Gravity is one of the oldest and most celebrated topics in physical science. Early benchmarks about the earth's gravity, the relationship between forces and motion, and observations of the sky support more sophisticated benchmarks about orbits and universal gravitation in later grades. Often, the largely descriptive benchmarks in the other three maps of this cluster depend on the gravitational explanation presented here, and, reciprocally, benchmarks about gravity build on observations of the sky and ideas about motion and orbits.
Several historical episodes in Science for All Americans and Benchmarks Chapter 10: HISTORICAL PERSPECTIVES could extend students' understanding of gravity. They include the account of heavenly motion in Displacing the Earth from the Center of the Universe, Newton's use of gravity to explain heavenly motion in Uniting the Heavens and Earth, and Einstein's theory of general relativity in Relating Matter & Energy and Time & Space, which pictures gravity as a distortion of space and time.
Universal gravitation, introduced in 6-8, is a very abstract idea and is especially hard to grasp because gravitational forces between objects such as soda cans, pencils, and people are not noticeable. Understanding changes in motion and observations of the solar system, and the fact that at least one very large mass must be involved for the effects to be easily detected, will make this idea more credible.
The counterintuitive idea that the earth is a sphere—necessary for students to understand that "down" is toward the earth's center—is supported by a K-2 benchmark about shapes. (Students will, of course, need to have experience with three-dimensional as well as two-dimensional shapes.)
In the early grades, students are probably not ready for a gravitational explanation of the difficult idea that the earth orbits the sun, but they should get a rough idea of the scientific description nonetheless. Even for this purpose, students need an early sense of how apparent motion depends on frame of reference. The relative motion strand includes a 3-5 benchmark from the relevant essay in The Earth section of Benchmarks Chapter 4 and a more sophisticated 6-8 benchmark from Benchmarks Chapter 10: HISTORICAL PERSPECTIVES.
The true nature of the motions of the sun, stars, and planets is not easily discovered just by observing them. This fact reveals the potential in instruction for relating the benchmarks in this map to models. Instructional strategies can also draw on the relationship between benchmarks in this map and benchmarks about systems and scale in Benchmarks Chapter 11: COMMON THEMES.
Research in Benchmarks
Student ideas about the shape of the earth are closely related to their ideas about gravity and the direction of "down" (Nussbaum, 1985a; Vosniadou, 1991). Students cannot accept that gravity is center-directed if they do not know the earth is spherical. Nor can they believe in a spherical earth without some knowledge of gravity to account for why people on the "bottom" do not fall off. Students are likely to say many things that sound right even though their ideas may be very far off base. For example, they may say that the earth is spherical, but believe that people live on a flat place on top or inside of it—or believe that the round earth is "up there" like other planets, while people live down here (Sneider & Pulos, 1983; Vosniadou, 1991). Research suggests teaching the concepts of spherical earth, space, and gravity in close connection to each other (Vosniadou, 1991). Some research indicates that students can understand basic concepts of the shape of the earth and gravity by 5th grade if the students' ideas are directly discussed and corrected in the classroom (Nussbaum, 1985a).
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 or they ascribe to it an internal effort of the object that is falling (Ogborn, 1985). If students do view weight as a force, they usually think it is the air that exerts this force (Ruggiero et al., 1985). Misconceptions about the causes of gravity persist after traditional high-school physics instruction (Brown & Clement, 1992) but can be overcome by specially designed instruction (Brown & Clement, 1992; Minstrell et al., 1992).
Students of all ages may hold misconceptions about the magnitude of the earth's gravitational force. Even after a physics course, many high-school students believe that gravity increases with height above the earth's surface (Gunstone & White, 1981) or are not sure whether the force of gravity would be greater on a lead ball than on a wooden ball of the same size (Brown & Clement, 1992). High-school students also have difficulty in conceptualizing gravitational forces as interactions. In particular, they have difficulty in understanding that the magnitudes of the gravitational forces that two objects of different mass exert on each other are equal. These difficulties persist even after specially designed instruction (Brown & Clement, 1992).