PURPOSE:
The purpose of this activity is to further uncover the students' notions of gravity as the main mover and shaper of the Universe. It should help them develop ideas about the enormous impact that gravity has on the Universe, e.g., the shape and movement of the planets, the Solar System, stars and galaxies.

OBJECTIVES:
• Students will be able to identify gravity as the main mover and shaper of the Universe.
• Students will identify gravity as a force and will cite examples of gravity throughout the universe.

BACKGROUND INFORMATION:
This activity was adapted from "Activity 2: The Earth's Shape and Gravity¹" from Earth, Moon and Stars. It is intended to follow as a continuation only after that lesson has been completed.

Early on in their development, children learn that dropped objects fall to the floor. This exercise will help students understand that: All objects that contain mass fall together as a result of their gravitational attraction, unless balanced by other forces.

As with the Earth, Moon and Stars lesson: "Please keep in mind that ideas and insights about the Earth's shape and gravity develop gradually. Getting the 'right answer' is not as important as the critical thinking skills that students develop as they struggle to apply their mental models of the Earth to real and imaginary situations."²

INTENDED AUDIENCE:
9th-12th grade

TIME REQUIRED:
45 min.

MATERIALS:
• 1 copy of worksheet per student

PROCEDURE:
1. Gather the same materials as the lesson in the book and copies of the "What are your ideas about Gravity?" worksheet for each student.
2. Follow the same procedures as in Part I of the lesson in the book with the new worksheets. Allow the students to use their worksheets from the lesson in the book and any other notes they may have taken for reference.
3. Follow the same procedures as in Part II of the lesson in the book using the information below to help facilitate the discussion.

EXPLANATION:
Question 5. The apple will hit the ground first on Earth because it is not slowed down by air resistance as the feather is.

Both objects will hit the ground at the same time on the Moon because there is no atmosphere to create air resistance. Think about a sky diver whose plummet slows after he or she opens the parachute. The diver still weighs the same, but his or her surface area increases, and the increased air resistance slows the diver. On the Moon, there is no atmosphere, so no matter how much you increase the surface area, the diver would fall at the same rate.

The acceleration due to gravity really does not depend on the object being dropped, but on the mass of the planet or moon that they are being pulled towards. Of course, the acceleration due to gravity is less on the Moon than it is on the Earth (because the Moon is less massive than the Earth and therefore has less gravitational pull on objects) so the apple and feather will both fall more slowly on the Moon than on the Earth. But both will reach the ground at the same time on the Moon. You can view a movie of an astronaut dropping a hammer and a feather at

http://www.vesuvius.jsc.nasa.gov/er/seh/feather.html

Students often think that a heavier object will fall faster than a lighter object. However, this is not true. When there is no air resistance, the objects are in free fall. The only force acting on those objects is the gravitational force between the object and the more massive body (the Earth or Moon). Newton's Second Law states that F=ma (Force equals mass times acceleration). Stated another way, the acceleration (a) is directly proportional to the net force (F) and inversely proportional to the mass (m) of the object. Since the gravitational force also depends on the mass of the falling object, the ratio of F to m is the same for both objects. On Earth, F/m = the acceleration due to gravity or 9.8 m/s². In other words, starting from a speed of zero, the object's speed will increase at the same rate no matter what the mass of the object is.

Since weight is defined as the mass of an object multiplied by the acceleration due to gravity, and since a pound is a unit of measurement of weight, a pound of feathers always weighs the same as a pound of bricks. (Of course, because the gravitational pull of the Moon is less than that of Earth, on the Moon it would take a lot more feathers to make up a pound than it would on Earth.)

Question 6. The rocks should fall just as in Question 3, towards the center of the group of rocks. They will stop falling when there is no more space between the rocks and they can support each other from the center of the group outwards. In other words, they will continue to fall due to the force of gravity until the force of another rock pushing up from below balances that force.

The balancing of gravity and other forces is what determines the shape of almost everything in the Universe. The stars gravitationally collapse until the force of gravity is balanced by the pressure of the hot gas inside. The galaxies are also shaped by the balance between the forces of gravity and the motion of the stars.

The Earth is round because each rock would pull on every other rock until they were all as close together as possible, resulting in a round Earth. This is also true of the other planets and stars.

Question 7. Neglecting air resistance, the rock should fall on a parabolic path, eventually hitting the surface of the Earth, and coming to a stop. The rock falls that way because once the rock leaves the person's hand, there is only the force of gravity pulling on the rock. The rock continues to move to the "right" with whatever velocity it left the person's hand, and it falls to the earth with the acceleration due to gravity of 9.8 m/s².

This question can serve as a check to make sure that students understood question 3 from the Earth, Moon and Stars lesson. If they still show the rock falling "down" on the page or continuing through the Earth, you may need to revisit the discussion from question 3.

Question 8. The rock should fall on a similar path as in question 7, only this time, traveling farther along the surface of the Earth before hitting the ground.

In the second part of this question, the rock is thrown so hard that it never touches the Earth's surface. The path of the falling rock matches the curvature of the Earth so that with each passing moment the distance between the Earth and rock stays the same. The rock travels around the Earth, in one sense falling because its path is curving towards the Earth, but in another sense flying because it never hits the surface. The rock has been sent into orbit! This is exactly how the Moon interacts with the Earth. The Moon falls constantly towards the Earth, but never hits the surface, because the surface of the Earth curves at the same rate.

Another acceptable answer could be that the rock curves slightly towards the Earth, but never comes back to the place it started. There is a minimum velocity (a threshold) that the rock would have to have to do this; this speed is called the escape velocity. This is an example of how we could send probes to other planets.

Be careful that students who draw their pictures showing the rock leaving the surface of the Earth but not going into orbit are not making a mistake and thinking that the rock is falling "down" on the page and "missing" the Earth.

RESOURCES:
Sneider, Cary I. Earth, Moon, and Stars. University of California, Berkeley: Great Explorations in Math and Science (GEMS), Lawrence Hall of Science, 1986. pp. 9-15. This book can be ordered from http://www.lhs.berkeley.edu/gems/GEM250.html
This book is also available in the Check Out Kit.

² Ibid., p. 9