them take 1 turn (or 2 turns) only of baby steps in which they will either get sucked into the new star or bump into each other to combine bigger planets, or remain solitary. On the next turn don't let the planets combine or fall into the star anymore but instead have them regular step (bigger than baby, smaller than gigantic) out their masses in circular orbits. Help them stay in a circular orbit as they walk by, explaining they should try to maintain the exact same distance from the star at all times. What you should find and discuss is that the outer planets will appear to be moving very slowly even if they are taking more steps because their orbits are so much larger. This is in fact how the solar system really works! (for more help review Solar System Size up).
The Solar System with Moons!
Works best if played after the The Solar System variation. Instruct all the planets to crash back into one big star and supernova outward as before. However this time, after the proto-planet children (masses > 9 ) have taken their 1-2 baby step turns and combined into larger planets, instruct the smaller mass child(ren) (there maybe more than 1) to baby step back away from the single largest mass child. Explain that the largest mass child in the proto-planet group becomes the planet while the other(s) become a moon or moons of that planet. Now for some fun and chaos! Instruct the moon children to make their orbits in their mass number of gigantic steps around their planets while the planets orbit the sun in their numbers of regular steps. Invariably (just as in real star systems) moons will come within arms reach of their planet and if so the planet should pull the moons in and add the moons mass to the planets. Don't let the learning aspect of this analogy get completely overshadowed by the fun. Ask questions that keep the kids creatively thinking and learning.
Using Real Math!
For older kids try using an actual formula appropriate to the student's math ability. Both of these variation require a measurement or at least an approximation of distance between children. Instead of actually measuring (which requires 1 tape for every 1-2 kids), play on a prepared gigantic grid (at least 20 x 20 in size) by marking with street chalk on a paved playground or parking lot. The number of squares between children can counted to measure of distance. This game can also be played in an empty auditorium (where it is safe and permissible for children to climb over the backs of chairs) where the number of seats between children can be measured as distance.
Formula 1 (simplified version):
(Mass of CS) - (Distance to CS) = # of steps you walk toward CS
CS = closest student or closest combined group of students
Note: If (Mass of CS) < (Distance to CS), 0 steps are taken because there's not enough gravitational pull.
Formula 2 (Newton's actual formula):
(Your Mass) x (Mass of CS) / (Distance to CS^2) = # of steps
CS = closest student or closest combined group of students
Note: Round off decimals or fractions
Encourage the students to do further research into the planets. Assign them research papers and or presentation on individual planets.
Included National Parks and other sites:
Utah Science Core:
3rd Grade Standard 3 Objective 1,2
3rd Grade Standard 4 Objective 1,2
6th Grade Standard 1 Objective 2
6th Grade Standard 3 Objective 2