GO IN DEPTH

# Gravity Launch

#### Materials

• 6 beanbags (or other non-breakable object that can be dropped to the floor for demonstration)
• Pencil
• Masking tape

### Purpose

To explore how the earth’s and moon’s gravity affects the path of a rocket launched into space.

### Context

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, angle, and thrust, and how these things relate to force.

At this age, students will also 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 an online interactive, in which students launch a rocket from earth on various space missions. Students will have control over the angle 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.)

### Planning Ahead

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.

### Motivation

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. Since they just saw a movie on gravity, they should know 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? (As the movie gave for an example, 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.)

### Development

In this part of the lesson, students will use Gravity Launch, an online interactive in which they send a rocket ship to a docking station. They will learn how the force of gravity can pull an object toward the earth and moon, and experiment with different thrust and angle settings to learn how these settings change the motion of the rocket.

First print out and give students the Gravity Launch student sheet. Then, they will use the Gravity Launch student esheet to access the interactive. 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 interactive. One student could put settings in while the other writes down observations. Be sure students switch off.)

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 angle. We also recommend that you ask individuals direct questions as they do the interactive to be sure they understand what it is they are trying to achieve, and more importantly, how the angle and thrust help in reaching the goal.

Here are some correct thrust and angle settings for each mission, 1-5, of the interactive:

 Mission Thrust Angle Thrust Angle 1 6.35 50.844 OR 6.7 17.699 2 6.9 31.5 OR 7.0 34.624 3 7.112 42.506 OR 8.0 37 4 7.0 45.9 OR 7.6 12.39 5 6.9 52.251

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 below six 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 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.)

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 angle and thrust have to do with the motion of the object? (Discuss how these things change the motion. The angle also worked with or against gravity because it helped aim the rocket ship.)
• What did you observe with number 4 where you were instructed to put in certain angle and thrust numbers? (This was a demonstration of the rocket ship orbiting around the moon several times. Discuss that it orbited due to the pull of the moon and was actually "falling." The speed it was traveling was due to the thrust and the pull.)

### Assessment

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 Gravity Launch 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.

### Extensions

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.