To explore how the earth’s and moon’s gravity affect the path of a rocket launched into space.
Two kinds of forces we are commonly aware of are gravitational and electromagnetic. This lesson focuses on the former. All objects in the universe exert gravitational force on all other objects, and the focus of this lesson will be mainly on earth's gravitational pull, as well as forces and how the motion of an object changes.
So far, students will have had experiences with learning about forces and motion in a very general way and without using scientific language. They will know that pushing and pulling an object will change how it moves and that "things near the earth fall to the ground unless something holds them up." (Benchmarks for Science Literacy, p. 94.)
This lesson will start with an introduction to forces. While students know that, for instance, pushes and pulls move things, it is important that they realize how the word and the definition of force are applied. They will build on this knowledge with the vocabulary and understanding of motion, speed, launch point, and thrust, and how these things relate to force.
At this age, students also will be ready to explore and "…work out for themselves some of the general relationships between force and change of motion..." (Benchmarks for Science Literacy, p. 89.) The heart of this lesson will focus on a science app in which students play the role of a space pilot engaged in missions of varying difficulty that require them to overcome the force of gravity in order to dock with one or more space stations. Students will have control over the launch point and thrust of the rocket and should see the relationship between the two as they change these settings.
Research suggests that elementary-school students typically do not understand gravity as a force. They see the phenomenon of a falling body as "natural" with no need for further explanation. While this lesson will point out that things fall because of gravitational force, or that they are pulled to earth, it will not delve into much more of a discussion. The concept of gravity, and that all objects pull on all other objects, is a concept that will be better explored in later grades. Research also states that students may not realize the magnitude of the earth's gravitational force. Though this will be brought up in discussion, students still may not have enough of a base understanding at this age to understand the magnitude. (Benchmarks for Science Literacy, p. 340.)
Note: For your own review, you may want to read the essays in Science for All Americans, under 4B, 4F, and 4G. These essays may give you ideas for further discussion throughout the lesson.
Start this lesson by doing an activity that will help you go over the concepts that gravity is a force that depends on mass and distance; that mass is not the same as weight but rather the amount of stuff that makes up a thing; and also that weight is a force. It is important you determine whether or not students have a good understanding of forces, motion, etc.
To do this, stand at the front of the classroom and drop a beanbag to the floor. Ask students:
- Why did the beanbag drop to the floor?
(If students think that things just "naturally" fall, this is a misconception and you will need to spend more time on this question. They should understand that it fell to the floor due to the "force of gravity." If they do not use the word force, ask them what gravity is.)
- Does the earth have to touch something to pull on it? Does everything fall to the ground when it is pulled on by earth?
(The answer to both is no. What about earth's atmosphere? What about the moon or orbiting objects? Earth doesn't pull everything to the ground, so the point here is that earth pulls on objects period.)
- What happens to something when force is put on it?
(This question is to enforce that a force exerted on something makes motion. Students may say "falling" since you've been discussing gravity. If so, push a chair and say that you've just exerted a force on the chair. What happened?)
- What if I pushed harder (put more force) on the chair?
(The chair would move faster or farther.)
- What does weight have to do with the force of gravity?
(Weight is a force, and its strength depends on the strength of gravity. You could give the example of the moon having six times less gravity than earth. Students would weigh only 1/6th of their weight on the moon.)
- What does distance have to do with the force of gravity?
(If two things are farther apart, there will be less of a pull.)
- Can you define the word force?
(Try to get students to define it in the broadest sense, basically that force causes motion. But do not discount more specific answers, such as force pulls on things or pushes on things.)
This time, toss the beanbag up into the air a bit and then let it fall to the ground. Ask students:
- Did the beanbag fall to the ground the same way as last time? Why or why not?
(Students will likely say no, which is correct, but be sure they understand that force pushed it up into the air, and also pulled it to the ground.)
- What do you predict would happen if more force were used in tossing the beanbag? What if the angle were just right and it got going fast enough?
(Students should realize that it would go much farther.)
In this part of the lesson, students will use the Gravity Launch App, a science app available for both iPad and Android-based tablets in which they pilot a rocket ship to one or more space stations. They will learn how the force of gravity can pull an object toward the earth and moon, and experiment with different thrust and launch point settings to learn how these settings change the motion of the rocket.
First print out and give students the Gravity Launch Mobile student sheet. Then, they will use the Gravity Launch Mobile student esheet to access the app. The esheet includes instructions on how to play, as well some more information about gravity. There is not just one setting for each mission. (You may want to have groups of two work on this app. One student could put settings in while the other writes down observations. Be sure students switch off.)
Before students begin the games, they should explore the About Gravity section. Discuss these questions with them once they have gone through that section:
- How does the size of an object affect its gravitational force?
(Larger objects have stronger gravitational force or pull.)
- How does the distance between objects affect how gravity acts on them?
The closer you are to an object, the more its gravity pulls on you.)
- What is the force produced by a rocket's engines called?
(It is called thrust.)
- What does it mean to be in orbit around something?
(It means to go around and around another object.)
We suggest that students start with Get to Orbit followed by Land on the Moon games to familiarize themselves with the thrust and launch point dials.
Since this is an answer-driven activity, we recommend that you closely monitor your students as they play this game. It takes a bit of time to get the numbers right on the thrust and launch point. We also recommend that you ask individuals direct questions as they do the app to be sure they understand what it is they are trying to achieve, and more importantly, how the launch point and thrust help in reaching the goal.
Please consult the Gravity Launch Mobile teacher sheet for some correct thrust and laungh point settings for the games within each mission of the app.
Give students about ten minutes to do the first launch. This is a generous amount of time, but unless they are being systematic about the settings, it could take this long. Below are questions you may want to discuss during and after the game. These questions are not on the esheet:
- Did anyone have their rocket ship pulled back to earth for a crash?
(This will likely happen to some of your students since any thrust setting that is too low for a game does not exert enough force to get the rocket ship far enough away from earth.)
- Why do you suppose it did that?
(If there is not enough thrust or force on the rocket ship, it will go up and just come right back down. Here you can discuss how enough thrust/force is needed to get beyond the gravitational pull, also a force.)
- Did anyone have their rocket ship crash on the moon?
(This also could happen to some students if the thrust still doesn't exert enough force to overcome the gravity of the moon.)
- Did anyone observe the earth or the moon pulling the rocket ship toward it? Did it change the motion of the rocket ship?
(Students may have noticed the moon having an effect on the rocket ship's path. Discuss that this pull/force also changed the motion of the object.)
- Did you have to use different strategies to get the rocket to two or more space stations as opposed to just one?
(Answers may vary. Encourage students to explain their answers.)
- In the Advanced mission, how did the movement of the moon affect how you chose the thrust and launch point settings?
(Answers may vary. Encourage students to explain their answers.)
Ask these questions after students are done with the interactive:
- What do you think the thrust of the rocket ship is?
(Basically, you want to be sure that students understand that when an object is moved, a force moves it, and thrust imposes a force great enough to get the rocket off the ground.)
- What do launch point and thrust have to do with the motion of the object?
(Discuss how these things change the motion. The launch point also worked with or against gravity because it helped aim the rocket ship.)
- What did you observe with Easy Game 1 where you were instructed to put in certain thrust and launch point numbers?
(This was a demonstration of the rocket ship going into orbit around the earth and moon several times. Discuss that it orbited due to the pull of the earth and moon and was actually "falling." The speed it was traveling was due to the thrust and the pull.)
The assessment will go back to what you demonstrated at the beginning of the class, only this time, students will do the beanbag tossing and explaining amongst themselves.
Divide your class into five or six groups and give each group a beanbag and two pieces of masking tape. Each group will need enough space to toss the bags about five feet. Weather permitting, outside is also an option.
Students should follow the directions on their Beanbag Toss student sheets to make a target and play with different angles and speeds of the beanbags to hit the target. They also are instructed to write down their observations, which is what you will use to determine what they have learned in this lesson.
The observations section should in one way or another incorporate these observations:
- That the beanbags are pulled toward earth by its gravity;
- That the earth does not need to touch the beanbags in order to pull them down; and
- That different changes in how the bag is thrown, meaning speed or direction, is caused by a force from a person's arm.
For another lesson that explores gravity, see the Science NetLinks lesson, Falling.
Falling for Gravity is a Science NetLinks Afterschool activity that allows students to do some experiments and put gravity to the test.